The photoperiodic regulation of FT in LDPs has been extensively investigated by using Arabidopsis. The external coincidence model well fits into the photoperiodic regulation of flowering time in Arabidopsis, wherein flowering is induced only when a clock-controlled internal rhythm coincides with an external factor. The expression of CO follows a rhythmic pattern controlled by the circadian clock, gradually increasing from the middle of the day in LD conditions (Imaizumi, 2010). This increase is facilitated by the action of GIGANTEA (GI) and FLAVIN-BINDING, KELCH REPEAT, F-BOX (FKF1). GI expression, also regulated by the circadian clock, peaks in the middle of the day (Fowler et al., 1999). In LD conditions, the blue light receptor FKF1 accumulates and activates in the midday, forming a complex with abundant GI protein, subsequently activating CO expression. The stability of CO protein is light-regulated. HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE1 (HOS1) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)-SUPRESSOR OF PHYA-105 1 (SPA1)/-SPA3-SPA4 complex destabilize CO protein in the morning and in the night, respectively (Shim et al., 2017). In LD conditions, high CO expression coincides with light period, which in turn induces FT expression. In contrast, in SD conditions, when CO expression level is high, the dark period prevents CO protein stabilization and subsequent FT activation. This mechanistic model is based on the observed FT expression profile in laboratory LD/SD conditions. However, in nature, a morning FT peak as well as the evening FT peak appears. It has been shown that different R/FR ratios between sunlight (R/FR = 1) and laboratory conditions (R/FR > 2) determines the expression profiles (Song et al., 2018). Indeed, supplementary FR light in laboratory LD conditions mimics the bimodal FT expression profile observed in nature. Although the supplementary FR light induces the FT expression levels both in the morning and at dusk, the mechanisms underlying FR light-mediated FT induction likely differ between morning and evening. Firstly, the evening FT expression is induced in response to the supplementary FR light exposure as short as 2 hours (Lee et al., 2023). However, unlike induction of PIL1 expression, a marker of shade responses enriched by FR light, the evening FT expression could be induced when the 2 hr supplementary FR light was irradiated 6 hr ahead of dusk. This suggests that the evening FT expression is influenced by light quality (R/FR ratio) and the circadian clock, which is consistent with the classical external coincidence model. Secondly, the morning FT induction requires prolonged FR light exposure. Neither a 2hr, 4hr, or 8hr FR light exposure is sufficient for full morning FT expression (Lee et al., 2023). phyA, but not phyB, has thought to be crucial for the morning FT induction under natural LD or FR light-enriched LD conditions. Mutation on phyA abolishes the morning FT induction under FR light supplemented LD conditions, while mutation on phyB does not affect the morning FT expression level in laboratory LD conditions (Song et al., 2018). Furthermore, end-of-day FR light treatment accelerates flowering, mediated by phyB but not phyA (Cerdán and Chory, 2003). These findings, combined with the requirement of prolonged FR light exposure, support the notion that the morning FT expression is a FR high-irradiance response (HIR) via phyA. As part of phyA-mediated FT expression regulation, PIF7, together with other PIFs including PIF4 and PIF5, directly activates FT expression (Lee et al., 2023). EARLY FLOWEIRING 3 (ELF3) has shown to antagonize phyA in the regulation of morning FT expression. elf3 mutant significantly increased FT expression in the morning under both FR light-enriched LD conditions and laboratory LD conditions. ELF3 affects FT expression by regulating the stability of CO protein, in which ELF3 interacts with the COP1/SPA1 complex (Laubinger et al., 2006; Jang et al., 2008; Yu et al., 2008; Huang et al., 2016). Under simulated natural LD conditions, the abundance of ELF3 protein is lower compared to laboratory LD conditions, and the stability of CO protein increases throughout the day when ELF3 is mutated (Song et al., 2018). This suggests that supplementary FR light reduces ELF3 protein stability, thereby stabilizing CO protein and resulting in morning FT expression. Based on previously reported FT regulators and their molecular mechanisms, phyA and ELF3 have been identified as regulators of morning FT expression. However, considering observations that none of mutants except phyA and fhy1fhl examined by Song et al., 2018 specifically affected morning FT expression, it is possible that the regulation of morning FT expression is complex. Indeed, in addition to the prolonged FR light exposure, the induction of morning FT expression appears to be controlled by the circadian clock, as indicated by the presence of time windows most effective for FT induction by the supplementary FR light (Lee et al., 2023). FR light treatment from the late afternoon in the previous day to the early morning is sufficient to fully induce morning FT expression. Thus, the regulation of morning FT expression is tightly governed by both the circadian clock and prolonged FR light signals. Further investigations are required to elucidate the molecular mechanisms underlying FT expression under FR light enriched LD conditions.

