Functional Divergence of the Arabidopsis Florigen-Interacting bZIP Transcription Factors FD and FDP.

Flowering of many plant species depends on interactions between basic leucine zipper (bZIP) transcription factors and systemically transported florigen proteins. Members of the genus Arabidopsis contain two of these bZIPs, FD and FDP, which we show have largely complementary expression patterns in shoot apices before and during flowering. CRISPR-Cas9-induced null mutants for FDP flower slightly earlier than wild-type, whereas fd mutants are late flowering. Identical G-box sequences are enriched at FD and FDP binding sites, but only FD binds to genes involved in flowering and only fd alters their transcription. However, both proteins bind to genes involved in responses to the phytohormone abscisic acid (ABA), which controls developmental and stress responses. Many of these genes are differentially expressed in both fd and fdp mutant seedlings, which also show reduced ABA sensitivity. Thus, florigen-interacting bZIPs have distinct functions in flowering dependent on their expression patterns and, at earlier stages in development, play common roles in phytohormone signaling

Vision, challenges and opportunities for a Plant Cell Atlas

With growing populations and pressing environmental problems, future economies will be increasingly plant-based. Now is the time to reimagine plant science as a critical component of fundamental science, agriculture, environmental stewardship, energy, technology and healthcare. This effort requires a conceptual and technological framework to identify and map all cell types, and to comprehensively annotate the localization and organization of molecules at cellular and tissue levels. This framework, called the Plant Cell Atlas (PCA), will be critical for understanding and engineering plant development, physiology and environmental responses. A workshop was convened to discuss the purpose and utility of such an initiative, resulting in a roadmap that acknowledges the current knowledge gaps and technical challenges, and underscores how the PCA initiative can help to overcome them.</jats:p

The WOX13 homeobox gene promotes replum formation in the Arabidopsis thaliana fruit

The Arabidopsis fruit forms a seedpod that develops from the fertilized gynoecium. It is mainly comprised of an ovary in which three distinct tissues can be differentiated: the valves, the valve margins and the replum. Separation of cells at the valve margin allows for the valves to detach from the replum and thus dispersal of the seeds. Valves and valve margins are located in lateral positions whereas the replum is positioned medially and retains meristematic properties resembling the shoot apical meristem (SAM). Members of the WUSCHEL-related homeobox family have been involved in stem cell maintenance in the SAM, and within this family, we found that WOX13 is expressed mainly in meristematic tissues including the replum. We also show that wox13 loss-of-function mutations reduce replum size and enhance the phenotypes of mutants affected in the replum identity gene RPL. Conversely, misexpression of WOX13 produces, independently from BP and RPL, an oversized replum and valve defects that closely resemble those of mutants in JAG/FIL activity genes. Our results suggest that WOX13 promotes replum development by likely preventing the activity of the JAG/FIL genes in medial tissues. This regulation seems to play a role in establishing the gradient of JAG/FIL activity along the medio-lateral axis of the fruit critical for proper patterning. Our data have allowed us to incorporate the role of WOX13 into the regulatory network that orchestrates fruit patterning.This work was supported by the Spanish Ministry of Science grants BFU2006-00771 and BFU2009-08325 to S.P., and by a National Science Foundation to M.F.Y(grant IOS-1121055). J.J.R. was recipient of a fellowship from the Generalitat Valenciana (BPOSTDOC06/060). S.P.'s group was recognized as consolidated by the Catalonia Government (2009 SGR 697) and the CRAG is supported by the CONSOLIDER Program (CSD2007-00036).Peer reviewe

FLOWERING LOCUS T interacts with a cell cycle regulator that affects growth and development in Arabidopsis thaliana

