Introduction

The critical need in shift towards sustainable energy resources made the sugarcane crop to catch researcher’s eye. Sugarcane crop is spread up to 35° N and 35° S latitudes occupying all tropical and sub-tropical regions of the world (Cheavegatti-Gianotto et al. 2011; Calderan-Rodrigues et al. 2021). India occupies the second position in global sugarcane production by contributing 20.8%, standing next to Brazil. In the Indian ecosystem of agro based industries, next to cotton, sugarcane occupies a prominent position and has contributed immensely towards employing around 50 million farmers and 5 lakh labourers in the rural population (Ahmed and Rahman 2014; Price Policy for Sugarcane 2022). According to the Directorate General of Commercial Intelligence & Statistics for 2020–2021, sugar exports from India are valued at 20,669 crore (Price Policy for Sugarcane for 2022). Ethanol, bagasse, and molasses are the secondary products from sugarcane; however, ethanol occupies prime position among secondary products as it serves as a potential biofuel, an alternate for fossil fuels. In Brazil, almost 47% of the crop is used for ethanol production, providing fuel to 40% of the vehicles in Brazil.

Sugarcane is a tall perennial grass, belongs to the Andropogoneae tribe, Poaceae family, and genus Saccharum, which has six species, of which S. spontaneum and S. robustum are wild species, while S. officinarum, S. barberi, S. sinense, and S. edule are cultivated species. The Saccharum complex includes five genera, namely, Saccharum, Erianthus, Sclerostachya, Narenga, and Miscanthus, which are closely interbreeding populations that are involved in the origin of the sugarcane (Daniels and Roach 1987; Daniels et al. 1975; Mukherjee 1957; Dlamini 2021). The interspecific crosses between S. officinarum (2n = 80, x = 10) and S. spontaneum (2n = 40–128, x = 8) followed by the continuous backcrosses with the S. officinarum, resulted in the modern, commercial cultivars with complex polyploid, aneuploid genomes with chromosome numbers varying between 2n = 99–130, comprising 70–80% of S. officinarum, 10–20% of S. spontaneum, and 10% of the recombinant chromosomes of the two species (Butterfield et al. 2001; D’Hont et al. 1996; Premachandran et al. 2011). The ploidy in the hybrid cultivar R 570 is estimated to be 10 × with a genome size of gigabase pairs (Gbp), while S. officinarum, S. spontaneum genome sizes are close to 7.5 and 6.7 Gbp, respectively (Zhang et al. 2012). These differences in genome size and ploidy make it imperative to sequence and gather information from both parents as reference genomes to assemble and analyse the genome of hybrid cultivars.

Flowering is an important stage in the lifecycle of angiosperms, necessary for the further propagation of species. The adaptation of different plant species to different geographical locations is determined by the dependence of the time of flowering on the daylength sensitivity that influences the internal circadian rhythms (Andres and Coupland 2012). This genetically controlled, complex multistage developmental process is regulated and influenced by various environmental factors that are generally categorised into three classes as primary, secondary and tertiary controlling factors. Primary controlling factors include photoperiod. Ambient temperature, irradiance, and water availability fall under secondary factors, while nutrient availability, light quality, and other factors that plants face locally are tertiary controlling factors (Bernier and Perilleux 2005). All these factors influence the expression and regulation of intricately connected of five gene networks under the name (age, autonomous, gibberellin, photoperiod, vernalisation) that are responsible for controlling the flowering phase (Hill and Li 2016; Srikanth and Schmid 2011). Artificial photoperiod, temperature, growth regulators, etc. have been used in different crops to induce early flowering and seed set for rapid advancement of generations during crop breeding (Rai 2022).

In the case of sugarcane, even in the presence of supportive uniform abiotic environment, the intensity of flowering among clones varies over the years, and only a few tillers flower, leading to a non-synchronous flowering pattern and thereby retarding the breeding programmes designed for varietal development. To improve the synchronised flowering for a performing higher number of desirable crosses, researchers started to use photoperiod facilities with controlled environmental conditions; however, building such a controlled photoperiod facility is expensive and is obstructing further research in sugarcane.

Although the flower developmental physiology includes numerous factors at the molecular level, of which few might be species specific, this review of the flowering mechanism in sugarcane at molecular level is written from the perspective of Arabidopsis believing that flower development is common in the angiosperms and in an expectation that concepts understood from the model plant Arabidopsis can be applied to diverse plant species as a rule. However, there is a possibility of evolving species-specific responses through different molecular networks involved in the regulation of endogenous responses to the exogenous signals.

To date, to the best of our knowledge, the present review will be highly beneficial for sugar scientists around the globe. The collection of significant literature in these aspects and its subsequent analysis are necessary to pave the way for future researchers in order to ensure biotechnology advancement.

Need of studying the flowering biology in sugarcane

Many of the sugarcane breeding initiative’s primary goals is to produce cultivars with delayed flowering, so that the crop season can be extended to increase the sucrose yields further (Cheavegatti-Gianotto et al. 2011). As conventional crop improvement programmes make use of the genetic diversity present in the gene pool of the crop species, which relies upon the onset of flowering to make desirable crosses and select useful recombinations. Therefore, synchronous flowering among the selected parents is a prerequisite for hybridization. However, in sugarcane, different genotypes show different responses to the environmental stimuli, which makes it difficult to achieve synchronous flowering and limits the choice of parents that can be selected for crossing (Glassop et al. 2014).

Flowering in sugarcane is influenced by numerous factors, such as daylength, day and night temperatures, humidity, physiological maturity of the cane, nutrient availability, and latitude of its habituated zone. However, the flowering stage depletes the sucrose reserves accumulated in the stalk, thereby a posing the detrimental impact on the commercial production. A void still exists regarding why different sugarcane genotypes and cultivars respond differently, even though the same inductive daylength is prevalent. Consequently, there is a high need for studying and understanding the different molecular pathways involved in the transition from the juvenile phase to the reproductive stage. Detailed studies in sugarcane regarding the development of flowers and their emergence help to manipulate the behaviour of the flowering varieties in commercial fields and thereby prevent the losses incurred by farmers and millers. Besides this knowledge, it is also useful to achieve consistent flowering in the hybridization plots for crop improvement programmes.

Flowering in sugarcane: a boon or a bane

Every stage of development in organisms is genetically determined so is flowering. In case of sugarcane, it is well-established that flowering is also influenced by many other factors. Although flowering is necessary for breeding, in sugarcane, it is known for its negative impacts on productivity, affecting farmers and millers. Under varying environmental conditions around the globe, various sugarcane varieties exhibit differing responses regarding flowering (Singh et al. 2019). Profuse flowering reduces the sucrose content drastically, because, with the transition to the reproductive phase, the sucrose stored in the stalks is remobilized to the apical portions, where it is used for flower development. This remobilization of the sucrose causes dehydration from in the internal tissues of the stalks, causing pithiness, and thereby leads to a significant loss of sucrose and biomass from the stalks (Endres et al. 2016). Cane weight, fibre and pith content, and sucrose percentage in juice are the various factors that are negatively affected by flowering, and loss in cane tonnage depends on the extent of flowering and age of the canes during flowering (Rao and Kumar 2003). In commercial fields practices like withdrawing irrigation, usage of chemicals such as glyphosate, ethephon, sulfometuron and night light break (NLB) are followed effectively to cut back the losses due to flowering (Endres et al. 2016).