Arabidopsis plants under canopy exhibit earlier flowering compared to those fully exposed to light. They perceive shaded conditions by sensing R/FR ratios, which is mediated by photoreceptors including phys. In shade, neighboring plants absorb R light but reflect FR light, leading to a decreased amount of R and an increased FR light. As a result, the R/FR ratio varies depending on the degree of shade. In response to the reduced R/FR ratio under shade, active phys turns into inactive state, triggering shade avoidance responses including acceleration of flowering (Casal, 2012). The early flowering phenotype in shade is predominantly dependent on phyB with contributions from phyD and phyE (Devlin et al., 1999, 1998). On the other hand, phyA has thought to be less effective in flowering regulation under shade, even though it mediates other shade avoidance responses (Franklin et al., 2003; Roig-Villanova and Martínez-García, 2016). It has been shown that both morning and evening FT expression gradually increase as the R/FR ratio decreases from normal laboratory white light (R/FR ratio = 2.0) to shade conditions (R/FR = 0.25) (Song et al., 2018). Given that the induction of FT expression requires functional phyA, these observations suggest that the phyA-mediated regulatory mechanism for FT induction under FR-enriched LD conditions may also partially contribute to early flowering under shade conditions.

Medicago truncatula (Medicago), belonging to the rosidae subclass along with Arabidopsis, is a model plant of legumes that can mediate nitrogen fixation. Its flowering is promoted by long days (Clarkson and Russell, 1975). Medicago possesses five FT ortholog genes, MtFTa1, MtFTa2, MtFTb1, MtFTb2, and MtFTc. Among these, MtFTa1, MtFTb1, and MtFTc exhibit flowering activation activity, as demonstrated by the complementation of the late flowering phenotype in Arabidopsis ft mutants upon overexpression of each FT ortholog gene (Laurie et al., 2011). The role of MtFTa1 in floral induction has been further confirmed in Medicago, however, mtftc mutants did not affect flowering in Medicago (Laurie et al., 2011). Except for MtFTa2, the expression of the four MtFT genes is dependent on photoperiod ( Table 1 ). In LD conditions, MtFTa1, MtFTb1, and MtFTb2 are expressed, while their expression is either absent or marginal in SD conditions (Laurie et al., 2011). This implies that MtFTa1 and MtFTb1 are responsible for photoperiodic flowering regulation, while MtFTb2 may contribute to photoperiodic responses other than flowering. MtFTb1 and MtFTb2 show bimodal expression profiles in LD conditions with peaks in the morning (Zeitgeber Time, ZT4) and at dusk (ZT16), reminiscent of the Arabidopsis FT expression profile observed in FR-enriched LD conditions. In contrast, MtFTa1 expresses consistently throughout the day in LD. The expression of MtFTc also appears to be regulated by photoperiod, but unlike MtFTa1, MtFTb1, and MtFTb2, its expression level is higher in SD than in LD conditions (Laurie et al., 2011)( Table 1 ). In SD conditions, MtFTc expression levels start to increase in the morning and peak after dusk (during the night-time period), which is a unique pattern among the FT orthologs examined in this review (Laurie et al., 2011) ( Table 1 ). Moreover, MtFTc expresses at a low level in leaves in LD conditions. This expression profile suggests that MtFTc may have little or weak contribution to flowering time regulation in LD, and its high expression in SD conditions implies a potential role in regulating flowering time in SD. Conversely, MtFTa2 is expressed in both LD and SD conditions. Its expression level in LD remains relatively constant throughout the day, while it peaks after dusk in SD conditions, similar to MtFTc. The inability of ectopic expression of MtFTa2 in Arabidopsis to rescue the late flowering of the ft mutant, combined with its high expression in tissues other than leaves such as roots, suggests diversified role(s) for MtFTa2 beyond flowering time regulation