International audiencePhosphatidylethanolamine-binding proteins (PEBP) play key roles in the regulation of plant growth and development. PEBPs are present in all plant species and regulate crucial biological processes such as flowering, tuberization, vegetative growth and plant architecture. Despite their importance, complete understanding of the biochemical functions of PEBP proteins remains elusive. In terms of flowering, PEBP proteins closely related to the Arabidopsis thaliana FLOWERING LOCUS T (FT) act as floral activators, whereas another group more similar to TERMINAL FLOWER1 (TFL1) has the opposite function acting as inhibitors of flowering. Recent studies suggested that this antagonistic activity of PEBPs is likely conferred by unknown proteins that specifically interact with FT and TFL1. Our yeast-two-hybrid and co-immunoprecipitation experiments showed that FT interacts with ANAPHASE PROMOTING COMPLEX 8 (APC8), a sub-unit of an E3 ubiquitin ligase complex (APC/C) that plays a major role in the progression of the eukaryotic cell cycle. APC/C targets specific proteins for destruction via 26S proteasome based on the presence of short conserved amino acid motifs known as D-box. We found that a predicted D-box motif in FT sequence is essential for its interaction with APC8, suggesting that FT is a substrate of the APC/C complex. However, TFL1-like proteins do not contain a D-box motif and do not interact with APC8. Furthermore, we constructed chimeric proteins and demonstrated that critical amino acids that distinguish between FT and TFL1 functions are important for the selectivity of the interaction with APC8. To investigate the connection concerning APC8 activity and FT function, we studied their genetic interaction. A weak allele of APC8 (apc8-1) showed pleiotropic phenotypes, including distorted leaf shapes, abnormal shoot meristem development, reduced main shoot elongation and delayed vegetative to reproductive phase transition. Interestingly, the double mutant ft-10 apc8-1 displayed a more severe phenotype than either of the single mutants as it was not able to flower under long day conditions. The same phenotype was observed in apc8-1 plants grown under short days. By contrast, overexpression of FT accelerated flowering of apc8-1 mutant and partially restored the capacity of the main shoot to elongate after floral transition. These observations indicate that loss-of-function and overexpression of FT enhances or partially overrides the phenotype of the apc8-1 mutant, respectively. Together, these data suggest an unexpected role of FT in the control of plant growth and development by interacting with cell cycle regulators. Current studies are being performed in order to clarify the molecular mechanism underlying the activity of the FT-APC8 complex in A. thaliana

Repression of the FT-signaling pathway mediated by organ patterning genes

Repression of the FT-signaling pathway mediated by organ patterning genes. Workshop on molecular mechanisms controlling flower developmen

The SVP transcription factor coordinates GA biosynthesis with floral induction during photoperiodic flowering

The timing of flowering is crucial in determining the final production of seeds and fruits. In plants the transition from vegetative growth to flowering is regulated by several environmental stimuli and by the age of the individual. This complexity is conferred by a network of genetic pathways that has been characterized best in the model species Arabidopsis thaliana. FLOWERING LOCUS T (FT) is a positive regulator of flowering which participates in different genetic pathways, such as vernalisation, thermosensory, gibberellin (GA) and potoperiod. In Arabidopsis, FT is induced by long days and has been placed at the core of the photoperiodic pathway, downstream of the GIGANTEA (GI) and CONSTANS (CO) genes. Under LDs, FT protein is transported from the leaves to the shoot apical meristem (SAM) where it induces the expression of several floral promoter genes (reviewed in Andrés and Coupland (2012)). Interestingly, recent studies have shown that GA also induces flowering at the SAM under LDs and plays an important role in the FT-mediated transcriptional activation of floral promoter genes. However, how the photoperiod and the GA signaling pathways are coordinated at the SAM to promote flowering is still unclear. The Arabidopsis MADS-box gene SHORT VEGETATIVE PHASE (SVP) is a well-known floral repressor which mRNA expression is reduced in the SAM by the action of the photoperiod signals, mainly represented by FT protein. We have performed ChIP-seq and expression analysis which resulted in the identification of genes and pathways regulated by SVP (Gregis et al., 2013). These analyses revealed an unexpected role of SVP in regulating components of different hormonal biosynthetic and signaling pathways, such as cytokinin and GA pathways. Remarkably, we found that the expression of GA20ox2, a gene encoding a rate-limiting enzyme in GA biosynthesis, is repressed by SVP. Up-regulation of GA20ox2 mRNA in the svp-41 mutant leads to elevated levels of GA that correlate with GA-related phenotypes such as early flowering and organ elongation (Andrés et al, submitted). Taking our results together, we propose that inductive LD conditions contribute to the reduction of SVP expression in the shoot apex which in turn, allows the de-repression of the gibberellin pathway and therefore accelerates the floral transition process