Single cane weight declined (Rao and Kumar 2003), while fibre content increased with age in the case of flowering cultivars (Long 1976). Flowered stalks showed higher sucrose percent juice (Rao and Kumar 2003); however, these results differ from the observations made by Rao (1977), where sucrose percentage was observed to be predominantly higher in non-flowered stalks than flowered stalks. Although, the reason behind these conflicting observations is not known. The juice quality did not deteriorate up to 4 months after tassel initiation (Singh et al. 2019), supporting the previous observations made by Thulijaram (1964), where yield was not affected when the canes are harvested within 3 months of flowering/arrowing and the quality of the canes was not affected until two months after flowering (Miah and Sarkar 1981).

These findings suggest that harvesting the crop within 2–3 months after arrowing can neutralize the negative impacts of flowering in commercial fields.

Biochemical changes during flowering in sugarcane

The apical meristem needs to undergo some essential biochemical changes prior to receiving the stimulus translocated from leaves and the inductive environment for transitioning to the floral meristem (Coleman 1969). Acid phosphatases, protein content, reducing sugars, amylose activity, total phenolics, and RNA content were a few biochemical factors chosen for a comparative analysis between the flowering and non-flowering cultivars of sugarcane. A rise in activity of acid phosphatases and amylose, along with an increase in protein content and reducing sugars, was observed in the flowering cultivars compared to non-flowering canes (Chandra et al. 2005). The apical portion of the canes revealed that RNA content among the profusely flowering cultivars, and this rise is positively correlated during pre-inductive and inductive stages. However, this rise in RNA content varied between different flowering varieties (Gururaja Rao et al. 2012). Surprisingly, the phenolic content was more prominent in the non-flowering cultivars than in the flowered canes (Chandra et al. 2005).

This high amount of phenolics could be one possible reason, for the obstructing of the flowering mechanism and non-uniform flowering in sugarcane, but how the flowered canes are overcoming the phenolics needs to be understood.

Effect of photoperiod and temperature on flowering in sugarcane

Sugarcane flowering can be considered erratic as it differs in flowering intensity, percentage of flowered canes, and variation in the repetitiveness of flowering varieties and genotypes flowered in different seasons even in the presence of the same supportive environment (Nayamuth et al. 2003). Diversified Saccharum cultivars have a range of inductive day-lengths depending on their habituated latitude. Sugarcane is classified an as the intermediate daylength plant (IDP), as the behaviour resembles a combination of short- and long-day plants. It is termed by a few researchers as a qualitative IDP (Moore and Berding 2014) for the absolute requirement of inductive photoperiod for the apical meristem to respond, while it is also termed quantitative short-day plant for the requirement of declining day-lengths for the complete evocation of inflorescence (Singh et al. 1986; Moore and Berding 2014). Flowering in sugarcane is a complex biological process that includes several stages of development, each stage requiring its own set of supportive environmental and physiological conditions (Nayamuth et al. 2003).

Sugarcane stalks must reach the stage of “ripeness to flower” (Coleman 1969), which corresponds to physiological maturity, with the formation of 2–4 mature internodes from the base of the stalks, which is necessary for the canes to be receptive to the photoinductive stimulus (Nayamuth et al. 2003). A specific sequence of daylength is required for the proper flower development in the Saccharum clones (Julien 1971). From the research conducted on the artificial induction of flowering, it was found that flower development is dependent on the diminishing day-length and concluded that a decrease of 50 s per day prompted an early flowering (Edwards and Paxton 1979). 15 cycles of night with a length of 11.5 h are considered the minimum number for initiating flowering (Moore and Berding 2014).

The flowering response in sugarcane was observed to change according to the changes in light intensities as well. Light intensity of 1399-lx delayed panicle emergence substantially then the 86-lx indicating that higher light intensities are not in favour of flower development (James and Smith 1969). However, the reason behind this differing response to high and low light intensities is unknown. In the parental clones selected for crossover, synchronized flowering is the major setback observed. Night light break (NLB) is using in the breeding initiatives, where light is provided for 1–2 h in the middle of the night in order to delay the flowering in early flowering clones, so that these clones will be in synchrony with late flowering clones (Midmore 1980).

Along with inductive photoperiod, temperature is also a necessary factor for this complex developmental phase. As the biochemical processes are associated, each stage of flower development is temperature dependent too (Coleman 1969). Two temperatures (28 °C and 23 °C) are verified as the optimum day and night temperatures, respectively (Clements and Awada 1967). Daytime temperatures exceeding 31 °C significantly delay the emergence (Nayamuth et al. 2003), and the sporadic occurrence of night temperatures below 18 °C is detrimental to flowering intensity (Gosnell 1973). Cold temperatures at night below 15°c are responsible for anther abortion and male sterility or may be responsible for the production of unviable pollen. Any deviation in these stimuli at any stage of floral development can cause a reversion to the vegetative stage (Julien 1971). The seasonal variations in the temperature can be one of the reasons for sugarcane clones to show irregular flowering behaviour.

Flowering in sugarcane has a functional relationship with the latitude too. The flower emergence in saccharum hybrids of the northern hemisphere begins at the equator by September and proceeds 10° N by October, 20° N by November, and 30° N by December, while in the case of southern hemispheres, the flowering is noticed from March to June (Moore and Berding 2014). In the Indian subcontinent, saccharum hybrids flower by 20th August in Coimbatore (11° N) and by 30 September in Karnal (29.7° N) (Panje and Srinivasan 1959).

Genus Saccharum is habituated across the globe, spread out in different zones of latitudes, as different latitudes have differing temperatures and day-lengths. This explains the reason behind varied flowering responses of saccharum clones to a wide range of photoperiods and temperatures.

Effect of nutrient levels and moisture availability on flowering in sugarcane

The flowering response also depends on the soil moisture availability, which is useful for the synthesis of the hormones, metabolites, and their translocation to the shoot apex for the meristem to perceive the stimulus (Nayamuth et al. 2003). Low soil moisture obstructs tassel formation and delays flowering. Flag leaf emergence is the first indication that supports the transition to the flowering phase, but this emergence does not support the prediction of date of inflorescence emergence (Melloni et al. 2015). The panicle’s emergence also depends on the availability of moisture.

At the flowering initiation stage, higher proportions of nitrogen (N) in canes are known to delay flowering (Clements and Awada 1967; Nuss and Berding 1999). High nitrogen levels showed a negative correlation with initiation, emergence, and delayed flowering. However, there is no association found between nitrogen application and the number of days to flower or flowering intensity. Before beginning the photoperiod treatment, nitrogen supply is withheld to achieve floral initiation in higher number of plants, and it is followed as a common practice for sugarcane in many breeding initiatives for sugarcane (Hale et al. 2017), but if the floral initiation process has already started, nitrogen (N) and potassium (K) promote flowering, so withholding N application after floral initiation would lead to suppression in further flower’s development, process. Previous studies on various species mentioned that sucrose promotes the flower development while K is involved in the translocation of sucrose to apical portion. However, calcium acts antagonistic and competes with K at the site of absorption, thereby reducing the concentrations of K and a causing negative impact on flowering. Manganese did not show any correlation in response to flowering stages. (Brunkhorst 2001; Endres et al. 2016; Menshawi 1978; Bell and Leigh 1996; Gajdanowicz et al. 2011).

The insights into nutrient levels and their effects on flowering behaviour help manipulate flowering responses and achieve flowering in shy and in non-flowering varieties to a certain extent by changing the fertilizer regimes accordingly, so that the number of individuals under the crossing programme can be higher.

Looking through the lens of grasses to understand molecular mechanisms underlying in sugarcane

The floral transition machinery is so closely linked to the signals from the habituated environment. The migration of mankind and the spread of crops to different latitudes resulted in their adaptation to a diversified environment, for instance, consider the photoperiodic behaviour of plants: in Arabidopsis, wheat, barley, and oats are developed as long-day plants; rice, sorghum is short day plants; and maize, a day-neutral plant. This led some plants to develop unique pathways for integrating and transmitting floral inductive signals, while others have adopted the already conserved pathways like their ancestors (Colasanti and Coneva 2009). As all these grasses are monocarpic in nature, appropriate timing for their transition to the reproductive phase is very important for the perpetuation of species as well as from an agronomic perspective, as time and season of flowering and the response to changes in the daylength affect the yield potential directly (Hill and Li 2016; Colasanti and Coneva 2009).