in Medicago. In addition to FT orthologs, orthologs in Medicago of photoperiodic flowering time-regulating factors such as CO and CO-like (COL), GI, and FKF1 genes have been identified, though their functions in flowering remain to be resolved or appear to be diversified (Hecht et al., 2005; Wong et al., 2014). Medicago possesses a single copy of PHYA gene. MtPHYA positively regulates flowering in LD, while its effect becomes weaker in SD, suggesting a photoperiod-dependent contribution of phyA to flowering (Jaudal et al., 2020). Despite the functional conservation of phyA between Medicago and Arabidopsis in flowering, it has been shown that flowering in Medicago sativa (M. Sativa), a close relative Medicago truncatula, is delayed under shade conditions. This shade-induced delay of flowering was also observed under a R/FR ratio of 0.8 (Lorenzo et al., 2019). Given the sunlight R/FR ratio close to 1, M. Sativa in its natural habitat may flower later than in laboratory LD conditions. Under shade with a R/FR ratio of 0.4–0.6, the expression levels of MsFTa1 and MsFTb1 were reduced around dusk compared to normal LD conditions, which is consistent with the reduction of expression of SQUAMOSA PROMOTER BINDING LIKE 3 (SPL3), a floral activator (Lorenzo et al., 2019). However, it should be noted that floral activators such as PIF3 and HB2 are up-regulated in the same condition. Further analysis is needed to understand how the regulatory pathway of flowering responds to supplementary FR light. The delay in flowering under shade in M. Sativa is likely a diverged strategy to increase survival rates by choosing the accumulation of resources instead of flowering under unfavorable conditions, as a characteristic of a perennial plant. However, Medicago truncatula, an annual plant, also exhibits similar responses to shade or supplementary FR light; flowering is delayed in shaded conditions (Mauro et al., 2014), and HB2 expression is up-regulated by supplementary FR light (Zhang et al., 2020). The delay in flowering due to shade has also been reported in another legume plant, soybean. Flowering of soybean is delayed under low R/FR light quality (Cober and Voldeng, 2001) (Discussed in Section 2.2.2). Thus, the determination of flowering time in response to different R/FR ratios may be governed by legume-specific mechanisms rather than whether it is an annual or a perennial plant.

Lactuca Sativa (Lettuce), a dicot like Arabidopsis, belongs to the asteridae subclass, and its flowering is induced with longer day lengths. Lettuce has been shown to have one copy of the FT ortholog gene, LsFT, which functions as a floral activator (Fukuda et al., 2011) ( Table 1 ). While it remains unclear whether photoperiod affects LsFT expression levels, the rates of its accumulation during development in different cultivars positively correlate with floral induction (Fukuda et al., 2017), further supporting the functional conservation of FT ortholog in Lettuce. Under LD conditions, LsFT exhibits high expression in the morning and around dusk (Sgamma et al., 2016), resembling the expression pattern observed in Arabidopsis grown under FR light-enriched LD conditions. However, this result was obtained from lettuce grown in a greenhouse, thus it needs to be elucidated if the expression profile of LsFT is also controlled by FR light that is enriched in natural light conditions. Nevertheless, the sole FT-like gene in lettuce, LsFT, behaves similarly to Arabidopsis FT. FR light likely accelerates flowering in lettuce, as supplementary FR light in the presence of R/Blue/Green light increased stalk heights, indicative of a premature flowering response (Alrajhi et al., 2023). However, little is known whether LsFT is responsible for floral induction by the supplementary FR light. Being one of the most popular leafy vegetables, delayed flowering in lettuce is preferred to prevent stem elongation and bitterness (Assefa et al., 2019). Therefore, understanding the flowering time pathway of lettuce is crucial.

