The Arabidopsis FD gene encodes a bZIP protein of 285 amino acid residues, which is identified as AtbZIP14 (At4g35900). Arabidopsis FD mRNA is distributed in the shoot apex and leaves with consistent 24-h (hour) rhythms, and its expression is significantly upregulated after seedling emergence (Abe et al., 2005; Park et al., 2023). The results of functional studies on loss and gain of function mutants suggest that FD regulates floral transition (Abe et al., 2005). Arabidopsis fd-2 mutant has a late flowering phenotype (Wigge et al., 2005). Overexpression of FD results in early flowering, 35S:FD enhances 35S:FT phenotype; therefore, the amount of FD activity is one of the limiting factors for 35S:FT plants (Abe et al., 2005).

In Arabidopsis, 14-3–3 proteins interact with FD and FT to form the ‘florigen activation complex’ (FAC) complex (Ho and Weigel, 2014). 14-3–3 proteins act as adaptor proteins to recognize and interact with the phosphorylated FD. FAC formation is dependent on the phosphorylation at position 282 (T282) of FD. That is, only after FD has been phosphorylated can active FAC be formed to induce flowering (Collani et al., 2019) ( Figure 2A ). Kawamoto et al. (2015) found that the calcium-dependent protein kinases (CDPKs) CPK4, CPK6, and CPK33 are good candidates for FD kinases. FD binds DNA (a strong preference for binding to G-box motifs) but does not activate transcription. FT acts as a transcriptional coactivator, increasing the enrichment of FD on floral time and floral homeotic genes such as AP1, LFY, SOC1, FRUITFULL (FUL), SEPALLATA1 (SEP1), SEP2, and SEP3 (Wigge et al., 2005; Ahn et al., 2006; Collani et al., 2019). AP1 is a class A gene, SEP1, SEP2, and SEP3 are class E genes during flower development (Krizek and Fletcher, 2005). TWIN SISTER OF FT (TSF), a paralog of FT, promotes flowering by enhancing the binding of FD to DNA (Collani et al., 2019).

In rice, FD homologs (OsFD1-OsFD7) share a conserved bZIP and SAP motif (Tsuji et al., 2013; Kaur et al., 2021). OsFD1-Hd3a-14-3–3 complex activates OsMADS15 (a homolog of AP1) and leads to early flowering (Tsuji et al., 2013). Silencing of OsFD7 correlates with late flowering and downregulation of MADS-box genes (e.g. OsMADS14, OsMADS15, and OsMADS18) involved in floral meristem development (Kaur et al., 2021) ( Figure 2B ). However, the regulation mechanism in rice is different from Arabidopsis. OsFD1 is located in the nucleus of shoot apex cells. The rice FT homolog, HEADING DATE 3a (Hd3a), translocates from the leaves to the shoot apex and binds 14-3–3 proteins in the cytoplasm. Then, the Hd3a-14-3–3 complex enters the nucleus and forms an FAC with OsFD1. The phosphorylated serine residue S192 in the OsFD1 SAP motif is recognized by 14-3–3 to facilitate the association between OsFD1 and Hd3a. FAC activates the transcription of OsMADS15, leading to floral induction (Taoka et al., 2011) ( Figure 2B ). Similar to OsFD1, OsFD7 is located in the nucleus of shoot apex cells. The rice FT homolog, OsFTL, translocates from the leaves to the shoot apex and binds OsGF14 proteins in the cytoplasm. Then, the OsFTL-OsGF14 complex enters the nucleus and forms an FAC with OsFD7. OsFD7 is phosphorylated by OsCDPK41 and OsCDPK49, and the interaction between OsGF14b and OsFD7 is dependent on this phosphorylation. FAC (OsFTL-OsGF14-OsFD7) activates the transcription of some floral meristem identity genes, leading to floral transition (Kaur et al., 2021) ( Figure 2B ). Unlike OsFD1 and OsFD7, OsFD2 shuttles between the cytoplasm and the nucleus. Normally, OsFD2 is restricted to the cytoplasm of shoot apex cells via its interaction with cytoplasmic 14-3–3 proteins. When Hd3a moves from the leaves to the shoot apex, the interaction between Hd3a and 14-3–3 initiates its nuclear translocation. The putative phosphorylation site, S164, within the SAP motif of OsFD2 is critical for the interaction between OsFD2 and 14-3-3 ( Figure 2B ). These results indicate that the FD function diverges among OsFD1, OsFD2, and OsFD7, but the formation of an FAC is essential for its function (Tsuji et al., 2013).