Apical meristem differentiation into a floral meristem and flower formation is ancient characteristics shared by all flowering plants, and the genetic conduits that regulate this chain of events are substantially preserved in angiosperms (Colasanti and Coneva 2009). Development of reproductive structures is a genetically regulated mechanism affected by environmental signals and endogenous conditions of plants. Shoot apical meristem being the site of response; floral inductive signals direct the gene expression pathways, thereby triggering the shoot apical meristem (SAM) transition to the floral meristem (Bernier and Perilleux 2005). The biology of flowering time in Arabidopsis thaliana has revealed that the responses to various external, internal stimulative conditions and how they are interconnected as a sophisticated gene regulatory network that governs this transition (Wellmer and Riechmann 2010), several studies conducted on Arabidopsis and various flowering plants helped in categorizing the different genes involved in the regulatory networks responsible for the floral induction into five responsive pathways, namely, autonomous, endogenous-age, gibberellin, photoperiod, and vernalization pathways (Srikanth and Schmid 2011; Coelho et al. 2013), all these pathways communicate with one another constructing an integrated regulatory network, and the respective signals from different concerted pathways are routed through several floral integrators leading to onset reproductive stage (Teotial and Tang 2015).

As sugarcane is one of the poaceae family members, exploring and understanding the already established genetic underpinnings of flowering mechanisms in the related grasses and in the model plant Arabidopsis helps unveil the powerful, interconnected, and may be novel genetic circuits involved in the flowering process of sugarcane.

Circadian rhythms and photoperiod pathway

As the earth rotates, the sun rises and sets, the seasons vary, days and nights wax and wane, plant being a sessile, photosynthetic organism, must respond and adapt to these changing environmental conditions by synchronising its developmental biology with the seasons. So as to maintain a harmonious relationship between a constantly changing environment and an organism developmental phase, one common response that has evolved in all living organisms, is circadian rhythms (Mc Watters and Devlin 2011). Circadian rhythms are a subset of biological rhythms, an autonomous mechanism generating endogenous rhythms in a cyclic manner with a requirement of a 24 h period to complete one cycle (Glassop and Rae 2019; Hayama and Coupland 2003; Mc Clung 2006).

The circadian clock regulates diverse plant physiological processes, and photoperiodism is one of them. The sequence of events that involves plants coordinating with the changing seasons by measuring and sensing the fluctuations in daylength, perceiving the light quality, that results in onset of flowering is referred to as the photoperiodic pathway (Srikanth and Schmid 2011; Hayama and Coupland 2003). The photoperiod pathway promotes the floral transition by harnessing inputs from light, sensing the daylength, and the circadian clock (Valverde 2011). A wide range of daylengths can be observed in different seasons as one move towards the poles, and plants have evolved the potential to perceive this difference and use it as an indicator to regulate flowering initiation. Environmental signals such as light and temperature help to synchronise circadian clocks, and daylength is the principal factor that fine tunes the flowering process by mediating with the circadian rhythms (Somers et al. 1998). Light qualities, along with daylength, are important factors regarding the photoperiod dependent flowering mechanism.

The web of circadian system is divided into three layers of interconnected pathways, the first layer named input systems that perceive the environmental signals through the receptors and provide the input signals to the second layer, the central oscillator is the core that generates the 24 h rhythms, and the third layer called, outputs which constitute a range of physiological, biochemical, and developmental responses in a plant life cycle (Hayama and Coupland 2003; Mc Watters and Devlin 2011;Webb 2003), flowering responses of a plant can be considered as one such output pathway of circadian rhythms. A fitness advantage is attained by properly synchronising the endogenous circadian clock with the external light/dark cycles (Harmer 2009). Studies conducted in Arabidopsis thaliana helped in identifying and understanding various genes and their roles in circadian rhythms of plants. Environmental cues like light and temperature adjust the clock's phase to the correct time of day. As daylength, light intensity, and temperature vary according to the seasons, these external signals possess the ability to reset the circadian clock and also influence the rhythmic amplitude of clock outputs (Harmer 2009).

The central oscillator of the circadian system studied in Arabidopsis reveals the presence of three interconnected feedback loops called Transcription–Translation Feedback Loops (TTFLs) (McClung 2011) regulating the daily rhythms of the plants according to the changes in light and temperature. CCA1 (CIRCADIAN CLOCK ASSOCIATED1)/LHY (LATE ELONGATED HYPOCOTYL) are categorized as the morning phased genes (Mc Watters and Devlin 2011; Adams et al. 2015; Srivastava et al. 2019) and are considered as one functional unit instead of two different proteins as they exhibit redundant behaviour (Hayama and Coupland 2003; Imaizumi 2010). Light plays an important role in the direct activation of CCA1/LHY expression (Hayama and Coupland 2003) through the interaction between the PIF3 (PHYTOCHROME INTERACTING FACTOR3) and light-activated PHYs (PHYB in particular) (Bauer et al. 2004) thereby activates the CCA1/LHY as morning genes (Fig. 1).

Fig. 1
figure 1

Genes [PIF3 (PHYTOCHROME INTERACTING FACTOR3), CCA1 (CIRCADIAN CLOCK ASSOCIATED1)/LHY (LATE ELONGATED HYPOCOTYL), TOC1 (TIMING OF CAB EXPRESSION1), GI (GIGANTEA)] involved in regulating the Circadian clock in Arabidopsis

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CCA1, LHY are the core components responsible for creating three interlocking feedback loops in the central oscillator (Creux and Harmer 2019). CCA1/LHY proteins bind to the EE (evening elements) in the promoter region of TOC1 (TIMING OF CAB EXPRESSION1 is one of the family members of PRRs and is also termed PRR1) and directly repress the TOC1 transcription (Legnaioli et al. 2009; Srivastava et al. 2019) during morning hours; however, TOC1 acts as promoter of CCA1/LHY expression (Hayama and Coupland 2003; Mc Watters and Devlin 2011; Creux and Harmer 2019), thereby creating first negative feedback loop, called the “central loop” in the central oscillator of the circadian system (Alabadi et al. 2001). It is proposed that the rise of TOC1 in the evening is helpful for activation of CCA1/LHY indirectly in next morning (Hayama and Coupland 2003; Srivastava et al. 2019).

Insights into the PRR family members revealed the presence of second feedback loop called the “morning loop”, operated between the CCA1/LHY and PRR7 and PRR9 (Farre et al. 2005; Mc Watters and Devlin 2011; McClung 2011; Srivastava 2019). In the morning feedback loop, CCA1/LHY are bound to the PRR7 and PRR9 promoter regions and regulate the respective PRRs transcription positively (Farre et al. 2005; McClung 2006; Mc Watters and Devlin 2011; Imaizumi 2010). However, the PRR5, along with PRR7 and PRR9 proteins, act as negative regulators by suppressing the CCA1/LHY (Nakamichi et al. 2010; McClung 2011).