Beta vulgaris (Beet) belongs to Caryophyllidae, the third-largest subclass of the class Magnoliopsida (dicotyledons). Beet possesses two FT-like genes, BvFT1 and BvFT2, which interestingly exhibit opposing functions in flowering regulation. BvFT2 promotes flowering, while BvFT1 represses flowering in both beet and Arabidopsis when ectopically expressed (Pin et al., 2010) ( Table 1 ). The antagonistic roles of BvFTs appear to be conferred by sequence variations in segment B of the fourth exon, encoding an external loop of PEBP proteins. In this segment, both BvFT proteins share a conserved key amino acid, Glutamine (Q), at residue 144 and 140 in BvFT1 and BvFT2, respectively, that differentiates FT from its close PEBP protein TFL1, a floral repressor (Ahn et al., 2006). However, only BvFT2 has sequence conservation throughout the entire segment B. Indeed, swapping the segment B sequences in BvFTs altered their inherent activity in opposite ways (Pin et al., 2010). In addition to their opposing roles, expression profiles of the two BvFTs differ. Similar to Arabidopsis FT in laboratory conditions, BvFT2 expresses high in LD but not in SD conditions with a peak around dusk in annual beets (Pin et al., 2010). On the contrary, BvFT1 is expressed highly in SD with a peak after dawn (Pin et al., 2010). In biennial beets without vernalization, the expression of BvFTs is photoperiod-independently regulated. BvFT1 expresses in both SD and LD with the same diurnal profile, while BvFT2 does not express, which is consistent with essential role of vernalization for floral induction in biennial plants. After vernalization, the expression profiles of each BvFTs resemble those in annual beets. As a root crop, delaying flowering in the first year of the biennial plant’s life cycle is necessary for increasing leaf and root mass for sugar and bioethanol production. Several flowering-regulating genes have been identified and characterized in the regulation of beet flowering, such as BOLTING TIME CONTROL 1 (BvBTC1), a key determinant of vernalization requirements between annual and biennial plants (Pin et al., 2012). As a positive regulator of flowering, BvBTC1 is likely involved in the responsiveness to LD, as shown that reduced BvBTC1 expression by RNAi caused induction of BvFT1 expression and reduction of BvFT2 expression (Pin et al., 2012). Additionally, several reports have demonstrated the negative effect of shade on leaf and root biomass of beet (Artru et al., 2018; Schambow et al., 2019). In 1965, Lane et al. showed that by using different types of lamps, flowering time varied depending on R/FR ratios. In general, supplementary FR light promotes flowering in beets in the presence of R light. Thus, beet determines flowering time in response to the light quality, likely via mechanisms similar to phyA-mediated flowering regulation in Arabidopsis.

The mechanisms governing the photoperiodic regulation of flowering in SDPs have been demonstrated mostly in rice, one of the world’s major crops. Rice, a monocotyledonous flowering plant in the grass family Poaceae, initiates flowering in response to daylengths shorter than 13.5h (Itoh et al., 2010). In rice, two FT-like genes, Heading date 3a (Hd3a) and RICE FT1 (RFT1), play crucial roles as mobile florigens synthesized in leaves and essential for flowering (Kojima et al., 2002; Tamaki et al., 2007; Komiya et al., 2008) ( Table 1 ). The expression of these florigens is responsive to the photoperiod, with higher expression levels observed in SD conditions. Under SD conditions (10 h light, 30°C/14 h dark, 25°C), the expression of Hd3a and RFT1 peaks at or before dawn, and then gradually decrease during the day (Izawa et al., 2002; Komiya et al., 2008). The diurnal expression profiles of Hd3a and RFT1 are regulated by the activity of OsGI, whose expression is under the control of the circadian clock with a peak at the end of day (Hayama et al., 2003, 2002; Lee and An, 2015). OsGI is necessary for the diurnal expression of HEADING DATE 1 (Hd1), a rice ortholog of CO, which activates the expression of Hd3a and RFT1 in the morning (Hayama et al., 2003). This pathway resembles the CO-FT pathway in Arabidopsis. Additionally, a rice-specific pathway has been identified. EARLY HEADING DATE 1 (Ehd1) encoding a B-type response regulator is upregulated by OsGI when blue light coincides with the morning phase, and activates Hd3a and RFT1 expression (Doi et al., 2004; Itoh et al., 2010). Ehd1-mediated flowering regulation pathway has thought to be absent in Arabidopsis, although its homologs are found in Arabidopsis and other plants (Bar et al., 2008). It plays a central role in photoperiod-mediated flowering in rice, which is recently extensively reviewed in Vicentini et al., 2023. Conversely, in LD conditions, Ehd1 expression is repressed by GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (Ghd7), encoding a CCT-domain protein highly expressed in LD (Xue et al., 2008; Itoh et al., 2010). Despite lower expression levels of Hd3a and RFT1 in LD conditions compared to SD conditions, RFT1 has shown to be a major floral activator under LD conditions (Komiya et al., 2009). Indeed, RFT1 shows diurnal expression profile with a peak in the early morning and gradually increasing over the night (Komiya et al., 2009). The current model of photoperiodic flowering regulation mediated by Hd3a and RFT1 relies primarily on their on-off gene expression profiles between SD and LD, respectively. As the photoperiod lengthens in LD conditions, high expression of Hd1 coincides with the daytime phase. However, in contrast to the role of CO in Arabidopsis, Hd1 acts as a repressor for the expression of Hd3a and RFT1 in LD conditions (Hayama et al., 2003). Consequently, the accumulation of Hd1 suppresses the expression of Hd3a and RFT1, leading to reduced expression around dusk. Nevertheless, this model cannot explain the circadian clock-regulated RFT1 expression in LD, which is important for flowering in LD. The similar diurnal expression profiles of RFT1 between SD and LD suggest that photoperiodic regulation in rice differs from that in Arabidopsis. Ehd1 expression in LD showed similar expression patterns with RFT1 (Komiya et al., 2009), thus release of suppression of Ehd1 expression in LD conditions may accomplish with developmental processes.

FR light has been identified as a positive regulator flowering in rice, as demonstrated by experiments such as 2-hr FR light irradiation before a 10-hr dark period in LD conditions (14-hr light/10-hr dark) promoting flowering (Ikeda, 1985). In Arabidopsis, end-of-day (EOD)-FR light treatment counteracts the negative impact of R light by inactivating phyB (Smith, 1982; Franklin and Whitelam, 2005). Although the induction of flowering in Arabidopsis by EOD-FR light treatment is not solely dependent on phyB (Bagnall et al., 1995; Xie et al., 2020), it is possible that the 2-hr FR light irradiation before the dark period affects phyB activity in rice for floral induction. This hypothesis is supported by evidence that EOD-FR in SD conditions (10-hr light/14-hr dark) accelerated flowering in rice in a phyB-dependent manner (Takano et al., 2005). Another possibility is that the FR light signal itself, perceived by photoreceptors like phyA, plays a role in mediating flowering promotion, similar to Arabidopsis. Several observations support this idea: 1) 3-hr FR light was more effective at inducing flowering than 1-hr or 2-hr light treatment before the dark period in LD, and this FR light irradiation was more effective than extending the dark-period (Ikeda, 1985), 2) phyA mutant consistently exhibited delayed flowering in SD conditions (10-hr light/14-hr dark) (Takano et al., 2005; Ishikawa et al., 2009), which is different from the action of phyB, 3) simulated shade conditions accelerated flowering time, inducing RFT1 but not Hd3a expression (Cui et al., 2023). Given that phyA’s function in FR high-irradiance response (HIR) has been reported in photomorphogenesis in rice (Takano et al., 2005), similarly to Arabidopsis, these findings suggest a role of phyA in responding to prolonged FR light in flowering time regulation through the modulation of RFT1 (and Hd3a) expression.