Similar to Arabidopsis and rice, FD-like proteins in monocotyledonous plants such as maize, wheat, and bamboo also promote flowering ( Table 1 ). Maize delayed flowering1 (dlf1) mediates floral inductive signals transmitted from the leaves to the shoot apex, activated by indeterminate1 (id1) in the leaves (Kozaki et al., 2004). The targets of dlf1 might be the maize MADS-box homologs (ZM MADS) (Muszynski et al., 2006) ( Figure 2C ). The id1 defined floral induction pathway may be unique to monocots, because no clear id1 orthologs exist in the Arabidopsis genome (Colasanti et al., 2006). In wheat, the regulatory mechanism of TaFDL2 is similar to that in Arabidopsis FD. TaFDL2 interacts with TaFT and binds to the ACGT elements in the promoter of VRN1 (homolog of Arabidopsis AP1) ( Figure 2D ). TaFT does not directly interact with the VRN1 promoter but interacts with TaFDL2 proteins (Li and Dubcovsky, 2008). Overexpressing bamboo BtFD1 in Arabidopsis leads to early flowering. BtFD1 binds to the ACGT motif of AtAP1’s promoter and upregulates the expression of AtAP1 (Dutta et al., 2021). In barley (Hordeum vulgare), HvFDL4 and HvFDL5 have been shown to physically interact with 14-3–3 proteins in a phosphorylation-dependent manner. Serine-to-alanine substitutions at critical residues (S333A in HvFDL4 or S216A in HvFDL5) abolish their binding to 14-3–3 proteins, suggesting that phosphorylation at these sites is essential for complex formation (Li et al., 2015). However, the biological functions of these interactions remain uncharacterized.

The FD homologs in dicotyledons also have consistent functions ( Table 1 ). Overexpression of soybean GmFDc1 leads to early flowering, suggesting that GmFDc1 activates the floral transition (Yue et al., 2021). GmFT5a binds to GmFDc1 and enhances the positive effect of GmFDc1 on GmAP1 expression (Yue et al., 2021) ( Figure 2E ). Soybean GmFDL19-OE lines flower earlier than wild-type, which may be mediated by the direct up-regulation of GmAP1a. GmFDL19 also interacts with GmFT2a and GmFT5a to regulate flowering (Takeshima et al., 2019) ( Figure 2E ). GmFDL15 interacts with GmFT5b to promote flowering (Su et al., 2023). In poplar (Populus trichocarpa), the ectopic expression of PtFD1 (FDL2) results in early flowering (Tylewicz et al., 2015). Tobacco FD homologs participate in flowering regulation, NtFD1, NtFD3, and NtFD4 overexpression lines flower earlier than the wild-type (WT) (Beinecke et al., 2018). Overexpressing EdFD1 or EdFD2 in Arabidopsis results in early flowering. EdFT interacts with both EdFD1 and EdFD2 and regulates wild loquat flowering (Zhang et al., 2016).

SQUAMOSA PROMOTER BINDING LIKE (SPL) 3/4/5 are involved in the age pathway in Arabidopsis (Wang et al., 2009). SPL3/4/5 bind directly to the promoters of AP1, LFY, and FUL and recruit FD to these loci, mediating their activation by the FT-FD complex (Benlloch et al., 2011) ( Figure 2A ). SPL3/4/5 synergistically interact with the FT-FD module to induce flowering, linking the age and photoperiod pathways of flowering regulation (Jung et al., 2016). TCP5/13/17 (class II CIN TCP TFs) belonging to the Teosinte branched1/Cincinnata/proliferating cell factor (TCP) family act directly on the Telobox Motif cis-elements (GGACCA) of the AP1 promoter by interacting with FD ( Figure 2A ) (Li et al., 2019). They act synergistically and additively with the FT-FD module to positively regulate the initiation of flowering in Arabidopsis (Li et al., 2019). Chromatin remodeler HISTONE DEACETYLASE 19 (HDA19) preserves the identity of the inflorescence meristem (IM) in an age-dependent manner; in older hda19 inflorescence apices, floral organ identity genes are abnormally expressed, and the mutation of fd enhances the timing of these reproductive defects in hda19 (Gorham et al., 2018). FLOWERING H (FLH) is involved in the vernalization pathway of flowering. The early flowering Cape Verde Islands (CVI) allele of FLH requires the floral integrator FD to accelerate flowering (Seedat et al., 2013). These results confirm that FD is a central regulator of floral transition in the shoot meristem, which integrates signals from multiple pathways.