The third loop called, the “evening loop” is framed by CCA1/LHY, GI(GIGANTEA), and TOC1 (McClung 2011; Mc Watters and Devlin 2011; Srivastava et al. 2019). Although, GI is a known circadian clock component that is primarily involved in the promotion of flowering, the evening loop still needs to be explored to discover if there are any other novel factors involved in promoting GI expression. In case of evening loop, CCA1/LHY directly suppress the GI, indicates how the GI is negatively regulated by the central oscillator (Lu et al. 2012). ZTL (ZEITLUPE) on sensing the blue light gets activated and forms a stable complex with GI (Kim et al. 2007; McClung 2011) and this ZTL-GI complex breaks down the TOC1(Creux and Harmer 2019) through the 26S proteasomal degradation (Más et al. 2003; Srivastava et al. 2019).

LUX/PCL1 (LUX ARRHYTHMO also known as PHYTOCLOCK1 (PCL1), ELF3 (EARLY FLOWERING 3), and ELF4 are the three genes grouped as “Evening Complex” (EC) which are highly expressed from the beginning of evening hours and continue to express throughout the dark hours post dusk (Srivastava et al. 2019). The entire evening complex is negatively regulated by the CCA1/LHY, which binds to the EE or CBS (CCA1-binding sites) present on the EC promoter regions (Lu et al. 2012).

The evening complex is positively regulated by RVE8 (REVEILLE 8), RVE6 (REVEILLE 6) and RVE4 (REVEILLE 4) (Creux and Harmer 2019). RVE4/6/8 are expressed during the mid-day hours of the day (Creux and Harmer 2019) and promote the expression of TOC1 and PRRs. RVE8 is primarily involved in the positive regulation of TOC1 by promoting the hyperacetylation of Histone 3 in the promoter region of TOC1 (Farinas and Mas 2011) besides favouring TOC1 expression, RVE8 binds to the EE present in the PRR 5 and promotes its expression directly, however PRR5 suppresses the RVE8 (Rawat et al. 2011; Creux and Harmer 2019). To promote this transcriptional activation of TOC1 and PRR5 by RVE8, it needs to form a complex with LNK1 and LNK2 (NIGHT LIGHT INDUCIBLE AND CLOCK REGULATED 1&2) (Xie et al. 2014; Srivastava et al. 2019). RVE8 also favours the expression of LUX, ELF4 and GI (Farinas and Mas 2011; Rawat et al. 2011; Xie et al. 2014).

LHY/CCA1, ELF3, GI, TOC1, and ZTL are the chief circadian components influencing the flowering mechanism (Suárez-López et al. 2001). The mutated elf3 gene resulted in early flowering irrespective of daylength, making the plants insensitive to photoperiod (Zagotta et al. 1996), which could be explained in terms of elevated levels of CONSTANS (CO). A mutation in lhy and gi leading to their gain and loss of function respectively resulted in delay in flowering as LHY and CCA1 are involved in suppressing the GI during the early morning hours so mutation leading to gain in function of LHY would probably result in GI suppression throughout the day and the loss of function in gi resulted in transcriptional suppression of CO.

Sugarcane homologues for genes involved in the circadian clock were studied to analyse the expression levels (Glassop and Rae 2019) though the pattern of expression was observed to be the same as other plant species with CCA1/LHY followed by PRRs and TOC1 (Staiger et al. 2013; Glassop and Rae 2019) but the time of rise in the expression levels of individual genes varied in sugarcane compared to the other species (Glassop and Rae 2019). In sugarcane, ShPRR1/TOC1 started to show its expression with the beginning of light and risen to maximum expression within 7 h into light phase and reported lowest of expression after 2 h of dark phase (Glassop and Rae 2019). Brazilian cultivar RB855453 reported a rise in expression levels of PRR1/TOC1 during the transition to dark phase from light which is different from traditional expression pattern and made researchers to propose that this Brazilian cultivar might have alternate alleles for PRR1/TOC1 that differ from other cultivars (Glassop and Rae 2019).

Although the circadian system is well studied in Arabidopsis, this still needs to be understood in other crop species like sugarcane, which exhibit variable flowering behaviour to the same inductive daylengths, this helps in discovering the presence of any circadian system variants in sugarcane, that establish a mechanistic relation with the photoperiod-driven flowering. Understanding the chronobiology and circadian clock variants, if any, in sugarcane, might give us an answer to the erratic flowering behaviour in sugarcane which is distributed in various latitudes.

Flowering in long-day conditions

The photoperiod pathway can be defined in simple terms as the sequence of events that involves the plant’s ability to sense and measure the variation in daylength duration and thereby promote the flowering initiation. Studies conducted in Arabidopsis thaliana a long day plant, helped us in dissect and understand many molecular pathways involved in flowering mechanisms. Photoperiod led flowering in the case of long day plants can be explained in simple words as, gene GI (GIGANTEA) a circadian clock (Fig. 1) component that activates the CO (CONSTANS) gene transcription in leaves, which further promotes the transcription of FT (flowering locus T). Later, FT as a transcriptional factor interacts and forms a complex with another transcriptional factor FD (FLOWERING LOCUS D) in the apical meristem, that leads to activation of the floral meristem identity genes LFY and AP1, thereby regulating the transition of the SAM to the floral meristem through a cascade of events (Abe et al. 2005; Colasanti and Coneva 2009; Wellmer and Riechmann 2010; Wigge et al. 2005).

Light quality and daylength are important factors for the photoperiod dependent flowering plants. Plants have photoreceptors to detect light that optimizes the plants growth and development in different phases of their life. Crys, Phots, and Phys are the three well-studied photoreceptor families. Cryptochromes (Crys) and phototropins (Phots) perceive the blue light, and phytochromes (Phys) perceive red/far-red light (Lariguet and Dunand 2005; Srikanth and Schmid 2013). Photoperiodic flowering is mediated through co-ordination between the cryptochromes and phytochromes (Li and Yang 2007). PhyA, PhyB, PhyD, and PhyE are the red-light photoreceptors (Hayama and Coupland 2003; Devlin and Kay 2000). PhyB is mainly involved in perceiving high intensity red-light while PhyA involved in the low intensity red-light (Somers et al. 1998). cry1 and cry2 are the blue light photoreceptors (Hayama and Coupland 2003; Somers et al. 1998).

CONSTANS (CO) is the critical gene that keeps track of changing daylengths, whose rhythmic gene expression is modulated by a circadian clock factor GI (Suárez-López et al. 2001) and its protein stability is light dependent (Imaizumi 2010), and regulated by the through photoreceptors (Valverde et al. 2004; Song et al. 2015). Flowering under long days is exclusively dependent on the CO protein (Suárez-López et al. 2001). CO requires light for its protein stabilization, and which explains the reason why CO cannot actively participate in flowering in short days as the rise in CO protein expression coincides with the dark phase (Jang et al. 2008). CO is transcriptionally activated by GI and exhibits a peak in expression almost 16 h after dawn in LDs (Andres and Coupland 2012; Imaizumi et al. 2003). The oscillation of CO expression is regulated by CDFs (CYCLING DOF FACTORs), GI and FKF1 (FLAVIN-BINDING, KELCH REPEAT, F-BOX1). Late flowering phenotype was observed in co mutant under long days, besides early flowering phenotypes were noticed in both long and short days when CO was over expressed which can be related to fact that CO plays an important role in plants ability to differentiate between long and short days (Jang et al. 2008).

The CDF family contains CDF1–CDF5, of which CDF1 is best known flowering repressor that acts by suppressing CO transcription (Imaizumi et al. 2005; Sawa et al. 2007; Song et al. 2015). CDF1 reaches the peak of its expression during the morning hours before the GI and CDF1 along with other CDFs act redundantly in suppressing the CO transcription in the morning hours by directly binding to the CO promoter region (Fig. 2a) (Imaizumi et al. 2005; Imaizumi 2010; Sawa et al. 2007; Song et al. 2015). Although a strong and stable CDF transcripts were reported in the presence of fkf1 mutants which confirms the possible role of FKF1 in transcriptional repression of CDFs (Imaizumi et al. 2005).

