Etudes Moléculaires et Génétiques de la Morphogenèse du Pétale – PetalSize

In the flowering plants, it is well established that multimeric complex formed by MADS-box transcription factors (TFs) is responsible for the specification of petal organ identity. However, how organ identity is translated into specific growth patterns to generate organs with characteristic shapes and sizes and how cell proliferation and cell expansion are regulated to attain a cell-type-specific value, remain largely unknown. Organ final size can be influenced by rates of cell proliferation or cell expansion or both. We recently identified a novel petal-expressed basic-helix-loop-helix transcription factor (BIGPETALp, BPEp; Szecsi et al., EMBO J, 25 : 3912-3920) and demonstrated for the first time that (i) its accumulation is regulated at the post-transcriptional level downstream of flower organ identity genes and that (ii) such regulation is indirect requiring the plant hormone jasmonate as signalling intermediate. We further demonstrated that BPEp specifically limits petal size by controlling post-mitotic cell expansion. BPEp is therefore part of the network that links the patterning genes to final morphogenesis. Interestingly, petals of the bpe-1 loss-of-function mutant show a modified veination pattern as compared to the wild type. It was suggested in leaves that veination pattern and organ size share common regulatory mechanisms and are controlled by auxin polar transport. We recently identified (in a yeast two hybrid screen) that BPEp interacts with a novel transcription factor of the Auxin Response Factors family (we named BiARF), thus in agreement with a putative role of auxin in petal morphogenesis. Furthermore, like for bpe-1, biarf loss-of function also exhibits petal overgrowth compared to the wild-type, thus shares similar phenotype as bpe-1. This study also suggested that BiARF may control cell proliferation at early petal development stages. BPEp and its interacting protein BiARF are therefore ideal candidates for molecular and genetic characterization of petal morphogenesis to generate petal organ with specific shape and patterns, an area for which very little information is available. Within this proposal we will address the following points: (1) In a first study, we propose to investigate the role of jasmonate pathway in the post-transcriptional regulation of BPEp (via intron retention) and its impact on cell expansion and petal growth by performing gain-of-function studies using over-expression lines as well as exogenous jasmonate treatments. To complete this study, genetic interactions between BPE and a gene (OPR3) involved in jasmonate biosynthesis will be examined. The proposed experiments will demonstrate whether jasmonate controls cell expansion and petal growth in a BPEp-dependent manner. (2) We will complete the functional characterisation of BPEp by studying the biological significance of the interaction of BPEp with BiARF. We will focus on the role of such interactions in the control of cell expansion, petal size and vein patterning. We will also address the question if and how BiARF controls cell proliferation in BPEp-independent manner at early petal development stages. Within this study, molecular and genetic interactions between BPEp and BiARF will be examined using double (bpe/biarf) and sesquimutant loss-of-function lines, as well as BPEp and/or BiARF over-expressing lines. (3) In a third study we will address whether BPEp localisation and function is influenced by BiARF and/or vice versa and (ii) whether BPEp function is influenced by auxin transport and/or concentration' The resulting data will serve to dissect the interactions between auxin, vein formation and petal organ size control. (4) The fourth task will focus on the identification and functional characterization of the target genes of BPEp and BiARF. These experiments will therefore help to unravel the network of downstream structural genes (realizators) involved in the control of petal morphogenesis. The present proposal that focuses on the functional characterisation of BPEp and its putative interacting factors will help in getting insight into the molecular characterization of morphogenesis to generate petal organ with specific shape and patterns. Furthermore, the proposed combination experiments may help understanding organ growth and development in general, an area for which very little information is available.