In citrus, CiFD forms two different proteins (CiFDα and CiFDβ) by alternatively splicing. Overexpressing CiFDα or CiFDβ in tobacco and trifoliate orange leads to early flowering (Ye et al., 2023). CiFDα and CiFDβ are induced by low temperatures and drought, respectively. Under low temperatures, CiFDα can interact with CiFT and Ci14-3–3 to form an FAC complex, which binds to the C-box element on the promoter of the floral meristem organization gene CiAP1, promoting its expression. In contrast, CiFDβ can bind directly to the CiAP1 promoter independently of CiFT and Ci14-3-3. The transcription factor CibHLH96 binds to the E-box of the CiFD promoter to promote CiFD expression. CibHLH96 is induced by drought but not at low temperatures. Under drought conditions, CibHLH96 activates the expression of CiFD and forms more CiFDβ ( Figure 2F ) (Ye et al., 2023). These results show that both CiFDα and CiFDβ are involved in the regulation of citrus flowering, but they have different mechanisms (Ye et al., 2023). In Kiwifruit, AcFD regulates cold signal response. It is downregulated in dormant buds in response to cold treatment (Varkonyi-Gasic et al., 2013). A member of soybean bZIP family group A, GmFDL19, is involved in abiotic stress tolerance and floral transition (Nan et al., 2014; Li et al., 2017; Yue et al., 2023). The tolerance to drought and salt stress is enhanced in GmFDL19-OE lines by upregulating ABA/stress-responsive genes and reducing the accumulation of Na⁺ ion content, and ectopic expression of GmFDL19 in soybean causes early flowering (Nan et al., 2014; Li et al., 2017).

In pea, VEGETATIVE2 (VEG2), which is homologous to FD, plays a central role in regulating meristem identity throughout the development of the compound inflorescence. VEG2 interacts with FTb2 in the shoot apex to promote primary (I₁) inflorescence meristem identity through DETERMINATE (DET), LATE FLOWERING (LF), FTa1, and FTc. VEG2 interacts with FTa1 at the shoot apex to promote secondary (I₂) inflorescence meristem identity via VEG1 and FTc. veg2 mutant transforms I₂ into I₁ inflorescence meristems (Sussmilch et al., 2015). VEG2 is also involved in the regulation of floral architecture through the regulation of MADS-box genes such as PIM (AP1), AP3, and SEP1. veg2 mutant has defective sepals and petals, fused floral organs, reduced organ numbers, and malformed organs (Sussmilch et al., 2015) ( Figure 3A ).

MtFDa, an ortholog of pea VEG2/PsFDa, plays a key role in inflorescence development in Medicago truncatula. mtfda has tertiary branches and bracts that transform into compound leaves, suggesting that MtFDa is required for I₂ inflorescence meristem identity and development. mtfda and mtfta1 flower later than WT, the double mutant mtfda/mtfta1 never forms flowers, and no floral transition in mtfda/mtfta1 happens (Cheng et al., 2021). The phenotype of mtfda/mttfl1 double mutant phenotype is similar to that of mtfda. The I₁ inflorescence shows indeterminate growth, indicating that MtFDa is epistatic to MtTFL1 for I₁ indeterminacy. MtFDa and MtFULc co-determine I₂ identity. The I₂ inflorescence of the mtfda/mtfulc double mutant transforms into an I₁-like vegetative structure, producing compound inflorescences, compound leaves, and indeterminate apices. The mtfda/mtfulc/mttfl1 triple mutant has a similar flowering time, inflorescence, and flower as mtfda, indicating that MtFDa has an epistatic effect on MtFULc (Cheng et al., 2018). Collectively, MtFDa plays a key role in inflorescence development, functions in coordination with MtFULc for I₂ inflorescence meristem identification, and is epistatic to MtTFL1 for I₁ indeterminacy (Cheng et al., 2018, 2021).