Fig.2
figure 2

Flowering gene regulation during day time. (a) Morning time (transcriptional activation of CDFs due to absence of repressors GI & FKF1, Transcriptional inactivation of CO due to repressors CDFs), (b) Transcriptional activation PRRs, Transcriptional inactivation of CDFs by PRRs, and promotion of CO transcription through degradation of CDFs. (c) Afternoon (Blue light activated FKF1 forms complex with GI)

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As the intensity of daylight starts progressing, GI expression gradually increases achieving the peak of its expression by 12–13 h after dawn breaks in. This rise in GI promotes the interaction with the CDF1 which is already bound to the CO promoter region. FKF1, a E3 Ubiquitin ligase (Imaizumi et al. 2005) is activated by absorbing blue light through its LOV (Light, oxygen, voltage) domain, and assists in complex formation with the GI (Fig. 2c); now FKF1 through its KELCH REPEAT domain recognises the CDF1 protein (Sawa et al. 2007) bound to the CO promoter region and initiates the ubiquitin mediated degradation of CDF1 through F-box domain (Imaizumi et al. 2005) and relieves the suppression of CO transcription due to CDF there by initiating the CO transcription (Song et al. 2015) (Fig. 2b).

During long days, both GI and FKF1 are in synchronous expression and abundance of their respective proteins occurs by 12–13 h after dawn break (Fornara et al. 2009; Sawa et al. 2007), which is an opportunistic timing for FKF1-GI complex formation by late in afternoon (Imaizumi et al. 2005) that leads to CO transcription initiation, and by 16 h after dawn break CO transcripts reach their higher levels. Hence, daytime expression of CO is dependent on the timing of circadian regulated expression of CDFs, GI and FKF1 proteins (Sawa et al. 2007; Srikanth and Schmid M 2011).

The availability of CO protein tends to vary with daylength and fluctuates dynamically between day and night (Song et al. 2015). Not just at the transcription of CO, it is also regulated at post transcriptional stages regulating the stability and accumulation thresholds of CO protein once the CO protein gets accumulated to higher levels by the late afternoons in LDs (Srikanth and Schmid 2011; Song et al. 2015). This post transcriptional regulation of CO protein is driven by blue, red, far-red light and the respective photoreceptors sensing their wavelength. CO is stabilised by far-red and blue light but destabilized by red light (Song et al. 2015).

PhyB reduces CO protein abundance in the presence of red light in the early morning hrs independent of COP1-SPA interaction (Jang et al. 2008), while cryptochromes (CRY1&CRY2) under blue light inhibit CO protein degradation mediated by PhyB, while PhyA in far-red light during LDs stabilizes CO protein. As a result, blue and far-red light along with their respective photoreceptors, post-transcriptionally regulate CO protein activity (Fig. 3b), which is necessary for activation of FT transcription (Song et al. 2015; Valverde et al. 2004). Few other proteins, COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1) and HOS1(HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1), SPA (SUPPRESSOR OF PHYA-105 1) family members also mediate the CO degradation (Song et al. 2015; Lazaro et al. 2012) (Fig. 3a). COP1 upon receiving the inductive signal from SPA, initiates the ubiquitination thereby degrading the CO protein (Liu et al. 2008; Laubinger et al. 2006).

Fig.3
figure 3

Effect of different lights on flowering genes (a) CO protein destabilized under red-light, (b) CO protein stabilized under far-red & blue light, (c) CRY2- COP1-SPA1 complex releases the CO protein from degradation, (d) Regulation CO transcription and CO degradation at night/dark phase

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During daytime, CRY2 interacts with SPA1 in response to blue light, this interaction with SPA1 mediates the CRY2 binding to COP1 and results in a complex formation CRY2/ SPA1/COP1 suppresses the inhibitory activity of COP1/SPA1 on CO and helps in the accumulation of CO protein towards the end of LDs (Fig. 3c) (Zuo et al. 2011). A complex formed by COP1 and SPA1 actively degrades CO protein during the night in both LDs and SDs (Jang et al. 2008); SPA3, SPA4 along with SPA1 destabilize the CO protein in redundant manner (Laubinger et al. 2006; Song et al. 2015), due to the absence of light during night hrs, CRYs are not in active state so there will be no suppressive pressure on the COP1/SPA1 (Fig. 3d). This degradation of CO in the dark phase is helpful for the plants to differentiate between long and short days (Song et al. 2015). Plants with mutated Spa1 exhibited early flowering surprisingly in short day conditions by upregulating FT transcription but not in long day; spa1 and co double mutants didn’t exhibit any rise in FT transcripts, which explains that early flowering under short days in spa1 mutant plants is through regular CO inducing FT expression. Mutations in spa resulted in rise in FT transcript levels without affecting the regular transcription of CO, which explains that SPA regulates CO protein but not its transcription (Laubinger et al. 2006).

Though genes, homologous to the GI, CO, FT are found in many other agronomically important species (Colasanti and Coneva 2009) rhythmic expression of CO, FT; when and how this CO influences the gene FT vary considerably in short-day and long-day plants. CO as a master regulator, coordinates the daylength with circadian clock inputs thereby interconnecting photoperiodism and circadian rhythms (Valverde et al. 2004). However, the real picture of GI → CO → FT pathway involves multitude of transcription factors as mentioned above and some might be species specific regulate the transcription of CO, FT and stability of their respective proteins post translationally.

Age pathway

Among the 5 major molecular pathways involved in flowering mechanism, age and gibberellin pathways are the endogenous factors (Fig. 4). Micro RNAs can be dubbed as the wizards regulating gene expression post transcriptionally (Waheed and Zeng 2020), are significantly involved in many stages of plant growth and development and during defence against biotic and abiotic stresses (Jones-Rhoades 2006; Teotia and Tang 2015). miRNAs are 21–24 nucleotide long, present extensively in animals and plants, and belong to non-coding class of RNAs. miR156 is one of the conserved Micro RNA present in plants (Yamaguchi and Abe 2012; Ma et al. 2020) (Table 1).

miRNAs in the plants regulate the gene expression either through the translational inhibition or cleavage of the targets (Sun 2012). miR156 and miR172 are the chief members of the aging pathway, suppress their respective targets and ensure that plants flower when they are reproductively competent. It was discovered that miR156 expression is strongest during the early seedling, juvenile phase and it starts to decrease as the plant development progresses and reaches significantly lower levels of expression in the reproductively viable stage (Ma et al. 2020); whereas miR172 expression is low during the juvenile phase and then increases in due course of the flowering stage (Wu and Poethig 2006; Wu et al. 2009) this pattern of expression is conserved in Arabidopsis, maize, and rice (Teotia and Tang 2015).

Fig.4
figure 4

Transcription activation of miR156 (expression is strongest during the early seedling, juvenile phase, low expression of SPLs (SQUAMOSA PROMOTER BINDING LIKEs), and AP2 upregulates the expression of miR156) and miR172 (expression is low during the juvenile phase and then increases in due course of flowering stage with high expression of SPLs and AP2 downregulates the expression of miR172)

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Table 1 Normal and alternative pathway adopted by Saccharum species during their flowering mechanism
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In Arabidopsis, miR156 family consists of total eight members, miR156(a) to miR156(h) out of these, miR156(a) and miR156(c) play crucial role in governing the flowering time in arabidopsis (Teotia and Tang 2015; Waheed and Zeng 2020). AGL15 (AGAMOUS-LIKE 15) together with AGL18 forms a heterodimer and binds to the promoters of MIR156a and MIR156c genes and regulate the expression of miR156a &miR156c (Serivichyaswat et al. 2015; Teotia and Tang 2015). miR156 targets are the transcription factors named SPLs (SQUAMOSA PROMOTER BINDING LIKEs) (Rhoades et al. 2002; wang 2014). miR156 & miR157 binds at the 3ˈUTRs of SPLs and inhibit them by translational repression (Gandikota et al. 2007; Wang 2014; Waheed and Zeng 2020). The extended juvenile phase resulting from the overexpression of miR156 has a profound effect at 16 °C than at 23 °C, suggesting that miR156 expression is dependent on temperature too (Kim et al. 2012).