Soybean (Glycine max) belongs to the rosidae subclass together with Arabidopsis and Medicago, however, unlike Arabidopsis and Medicago, its flowering is induced under short days. Soybean possesses ten FT-like genes. GmFT2a/2b, GmFT3a/3b, GmFT5a/5b are known to activate flowering, while GmFT1a/1b, GmFT4, and GmFT6 function as floral repressors (Lee et al., 2021) ( Table 1 ). Their expression is controlled by photoperiod (Lee et al., 2021). The floral activators, GmFT2a/2b and GmFT5a, exhibit high expression levels in SD, peaking in the morning. On the other hand, GmFT1a and GmFT4, the floral repressors, are suppressed in SD but highly induced in LD. Interestingly, despite GmFT1a and GmFT4 showing high expression in LD conditions, their expression peaks are observed only in the morning. This is in contrast to FT orthologs of LDPs, where expression peaks either once dusk (e.g., Arabidopsis FT in the laboratory LD conditions) or twice in the morning as well as in the dusk (e.g., Arabidopsis FT in natural LD conditions). Moreover, in contrast to the role of phyA in flowering in Arabidopsis, soybean phyAs (GmPHYA2 and GmPHYA3) are known to negatively regulate flowering time. They achieve this by increasing the expression of the legume-specific regulator E1, which, in turn, represses the expression of the floral activators GmFT2a and GmFT5a while activating the expression of the floral repressors GmFT1a and GmFT4 (Zhai et al., 2014; Cao et al., 2017; Liu et al., 2018). E1 is specifically expressed in LD, but not in SD, with two peaks at both dawn and dusk in LD conditions (Xia et al., 2012), resembling the expression pattern of FT in Arabidopsis plants grown under FR light enriched-LD conditions. GmPHYAs promote E1 expression, partially through the regulation of circadian clock genes, the GmPRR3a/GmPRR3b(Tof11/Tof12)-GmLHY module. GmPHYs upregulate the expression of GmPRR3a/GmPRR3b, which then directly represses the expression of GmLHY genes. Subsequently, this relieves GmLHY-mediated suppression of E1, inducing its expression (Lu et al., 2020). Notably, mutations in GmLHY genes (lhy1a lhy1b lhy2a lhy2b) induces E1 expression at dusk in LD, while the morning E1 expression remains unaffected in the mutant (Lu et al., 2020), implying that E1 expression in the morning and at dusk may be regulated by at least two different modes of action, similar to the regulation of Arabidopsis FT expression. The soybean case study suggests that although the activity of FT orthologs has diversified as either activators or repressors, regulatory mechanisms for bimodal expression in flowering may have been conserved through the combined action of photoperiod and circadian clock. In addition to the negative effect of phyA on soybean flowering, low R/FR ratios differentially affect soybean flowering compared to Arabidopsis ( Table 1 ). Although shade avoidance responses such as elongation of seedlings and stem, and reduction of leaf area and size are observed in soybeans under various growth conditions similar to those in Arabidopsis (Green-Tracewicz et al., 2012; Yang et al., 2014; Wu et al., 2017), onset of soybean flowering in LD (20 h Light/4 h Dark) is delayed as the R/FR ratio decreases, in which E7 is required (Cober and Voldeng, 2001). Under LD (20 h Light/4 h Dark) with a R/FR ratio of 2.7, flowering time in near-isogenic lines with E7 allele and e7 allele is indistinguishable. However, when the R/FR ratio is similar to or lower than the sunlight R/FR ratio, soybean lines with the E7 allele significantly delay flowering compared to those with the e7 allele. Although a low R/FR ratio in SD (12 h Light/12 h Dark) slightly delays flowering, the responsiveness is much weaker compared to that in LD. As phyA is thought to mainly function in LD to repress flowering in soybean, the different responsiveness to R/FR ratios between LD and SD may be due to activity of phyA. Not only soybeans but also other legume plants, including winged bean (Psophocarpus tetragonolobus) (Raai et al., 2020) and Medicago (as described in Section 2.1.2), have shown to delay flowering in response to shade. Lentil (Lens culinaris) have been reported to flower earlier in low R/FR conditions than in high R/FR conditions (Yuan et al., 2017), however, it should be noted that the R/FR ratio in this low R/FR condition is 1.6, even higher than the sunlight R/FR ratio. Thus, it is possible that legumes can induce a series of typical shade avoidance responses but not early flowering. Instead, they may have evolved to delay flowering under shade in a legume-specific manner.