PaFDL1 and PaFDL2 are FD homologs of the London plane that participate in flower organ development. Overexpression of PaFDL1 and PaFDL2 in tobacco leads to extended stigmas, and curled petals at the tips (Cai et al., 2021).

FD-like proteins participate in inflorescence development, stem growth, and seed development. The inflorescences of Nicotiana tabacum NtFD1, NtFD3, and NtFD4 overexpression lines are condensed and the pedicles, peduncles, and internodes are short, resulting in a bushy, bunch-like architecture. They flower earlier than the WT, fewer leaves are produced before flowering, and differentiation of axillary meristems is also premature compared with WT (Beinecke et al., 2018).

Rice OsFD7 RNAi lines have longer and denser panicles, more florets, elevated seed size and weight, and more seeds. The transcription levels of OsMADSs are down-regulated in OsFD7 RNAi lines (Kaur et al., 2021). OsFD2 inhibits the developmental shift from inflorescence branch meristem to floral (or spikelet) meristem in panicle branches, which leads to plentiful spikelets or secondary branches and a dense panicle phenotype with smaller leaves (Tsuji et al., 2013). The overexpression of bamboo BtFD1 in Arabidopsis also leads to dwarfism and an apparent reduction in the length of the flowering stalk and number of flowers per plant (Dutta et al., 2021). The overexpression of FD by the 35S promoter in Arabidopsis results in dwarf plants (Abe et al., 2005). Overexpression of FD and FDP (At2g17770/AtbZIP27) in rice causes a reduction in plant height and spikelet size with decreased expression of genes involved in cell elongation without significant flowering time alteration, which is linked to impaired gibberellin biosynthesis in plants (Jang et al., 2017). Romera-Branchat et al. (2020) reported that FD and FDP bind to genes involved in water deprivation and hormonal pathways, including gibberellic acid, ABA, and jasmonic acid. These results provide evidence of crosstalk between the regulation of plant morphogenesis and hormone signaling pathways.

In soybean, overexpression of GmFDc1 leads to fewer nodes (Yue et al., 2021). GmFT5a interferes with the binding of Dt1 to GmFDc1 and enhances the positive effect of GmFDc1 on GmAP1 expression (Yue et al., 2021) ( Figure 2E ). Dt1 controls stem growth habit and flowering time and strongly influences soybean grain yield (Liu et al., 2010; Yue et al., 2021). Mutations in the recessive alleles of gmft2a and gmft5a delay flowering and increase node number, branch number, and yield (Li et al., 2021). Thus, GmFDc1 appears to contribute significantly to soybean plant architecture and yield.

In poplar, overexpression of PtFD1 (FDL2) results in severe dwarfing under a LD photoperiod, however, SD-induced growth arrest and bud formation are lost in PtFD1 (FDL2)-overexpressing lines (Parmentier-Line and Coleman, 2015; Tylewicz et al., 2015). PtFD2 (FDL1) overexpression resulted in a delayed SD response compared to WT. FT- PtFD2 (FDL1) complex mediates photoperiodic growth by regulating Like AP1 (LAP1). PtFD2 (FDL1) also participates in controlling adaptive responses and bud maturation pathways by interacting with ABSCISIC ACID INSENSITIVE 3 (ABI3), a component of ABA signaling (Tylewicz et al., 2015) ( Figure 3B ). Loquat EjFD suppresses EjAP1–1 expression by interacting with EjTFL1s or EjFT1, inhibiting loquat flower bud differentiation. Conversely, EjFD-EjFT2 promotes floral bud formation under the regulation of photoperiod and GA signals (Jiang et al., 2020, 2024) ( Figure 3C ).