Based on the studies from Arabidopsis, miR156 targets SPLs are divided into 2 groups, one group represents SPL3, SPL4 and SPL5 and the other group holds SPL2, SPL6, SPL9, SPL10, SPL11, SPL13 and SPL15 (Wang 2014). SPL9, SPL10, SPL15 act in opposite manner to miR156, where these 3 SPLs are primarily involved in plant’s transition towards the reproductive phase (Ma et al. 2020). SPL9 and SPL10 act redundantly to promote transcription of miR172 directly (Wu et al. 2009; Zhu and Helliwell 2011); however, they are also involved in activating miR156 expression (Wu et al. 2009). In long-day conditions, in leaves, SPL9,10,15 promote the expression of miR172, while in the apical meristem, SPL9,10,15 activate the expression of AP1, SOC1(Ma et al. 2020; Hyun et al. 2017). Overexpression of SPL7 and SPL8 resulted in early flowering, while downregulation of either SPL7 or SPL 8 resulted in a phenotype with a severe delay in flowering in switch grass (Gou et al. 2019). However, the effects of overexpressed and downregulated SPLs in case of sugarcane need to be identified to understand the complete picture of age driven flowering mechanisms.

GI is involved in the processing of the primary MIR172 (priMIR172) transcript levels (Jung et al. 2007). The direct targets of miR172 are AP2, TOE1, TOE2, TOE3 (TARGET OF EAT), SNZ (SCHNARCHZAPFEN), SMZ (SCHLAFMUTZE) and regulate their levels through cleavage and translational inhibition (Jung et al. 2007; Waheed and Zeng 2020). A loop of inhibition and promotion of genes is established between AP2, miR156 and miR172, where AP2 upregulates the expression of miR156 and downregulates miR172 (Wang 2014; Waheed and Zeng 2020).

Interestingly, it was reported that SPL3, SPL4, SPL5 are regulated in a photoperiod dependent manner through FT and SOC1 (Fig. 5). CO mediated transcription of FT, forms a complex with FD and this FT-FD complex activates the SPL3,4,5 expression (Jung et al. 2012a, b); SOC1 also reported to upregulate the SPL3 expression, whereas SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CO1) & FUL (FRUITFUL) together regulate SPL4 (Teotia and Tang 2015). SPL3 known to directly regulate and promote the expression of genes, LFY(LEAFY), AP1(APATELA1), FUL (Yamaguchi et al. 2009) whereas SPL9 activates the expression of AP1, FUL, AGL24, SOC1 (Wang et al. 2009) by directly binding on to the promoter regions and integrating the photoperiod and endogenous age pathways (Jung et al. 2011).

Fig. 5
figure 5

Photoperiod dependent activation of SPLs and targets of SPLs (CO mediated transcription of FT, forms complex with FD and this FT-FD complex activates the SPL3, SPL4, and SPL5 expression)

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Role of ambient temperature in flowering

Temperature is the crucial exogenous factor next to photoperiod that influences the plants transition from vegetative stage to reproductive stage. As plants are sessile, they need to discriminate between extended exposure to cold temperatures (vernalization) and the ambient temperatures that support normal growth and development (Capovilla et al. 2015). The role of ambient temperature at the molecular level response in plants is a road that still needs to be toured. As the global temperatures are on the rise, the effect of temperature on the flowering phenology is one of the concerning subjects that needs to be understood. Warm temperatures compensate for the long day photoperiods and promote flowering in the short-day environment, which signifies the importance of temperature in flowering (Wigge 2013). Plants have receptors for light sensing under the names phytochromes and cryptochromes, but no receptors are identified in particular for temperature sensing, however, the PRR7 & PRR9 (Samach and Wigge 2005) and ELF3 of the evening loop (Thines and Harmon 2010; Wigge 2013) are identified to play role in the temperature entrainment to the circadian clock.

Floral repressor FL-C (FLOWERING LOCUS C) prevents the transcriptional activation of FT, SOC1 (Searle et al. 2006) and FD by binding to their promoter regions (Deng et al. 2011) and this FL-C delays the flowering in an SVP dependent manner too and suppresses SOC1 & FT (Li et al. 2008; Capovilla et al. 2015). Studies on the plant response to vernalisation revealed that, FL-C gets suppressed at the chromatin level after prolonged exposure to lower temperatures, and the respective targets of FL-C are de-repressed. Although molecular underpinnings of vernalization dependent flowering are well understood, role of ambient temperature in flowering is mostly unknown. The thermo sensory pathway of flowering as a response to ambient temperature is suggested to be mediated via FL-C independent mechanisms (Blázquez et al. 2003) involving PIF4, splice variants of FL-M (Jin and Ahn 2021), TFL1 & ELF3 (Strasser et al. 2009) and also FVE & FCA (Blázquez et al. 2003).

FT gene is found to be the key component that amalgamates the different pathways in response to the ambient temperature variations (Capovilla et al. 2015) and a temperature mediated flowering mechanism upregulates the FT expression independent of daylength through a transcription factor called PIF4 (Kumar et al. 2012). PIF4 member of PIFs family, which is generally involved in the light entrainment into the circadian clock, has also been shown to regulate flowering in a temperature dependent way by strongly and directly binding to the FT promoter in high temperatures (Capovilla et al. 2015; Kumar et al. 2012). The binding ability of the PIF4 transcription factor depends on the presence of H2A.Z in the chromatin of the FT gene promoter region (Kumar et al. 2012).

H2A.Z is a histone variant that is known to play an important role in DNA repair, regulating the heterochromatin in the centromere (Giaimo et al. 2019) and also modulates the transcription of the genes as H2A.Z histones wrap around DNA tightly, thereby effecting the accessibility the DNA to the RNA Pol II and transcription promoters (Kumar and Wigge 2010). Nucleosomes with H2A.Z wrap around DNA more tightly in lower temperatures, as the temperature increases, this tight wrapping decline and provide the way for transcription factors and RNA Pol II and thereby genes that are expressed at elevated temperatures are upregulated (Kumar and Wigge 2010). The same is seen in case of PIF4-regulated expression of FT. As the temperature rises to the conducive ambient range, H2A.Z wrapping in the promoter of FT unfastens and promotes the binding of PIF4 and upregulates the FT expression (Kumar et al. 2012; Wigge 2013). Overexpression of PIF4 in co mutant plants lead to early flowering, which proved that temperature mediated flowering acts in a route independent of photoperiod (Kumar et al. 2012).

FL-M (FLOWERING LOCUS M) and SVP (SHORT VEGETATIVE PHASE) suppress the transcription of FT, SOC1, and the level of transcription suppression is believed to be function of changes in temperature (Samach and Wigge 2005). SVP, one of the flowering repressors, targets the SOC1 directly and downregulates the SOC1 transcription, and thereby SVP is known to maintain the vegetative stage (Li et al. 2008) and is known to be a part of temperature signalling too (Strasser et al. 2009; Lee et al. 2007). FLM has 2 prominent splice variants, FLM-β & FLM-δ, and their abundance is dependent on the temperature. FLM-β forms a complex FLM β-SVP which is abundant at low temperatures and represses flowering, on the other hand, SVP forms complex with FLM- δ and obstructs flowering at higher temperatures (Pose et al. 2013; Capovilla et al. 2015).

