Two modes of gene regulation by TFL1 mediate its dual function in flowering time and shoot determinacy of Arabidopsis.

Plant organ primordia develop successively at the shoot apical meristem (SAM). In Arabidopsis, primordia formed early in development differentiate into vegetative leaves, whereas those formed later generate inflorescence branches and flowers. TERMINAL FLOWER 1 (TFL1), a negative regulator of transcription, acts in the SAM to delay flowering and to maintain inflorescence meristem indeterminacy. We used confocal microscopy, time-resolved transcript profiling and reverse genetics to elucidate this dual role of TFL1. We found that TFL1 accumulates dynamically in the SAM reflecting its dual function. Moreover, TFL1 represses two major sets of genes. One set includes genes that promote flowering, expression of which increases earlier in tfl1 mutants. The other set is spatially misexpressed in tfl1 inflorescence meristems. The misexpression of these two gene sets in tfl1 mutants depends upon FD transcription factor, with which TFL1 interacts. Furthermore, the MADS-box gene SEPALLATA 4, which is upregulated in tfl1, contributes both to the floral transition and shoot determinacy defects of tfl1 mutants. Thus, we delineate the dual function of TFL1 in shoot development in terms of its dynamic spatial distribution and different modes of gene repression.

The bZIP transcription factor AREB3 mediates FT signalling and floral transition at the Arabidopsis shoot apical meristem.

The floral transition occurs at the shoot apical meristem (SAM) in response to favourable external and internal signals. Among these signals, variations in daylength (photoperiod) act as robust seasonal cues to activate flowering. In Arabidopsis, long-day photoperiods stimulate production in the leaf vasculature of a systemic florigenic signal that is translocated to the SAM. According to the current model, FLOWERING LOCUS T (FT), the main Arabidopsis florigen, causes transcriptional reprogramming at the SAM, so that lateral primordia eventually acquire floral identity. FT functions as a transcriptional coregulator with the bZIP transcription factor FD, which binds DNA at specific promoters. FD can also interact with TERMINAL FLOWER 1 (TFL1), a protein related to FT that acts as a floral repressor. Thus, the balance between FT-TFL1 at the SAM influences the expression levels of floral genes targeted by FD. Here, we show that the FD-related bZIP transcription factor AREB3, which was previously studied in the context of phytohormone abscisic acid signalling, is expressed at the SAM in a spatio-temporal pattern that strongly overlaps with FD and contributes to FT signalling. Mutant analyses demonstrate that AREB3 relays FT signals redundantly with FD, and the presence of a conserved carboxy-terminal SAP motif is required for downstream signalling. AREB3 shows unique and common patterns of expression with FD, and AREB3 expression levels are negatively regulated by FD thus forming a compensatory feedback loop. Mutations in another bZIP, FDP, further aggravate the late flowering phenotypes of fd areb3 mutants. Therefore, multiple florigen-interacting bZIP transcription factors have redundant functions in flowering at the SAM.

Unraveling the role of MADS transcription factor complexes in apple tree dormancy.

A group of MADS transcription factors (TFs) are believed to control temperature-mediated bud dormancy. These TFs, called DORMANCY-ASSOCIATED MADS-BOX (DAM), are encoded by genes similar to SHORT VEGETATIVE PHASE (SVP) from Arabidopsis. MADS proteins form transcriptional complexes whose combinatory composition defines their molecular function. However, how MADS multimeric complexes control the dormancy cycle in trees is unclear. Apple MdDAM and other dormancy-related MADS proteins form complexes with MdSVPa, which is essential for the ability of transcriptional complexes to bind to DNA. Sequential DNA-affinity purification sequencing (seq-DAP-seq) was performed to identify the genome-wide binding sites of apple MADS TF complexes. Target genes associated with the binding sites were identified by combining seq-DAP-seq data with transcriptomics datasets obtained using a glucocorticoid receptor fusion system, and RNA-seq data related to apple dormancy. We describe a gene regulatory network (GRN) formed by MdSVPa-containing complexes, which regulate the dormancy cycle in response to environmental cues and hormonal signaling pathways. Additionally, novel molecular evidence regarding the evolutionary functional segregation between DAM and SVP proteins in the Rosaceae is presented. MdSVPa sequentially forms complexes with the MADS TFs that predominate at each dormancy phase, altering its DNA-binding specificity and, therefore, the transcriptional regulation of its target genes.

Type-B cytokinin response regulators link hormonal stimuli and molecular responses during the transition from endo- to ecodormancy in apple buds.

KEY MESSAGE: Cytokinin together with MdoBRR1, MdoBRR8 and MdoBRR10 genes participate in the downregulation of MdoDAM1, contributing to the transition from endo- to ecodormancy in apple buds. The final step of cytokinin (CK) signaling pathway culminates in the activation of type-B response regulators (BRRs), important transcriptional factors in the modulation of CK-responsive genes. In this study, we performed a genome-wide analysis aiming to identify apple BRR family members and understand their involvement in bud dormancy control. The investigation identified ten MdoBRR protein-coding genes. A higher expression of three MdoBRR (MdoBRR1, MdoBRR9 and MdoBRR10) was observed in dormant buds in comparison to other developmental stages. Interestingly, in ecodormant buds these three MdoBRR genes were upregulated in a CK-dependent manner. Transcription profiles, determined during dormancy cycle under field and artificially controlled conditions, revealed that MdoBRR1 and MdoBRR8 played important roles in the transition from endo- to ecodormancy, probably mediated by endogenous CK stimuli. The expression of MdoBRR7, MdoBRR9, and MdoBRR10 was induced in ecodormant buds exposed to warm temperatures, indicating a putative role in growth resumption after chilling requirement fulfillment. Contrasting expression patternsin vivo between MdoBRRs and MdoDAM1, an essential dormancy establishment regulator, were observed during dormancy cycle and in CK-treated buds. Thereafter, in vivo transactivation assays showed that CK stimuli combined with transient overexpression of MdoBRR1, MdoBRR8, and MdoBRR10 resulted in downregulation of the reporter gene gusA driven by the MdoDAM1 promoter. These pieces of evidences point to the integration of CK-triggered responses through MdoBRRs that are able to downregulate MdoDAM1, contributing to dormancy release in apple.

Transcriptome analyses of the Dof-like gene family in grapevine reveal its involvement in berry, flower and seed development.

The Dof (DNA-binding with one finger) protein family spans a group of plant transcription factors involved in the regulation of several functions, such as plant responses to stress, hormones and light, phytochrome signaling and seed germination. Here we describe the Dof-like gene family in grapevine (Vitis vinifera L.), which consists of 25 genes coding for Dof. An extensive in silico characterization of the VviDofL gene family was performed. Additionally, the expression of the entire gene family was assessed in 54 grapevine tissues and organs using an integrated approach with microarray (cv Corvina) and real-time PCR (cv Pinot Noir) analyses. The phylogenetic analysis comparing grapevine sequences with those of Arabidopsis, tomato, poplar and already described Dof genes in other species allowed us to identify several duplicated genes. The diversification of grapevine DofL genes during evolution likely resulted in a broader range of biological roles. Furthermore, distinct expression patterns were identified between samples analyzed, corroborating such hypothesis. Our expression results indicate that several VviDofL genes perform their functional roles mainly during flower, berry and seed development, highlighting their importance for grapevine growth and production. The identification of similar expression profiles between both approaches strongly suggests that these genes have important regulatory roles that are evolutionally conserved between grapevine cvs Corvina and Pinot Noir.