Floral Induction in Arabidopsis by FLOWERING LOCUS T Requires Direct Repression of BLADE-ON-PETIOLE Genes by the Homeodomain Protein PENNYWISE

Flowers form on the flanks of the shoot apical meristem (SAM) in response to environmental and endogenous cues. In Arabidopsis (Arabidopsis thaliana), the photoperiodic pathway acts through FLOWERING LOCUS T (FT) to promote floral induction in response to day length. A complex between FT and the basic leucine-zipper transcription factor FD is proposed to form in the SAM, leading to activation of APETALA1 and LEAFY and thereby promoting floral meristem identity. We identified mutations that suppress FT function and recovered a new allele of the homeodomain transcription factor PENNYWISE (PNY). Genetic and molecular analyses showed that ectopic expression of BLADE-ON-PETIOLE1 (BOP1) and BOP2, which encode transcriptional coactivators, in the SAM during vegetative development, confers the late flowering of pny mutants. In wild-type plants, BOP1 and BOP2 are expressed in lateral organs close to boundaries of the SAM, whereas in pny mutants, their expression occurs in the SAM. This ectopic expression lowers FD mRNA levels, reducing responsiveness to FT and impairing activation of APETALA1 and LEAFY. We show that PNY binds to the promoters of BOP1 and BOP2, repressing their transcription. These results demonstrate a direct role for PNY in defining the spatial expression patterns of boundary genes and the significance of this process for floral induction by FT

Short vegetative phase reduces gibberellin biosynthesis at the arabidopsis shoot apex to regulate the floral transition

In Arabidopsis thaliana environmental and endogenous cues promote flowering by activating expression of a small number of integrator genes. The MADS box transcription factor SHORT VEGETATIVE PHASE (SVP) is a critical inhibitor of flowering that directly represses transcription of these genes. However, we show by genetic analysis that the effect of SVP cannot be fully explained by repressing known floral integrator genes. To identify additional SVP functions, we analyzed genome-wide transcriptome data and show that GIBBERELLIN 20 OXIDASE 2, which encodes an enzyme required for biosynthesis of the growth regulator gibberellin (GA), is upregulated in svp mutants. GA is known to promote flowering, and we find that svp mutants contain elevated levels of GA that correlate with GA-related phenotypes such as early flowering and organ elongation. The ga20ox2 mutation suppresses the elevated GA levels and partially suppresses the growth and early flowering phenotypes of svp mutants. In wild-type plants, SVP expression in the shoot apical meristem falls when plants are exposed to photoperiods that induce flowering, and this correlates with increased expression of GA20ox2. Mutations that impair the photoperiodic flowering pathway prevent this downregulation of SVP and the strong increase in expression of GA20ox2. We conclude that SVP delays flowering by repressing GA biosynthesis as well as integrator gene expression and that, in response to inductive photoperiods, repression of SVP contributes to the rise in GA at the shoot apex, promoting rapid induction of flowering.Fil: Andrés, Fernando. Max Planck Institute for Plant Breeding Research; AlemaniaFil: Porri, Aimone. Max Planck Institute for Plant Breeding Research; AlemaniaFil: Torti, Stefano. Max Planck Institute for Plant Breeding Research; AlemaniaFil: Mateos, Julieta Lisa. Max Planck Institute for Plant Breeding Research; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Bioquimicas de Buenos Aires; Argentina. Fundación Instituto Leloir; ArgentinaFil: Romera Branchat, Maida. Max Planck Institute for Plant Breeding Research; AlemaniaFil: García Martínez, José Luis. Universidad Politécnica de Valencia. Instituto de Biología Molecular y Celular de Plantas; EspañaFil: Fornara, Fabio. Università degli Studi di Milano. Department of Bioscience; Italia. Max Planck Institute for Plant Breeding Research; AlemaniaFil: Gregis, Veronica. Università degli Studi di Milano. Department of Bioscience; ItaliaFil: Kater, Martin M.. Università degli Studi di Milano. Department of Bioscience; ItaliaFil: Coupland, George. Max Planck Institute for Plant Breeding Research; Alemani