FCA and FVE earlier categorized especially under the autonomous pathway (Capovilla et al. 2015); however, their expression levels are dependent on the ambient temperature and are identified to play a role flowering mediated through thermo sensory signalling (Blázquez et al. 2003). Both FVE and FCA are responsible for the upregulation of the FT gene independent of CO mediation and FL-C suppression (Blázquez et al. 2003). PIF5, a close homolog of PIF4, together upregulate the flowering response via FT promotion during warm nights and independently of FT in warm days (Thines 2014).

Gibberellin pathway

Optimal plant growth and development needs phytohormones too. Gibberellins are essential for many of the plant developmental milestones such as seed germination, stem elongation, leaf expansion, and flowering as they promote cell division and elongation. Through DELLA proteins or by regulating GA homeostasis, gibberellins participate in phytohormone-mediated flowering and in the cross talk established between multiple genetic circuits established in flowering mechanism (Bao et al. 2019). GA being one of the factors in the well-established four interconnected flowering pathways, the role of GA still needs to be understood in sugarcane, during the reduction in inductive daylengths from 12 h 55 min to 12 h 30 min. GA is extensively studied in Arabidopsis to understand its role in flowering. Components in the GA signalling pathway include a soluble GA receptor, GID1 (GIBBERELLIN INSENSITIVE DWARF1) and growth repressor proteins, namely, DELLAs (Bao et al. 2019; Hauvermale et al. 2012). The receptor GID1 recognizes the GA signals and forms the GA–GID1 complex, and through protein–protein interactions, the GA–GID1 complex degrades DELLAs (Hauvermale et al. 2012).

Degradation of these DELLAs is the initiating step for plant growth promoted by gibberellin signalling (Mutasa-Gottgens and Hedden 2009). GAI (GA-INSENSITIVE), RGA (REPRESSOR OF GA), RGL1 (RGA-LIKE1), RGL2, RGL3 are the 5 DELLAs present in the Arabidopsis. Sugarcane contains only one known DELLA protein to date named, ScGAI (Fang et al. 2021). DELLAs interact with CO protein and seize its activity of promoting FT transcription (Xu et al. 2016). Apart from the FT, transcriptional activation of SOC1 (Moon et al. 2003) and LFY is modulated by endogenous GA levels, especially in short days (Moon et al. 2003; Eriksson et al. 2006). In ambient temperature conditions, DELLAs bind to PIF4 thereby preventing PIF4 from binding on to the FT promoter (de Lucas et al. 2008; Bao et al. 2019); therefore, if plants have reduced levels of endogenous gibberellins, DELLAs get accumulated in higher levels and participate actively in downregulating the flowering mechanism in LDs and in ambient temperatures (Bao et 2019; Galvao et al. 2015). The rise in gibberellin levels promotes the degradation of DELLAs and releases the flowering promoters from degradation by DELLAs (Bao et 2019) (Table 2).

Table 2 List of promoters and inhibitors involved in flowering mechanism
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What are FTs and how are they related to PEBPs-

All the five pathways involved in the regulation of the flowering process meet up at a few factors called floral pathway integrators, namely, FT, SOC1. These two genes activate the floral meristem identity genes LFY and AP1 which are placed downstream to FT, SOC1 thereby promoting the formation of floral primordia (Bao et al. 2019). FT is one of the members in PEBP protein family, and FT is observed to be conserved among most of the angiosperms. It is the most widely investigated floral pathway integrator, believing it to be the chief component of florigen, a mobile floral inductive signal.

During long days, FT transcriptional activation is mediated by blue light stabilization of CO protein in the evening hours. This CO protein interacts with the CORE (CONSTANS-responsive element) region of FT promoter, thereby promoting FT expression (Song et al. 2015). FT is strongly expressed in long days, and it is rhythmically controlled through circadian pathways with maximal expression during the evening (Suarez-Lopez et al. 2001). Besides CO acting as promoter of FT expression, there are repressors that suppress the transcription of FT. CDFs being the suppressors of CO transcription they also suppress the FT transcription during morning hours. By afternoon, formation, and stabilization of FKF1–GI complex, degrades the CDFs on the FT promoter and thereby FT is relieved from CDFs suppression (Song et al. 2015) which resembles the derepressing mechanism of CO from CDFs. TEM1 and TEM2 act redundantly in suppressing the FT transcription by binding to the 5ʹ UTR region of FT and the presence of higher FT levels is regulated by the balance between CO and TEM (Castillejo and Pelaz 2008). Another FT repressor, SMZ (SCHLAFMUTZE) targets the 3ʹ UTR of FT (Song et al. 2015).

The phosphatidyl ethanolamine-binding proteins are one of the evolutionary conserved family of proteins present in all taxa of living organisms ranging from bacteria to animals and plants. Though there is an extensive conservation in PEBP genes, they are involved in diversified biological functions from signalling pathways to regulating growth and differentiation (Banfield et al. 1998). Two types of PEBPs are present in the gymnosperms, one resembling MFT and other filling up the intermediate position between FT-like/TFL1-like in phylogenetic analysis. From the functional and evolutionary studies conducted on the non-seed producing plants, it was proposed that, as the club mosses and seed producing plants got diverged, possibly due to the occurrence of first duplication event, which resulted in the formation of MFT-like clade and another clade depicting the ancestor of FT-like/TFL1-like genes present in the gymnosperm, and a second duplication possibly occurred, leading to the diversification of the FT-like clade and TFL1-like clade supporting the angiosperms evolution (Karlgren et al. 2011).

In case of flowering-seed producing plants, PEBP members are primarily involved in the transition of plants to flowering phase and determining the architecture of the plant (Bradley et al. 1996; Conti and Bradley 2007). According to the phylogenetic studies conducted by Chardon and Damerval 2005, in the cereals, the PEBP gene family is split into three clades: FT-like, TFL-like, and MFT-like genes, with different roles in regulating the flowering mechanism in the angiosperms. The molecular studies revealed the presence of FT (FLOWERING LOCUS T), TFL1 (TERMINAL FLOWERING1) and MFT (MOTHER OF FT and TFL1), BROTHER OF FT and TFL1 (BFT), TWIN SISTER OF FT (TSF) and ARABIDOPSIS THALIANA CENTRORADIALIS (ATC), making PEBPs a six membered gene family in the arabidopsis (Karlgren et al. 2011). FT promotes the transition to flowering phase in an environment of inductive photoperiod and temperatures, while TFL1 repress this transition, functioning antagonistically albeit sharing almost 60% amino acid sequence identity (Ahn et al. 2006). TSF when overexpressed promoted flowering through binding to the FD (Yamaguchi et al. 2005); MFT induces flowering, and it shows homology to both FT and TFL1 (Yoo et al. 2004).

Flowering time is also regulated by the repressor FLOWERING LOCUS C (FLC), which suppresses the expression of FT and SOC1 in a pathway unrelated to photoperiodic flowering (Hepworth et al. 2002).

PEBPs in sugarcane

From studies conducted in plant PEBPs FT, TSF and MFT are identified as floral promoters, while TFL clade exhibits the flower repression function (Ahn et al. 2006; Harig et al. 2012; Wickland and Hanzawa 2015). TFL1 restricts the transition to floral meristem by suppressing the transcription of LFY and AP1 (Coelho et al. 2014; Hanzawa et al. 2005) thereby functioning in an opposite manner to the floral promoters. The gene duplication event, together with acquiring mutations, can be the possible reason for the development of opposite functions in the PEBP proteins (Hanzawa et al. 2005). However, few members of FT-like clade were identified with floral repressor function in sugar beet and tobacco (Pin et al. 2010; Harig et al. 2012) (Fig. 6).

