Components and Functional Diversification of Florigen Activation Complexes in Cotton

In shoot apex cells of rice, a hexameric florigen activation complex (FAC), comprising flowering locus T (FT), 14-3-3 and the basic leucine zipper transcription factor FD, activates downstream target genes and regulates several developmental transitions, including flowering. The allotetraploid cotton (Gossypium hirsutum L.) contains only one FT locus in both of the A- and D-subgenomes. However, there is limited information regarding cotton FACs. Here, we identified a 14-3-3 protein that interacts strongly with GhFT in the cytoplasm and the nuclei, and five FD homoeologous gene pairs were characterized. In vivo, all five GhFD proteins interacted with Gh14-3-3 and GhFT in the nucleus. GhFT, 14-3-3 and all the GhFDs interacted in the nucleus as well, suggesting that they formed a ternary complex. Virus-induced silencing of GhFD1, -2 and -4 in cotton delayed flowering and inhibited the expression of floral meristem identity genes. Silencing GhFD3 strongly decreased lateral root formation, suggesting a function in lateral root development. GhFD overexpression in Arabidopsis and transcriptional activation assays suggested that FACs containing GhFD1 and GhFD2 function mainly in promoting flowering with partial functional redundancy. Moreover, GhFD3 was specifically expressed in lateral root meristems and dominantly activated the transcription of auxin response factor genes, such as ARF19. Thus, the diverse functions of FACs may depend on the recruited GhFD. Creating targeted genetic mutations in the florigen system using Clustered regularly interspaced short palindromic repeats (CRISPR) and their associated proteins (Cas) genome editing may fine-tune flowering and improve plant architecture.

Phosphorylation of OsFD1 by OsCIPK3 promotes the formation of RFT1-containing florigen activation complex for long-day flowering in rice

Heading date is a critical trait that determines the regional adaptability and grain productivity of many crops. Although rice is a facultative short-day plant, its domestication led to the Ghd7-Ehd1-Hd3a/RFT1 pathway for adaptation to long-day conditions (LDs). The formation of the "florigen activation complex" (FAC) containing florigen Hd3a has been characterized. However, the molecular composition of the FAC that contains RFT1 for long-day flowering is unclear. We show here that RFT1 forms a ternary FAC with 14-3-3 proteins and OsFD1 to promote flowering under LDs. We identified a calcineurin B-like-interacting protein kinase, OsCIPK3, which directly interacts with and phosphorylates OsFD1, thereby facilitating the localization of the FAC to the nucleus. Mutation in OsCIPK3 results in a late heading date under LDs but a normal heading date under short-day conditions. Collectively, our results suggest that OsCIPK3 phosphorylates OsFD1 to promote RFT1-containing FAC formation and consequently induce flowering in rice under LDs.

OsFD4 promotes the rice floral transition via florigen activation complex formation in the shoot apical meristem

In rice, the florigens Heading Date 3a (Hd3a) and Rice Flowering Locus T 1 (RFT1), OsFD-like basic leucine zipper (bZIP) transcription factors, and Gf14 proteins assemble into florigen activation/repressor complexes (FACs/FRCs), which regulate transition to flowering in leaves and apical meristem. Only OsFD1 has been described as part of complexes promoting flowering at the meristem, and little is known about the role of other bZIP transcription factors, the combinatorial complexity of FAC formation, and their DNA-binding properties. Here, we used mutant analysis, protein-protein interaction assays and DNA affinity purification (DAP) sequencing coupled to in silico prediction of binding syntaxes to study several bZIP proteins that assemble into FACs or FRCs. We identified OsFD4 as a component of a FAC promoting flowering at the shoot apical meristem, downstream of OsFD1. The osfd4 mutants are late flowering and delay expression of genes promoting inflorescence development. Protein-protein interactions indicate an extensive network of contacts between several bZIPs and Gf14 proteins. Finally, we identified genomic regions bound by bZIPs with promotive and repressive effects on flowering. We conclude that distinct bZIPs orchestrate floral induction at the meristem and that FAC formation is largely combinatorial. While binding to the same consensus motif, their DNA-binding syntax is different, suggesting discriminatory functions.

Factorial combinations of protein interactions generate a multiplicity of florigen activation complexes in wheat and barley

The FLOWERING LOCUS T (FT) protein is a central component of a mobile flowering signal (florigen) that is transported from leaves to the shoot apical meristem (SAM). Two FT monomers and two DNA-binding bZIP transcription factors interact with a dimeric 14-3-3 protein bridge to form a hexameric protein complex. This complex, designated as the 'florigen activation complex' (FAC), plays a critical role in flowering. The wheat homologue of FT, designated FT1 (= VRN3), activates expression of VRN1 in the leaves and the SAM, promoting flowering under inductive long days. In this study, we show that FT1, other FT-like proteins, and different FD-like proteins, can interact with multiple wheat and barley 14-3-3 proteins. We also identify the critical amino acid residues in FT1 and FD-like proteins required for their interactions, and demonstrate that 14-3-3 proteins are necessary bridges to mediate the FT1-TaFDL2 interaction. Using in vivo bimolecular fluorescent complementation (BiFC) assays, we demonstrate that the interaction between FT1 and 14-3-3 occurs in the cytoplasm, and that this complex is then translocated to the nucleus, where it interacts with TaFDL2 to form a FAC. We also demonstrate that a FAC including FT1, TaFDL2 and Ta14-3-3C can bind to the VRN1 promoter in vitro. Finally, we show that relative transcript levels of FD-like and 14-3-3 genes vary among tissues and developmental stages. Since FD-like proteins determine the DNA specificity of the FACs, variation in FD-like gene expression can result in spatial and temporal modulation of the effects of mobile FT-like signals.

Hd3a promotes lateral branching in rice

Accumulating evidence indicates that the FLOWERING LOCUS T (FT) protein is the mobile floral signal known as florigen. A rice FT homolog, Heading date 3a (Hd3a), is transported from the phloem to shoot apical cells, where it interacts with 14-3-3 proteins and transcription factor OsFD1 to form a florigen activation complex (FAC) that activates a rice homolog of the floral identity gene APETALA1. Recent studies showed that florigen has roles in plant development beyond flowering; however, the exact nature of these roles is not well understood. It is not clear whether FT is transported to organs outside the shoot apex, and whether FAC formation is required for processes other than flowering. We show here that the Hd3a protein accumulates in axillary meristems to promote branching, and that FAC formation is required. Analysis of transgenic plants revealed that Hd3a promotes branching through lateral bud outgrowth. Hd3a protein produced in the phloem reached the axillary meristem in the lateral bud, and its transport was required for promotion of branching. Moreover, mutant Hd3a proteins defective in FAC formation but competent with respect to transport did not promote branching. Finally, we show that Hd3a promotes branching independently from strigolactone and FC1, a transcription factor that inhibits branching in rice. Together, these results suggest that Hd3a functions as a mobile signal for branching in rice.