In contrast to FT and TSF, TERMINAL FLOWER 1 (TFL1) and BROTHER of FT and TFL1 (BFT) are floral repressors in the PEBP family. These PEBPs have conserved 14-3–3 binding motifs and interact with FD (Yoo et al., 2010; Hanano and Goto, 2011; Ryu et al., 2014). TFL1 competes with FT for FD binding and represses the transcription of floral meristem identity genes such as LFY and AP1 (Hanano and Goto, 2011) ( Figure 2A ). TFL1 interacts with unphosphorylated FD via 14-3–3 proteins, suggesting that the inactive FD/14-3-3/TFL1 ternary complex may be present in the basal state of the SAM. Only when FD is phosphorylated can FT form an active complex with the 14-3–3 proteins to induce flowering. The efficient phosphorylation of T282 in FD is calcium-dependent. This requirement may help prevent the premature induction of flowering (Kawamoto et al., 2015). Under high salinity conditions, BFT delays flowering. The relative transcript levels of BFT are higher than those of FT and the high-level BFT protein competes with FT for FD binding in the SAM (Ryu et al., 2014) ( Figure 2A ). Similar to FD, FDP is phosphorylated by CPK33, forming a complex with FT and TFL1 in a phosphorylation-dependent manner. The weak late-flowering phenotype of cpk33–1 may be due to the combined effect of the florigen and anti-florigen complex formation of FD and FDP (Kawamoto et al., 2015).

The rice TFL1-like protein RICE CENTRORADIALIS (RCN) competes with Hd3a for 14-3–3 binding. RCN protein transports from the vasculature to SAM to form the “florigen repression complex” (FRC) with 14-3–3 and OsFD1 and then represses florigenic activity. The balance between FRC and FAC depends on the ratio of Hd3a to RCN and regulates the development of SAM. In the vegetative phase, FRC are formed. Upon reaching the SAM, Hd3a competes with RCN for FAC formation. When the balance shifts to FAC, the reproductive program begins (Kaneko-Suzuki et al., 2018) ( Figure 2B ).In soybean, Dt1 and GmFT5a have opposite functions. Dt1 complementary lines produce more nodes and flower later than WT (Yue et al., 2021). gmft5a delays flowering and increases node number (Li et al., 2021). Dt1 interacts with GmFDc1 and binds to ACGT cis-elements in the promoter of GmAP1a to repress its activity. GmFT5a interferes with the binding of Dt1 to GmFDc1 and enhances the positive effect of GmFDc1 on GmAP1 expression (Chen et al., 2020b; Yue et al., 2021). TFL1c and TFL1d, homologs of Dt1, interact with GmFDc1 and binds to ACGT cis-elements in the promoter of GmAP1a to repress its activity (Wang et al., 2023) ( Figure 2E ). TFL1c and TFL1d might also compete with GmFT5a for GmFDc1 binding. EjTFL1s inhibit loquat flower bud differentiation through EjFD binding and suppression of EjAP1-1 (Jiang et al., 2020). In contrast, EjFT1 and EjFT2 interact with EjFD but have opposing effects: EjFT2-EjFD activates EjAP1–1 and EjAP1-2, while EjFT1-EjFD represses EjAP1-1. EjFT1 may resemble EjTFL1s in promoting vegetative growth. The competitive interaction between EjFT1 and EjFT2 with EjFD regulates floral bud differentiation, with EjFT2 promoting flowering and EjFT1 supporting vegetative growth (Jiang et al., 2024) ( Figure 3C ). Protein structural analysis of EjFT1 and EjFT2 suggests that differences in amino acid residues at Val123/Leu123, Ser157/Ala157, and Val158/Ala158 may be the reason for their functional differences (Jiang et al., 2024).

In Brachypodium distachyon, FTL9 and FD1 form an FAC that induces VRN1 and FUL2 expression, promoting flowering under SD conditions. Under LD conditions, however, FTL9 inhibits flowering. The FTL9-FD1 complex is less potent than the FT1-FD1 complex in inducing flowering. Overexpression of FTL9 disrupts FT1-FD1 complex formation by competing for FD1 binding, leading to reduced VRN1 expression and delayed flowering (Qin et al., 2019).

Kiwifruit CEN and FT interact with AcFD but exhibit distinct temporal expression patterns. FT is specifically induced in dormant buds during winter chilling, promoting dormancy release, whereas CEN transcripts accumulate in latent buds during summer but decrease in autumn prior to dormancy establishment. These contrasting expression patterns and functional roles suggest that CEN and FT may act as antagonistic regulators in kiwifruit development (Varkonyi-Gasic et al., 2013).