Fig. 6
figure 6

Classification of Plant PEBPs based on the phylogenetic trees constructed (Chardon and Damerval 2005; Karlgren et al. 2011; Venail et al. 2022; Wolabu et al. 2016) At- Arabidopsis thaliana, Sb-Sorghum bicolor, Zcn- Maize, Sc-Sugarcane, Os-Oryza sativa, Hv- Hordeum vulgare, Ta-Triticum aestivum

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RNA-seq analysis in conjunction with mining of the SUCEST-FUN database and the sugarcane genome hub resulted in the identification of a total 19 PEBP members in sugarcane. These include, FT 1-FT 13, MFT 1, MFT 2 and TFL1-TFL4 (Venail et al. 2022), similar to 19 PEBPs identified in sorghum (Wolabu et al. 2016) and all of these have their comparable homologues in other monocot species, such as sorghum, maize, and rice. From Yeast-2 hybrid and bimolecular fluorescence complementation assays conducted with sorghum FT proteins, SbFT 1, SbFT 8, SbFT 10 interacted with Sb 14-3-3 protein in cytoplasm, while only SbFT 1 interacted with SbFD, hypothesising that FT1, 8, 10 along with 14-3-3 and FD proteins in sorghum are involved in formation of floral activation complex (Wolabu et al. 2016).

FT1 of sugarcane when expressed in Arabidopsis unexpectedly showed negative impacts on flowering, suggesting that FT1 could be the potential floral repressor in sugarcane (Coelho et al. 2014). Though Sorghum FT1 is considered to be a floral promoter (Wolabu et al. 2016), Sc FT1 was proved to be a floral repressor (Coelho et al. 2014). As FT interacts with FD (FLOWERING LOCUS D) to promote the transcriptional activation of downstream genes called floral meristem identity genes, in Arabidopsis TFL1 also reported to interact with FD and downregulate the LFY, AP1 thereby, TFL1 is involved in maintaining the meristem at indeterminant vegetative stage (Ahn et al. 2006; Coelho et al. 2014; Hanzawa et al. 2005; Linhares-Neto et al. 2021) and TFL1 loss of function resulted in early flowering in Arabidopsis (Hanzawa et al. 2005). Expression of sugarcane TFL1 in Arabidopsis resulted in delayed flowering suggesting that TFL1 in sugarcane has a similar role (Coelho et al. 2014).

Hd3a in Oryza has its homologue present in sugarcane as ScFT3 and ScFT3 along with ScFT5 were tested in Arabidopsis flowering mutants to identify the functional FT involved in promoting the flower development in sugarcane. From the transgenic studies conducted in Arabidopsis (Venail et al. 2022) using ScFT3 and 5, it was reported that ScFT3 accomplished the early flowering process in Arabidopsis flowering mutants and rise in ScFT3 under declining photoperiod conditions coincided with the morphological changes in shoot apical meristem of sugarcane. However, scFT5 did not affect the flowering behaviour in Arabidopsis flowering mutants, which suggests that ScFT 3 could be one possible functionally-promoter FT but not FT5 (Venail et al. 2022).

Duplication events in the gene could lead to evolution of homologous proteins, while the mutations in these proteins could be one possible reason for gain of novel functions by homologous proteins (Hanzawa et al. 2005). From studies conducted by Hanzawa et al. 2005, it was proposed that amino acid H at 88 of TFL1 and Y at 85 positions in FT could possibly play a crucial role in determining their antagonistic functions. From the sequence alignment in the PEBPs of Arabidopsis, Sorghum, and Sugarcane (Fig. 7), the similar difference in the amino acids is noticed at position 132 where all the FTs have Y except SbFT5 and ScFT 5 which have N (arginine) and all of the TFLs have H, while Sb MFT1and Sc MFT2 have Y residues resembling the FTs.

Fig. 7
figure 7

Multiple sequence alignment of PEBPs identified in Arabidopsis, Sorghum and Sugarcane showing the conserved and variations in amino acids at a specific position 132 (https://bioedit.software.informer.com/)

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Though all 19 PEBPs found in sugarcane have their corresponding homologues in sorghum, rice, and maize presumptions on functions of sugarcane PEBPs cannot be made as FT-clade includes both promoter and suppressor FTs, whose function varies considerably in different species (Venail et al. 2022). The PEBP gene members need to be functionally validated to understand the role of each gene in different stages of flower development.

Quantitative trait locus (QTL) and flowering

Molecular markers have been used to determine the location of QTL for a number of agronomic traits in sugarcane, including plant height and flowering (Ming et al. 2002). Nine quantitative trait locus (QTLs) were mapped in hybrid population (Saccharum officinarum × Saccharum spontaneum), whereas one from S. officinarum and eight from S. spontaneum. These nine markers [CSU415iGd, BCD1107aI, CDSB31eI, CDSC46bI, CDSR125aI, CSU415gI, CSU81cI, pSB101dI, and pSB188aI] showed remarkably strong association with flowering. The allele is effect of S. officinarum QTL delays flowering, in contrast to the allele effects of eight S. spontaneum QTLs that enhance or accelerate flowering. The authors further concluded that identification of QTLs controlling flowering in breeding populations could potentially assist breeders in selecting and maintaining low-flowering elite lines (Ming et al. 2002).

Non-flowering varieties

Collective efforts were made by different workers through conventional breeding methods to select parents, and crosses were made among them for selection of progenies suitable for particular area, with high productivity traits and non-flowering characters (Patil et al. 2014). The progenies with no flowering /late scattered flowering/profuse flowering were obtained. Two non-flowering genotypes (SNK 07680 and SNK 07337) were released for commercial cultivation. These two clones with high sucrose present along with late scattered flowering can be utilized in a breeding program for high sugar and non-flowering traits in future.

Concluding remarks and future prospects

Flowering in sugarcane is influenced by numerous factors, such as environmental conditions, nutrients, moisture, latitude, daylength etc. Nitrogen withdraw, a practice followed to initiate flowering. If this withdrawal is done even after initiation, it can cause detrimental effect on further development of flower. However, the role of ambient temperature and different light intensities (high and low) in flowering is mostly unknown. Further research is needed to understand the role of temperature and light intensities in flower development in sugarcane.

Being a highly polyploid crop, the control over flowering is also complex. There are a number of genes involved is this pathway like starting from light period perception to florigen production in leaf and its translocation to SAM and conversion of SAM to floral meristem. Maintenance of FT gene expression in plant leading to sequence of events controlled by several genes and transcription factors which ultimately results into flower development and production of seed. The role of multiple copies of each gene involved in flowering can be redundant. However, PEBPs are also involved in other developmental pathways apart from flowering, so each member of PEBP family involved in flowering needs to be functionally evaluated. This functional evaluation uncovers any species specific expression or flower developmental stage specific expression of FTs.

As flowering process is also a circadian regulated mechanism, exploring the gene pool of Saccharum species under different latitudes in search of circadian clock variants could possibly give us an answer at molecular level supporting how the flowering response varies at different latitudes with varying environment. Understanding the changes in gene expression among the clones that were reversed to vegetative stages after initiation of flowering due to a lack of supportive environmental conditions helps us in understanding how different genes play role in reversion.    Virus induced flowering (VIF), was conceptualized to promote the early flowering to speed up the breeding programmes. To date, VIF protocols are well established for dicots. However, VIF system developed using FoMV (Foxtail mosaic virus) promoted early flowering and spikelet development in proso millet (Yuan et al. 2020). This sparked the thoughts of employing this monocot specific FoMV-VIF system in sugarcane to achieve flowering in off season among selected parents that aids in conducting maximum number of desirable crosses.