Des composants centrosomaux dans les cellules végétales acentrosomales – CentroPlant

As autotrophic and sessile terrestrial organisms, land plants are unique in many aspects of their physiology, multicellular development and cellular organization. The plant cytoskeleton (microtubules and actin microfilaments) assembles several spatially independent arrays unique to plant cells, which follow one another during the cell cycle and differentiation. The cytoskeleton is involved in cell division, elongation, wall deposition and differentiation. It also plays a central role in many responses to endogenous factors like hormones, and to biotic and abiotic stresses. Unlike most other eukaryotes, cells of land plants lack a conspicuous MTOC like a centrosome, except basal bodies formed briefly in flagellate motile sperm cells of lower land plants. The absence of centrosomes and centrioles and the mechanisms of microtubule nucleation in higher plant cells have prompted many questions and hypotheses during the last decades. Indeed, very little information is available as to whether plant cells have retained/reorganized/reinvented functions associated with MTOCs in other eucaryotes. Our group initially focused on the fass and ton mutants of Arabidopsis, the only plant mutants unable to form the preprophase band of microtubule, a structure unique to embryophytes from mosses to angiosperms, and involved in the determination of division planes at G2/M transition. This defect correlates with defective phragmoplast guidance and abnormal positioning of the cell plate between daughter cells. These mutants allow to tackle essential questions in plant cell biology, like the role of the cytoskeleton in division and elongation and more generally the importance of these cell components in morphogenesis and organogenesis in land plants. Recent results obtained in our group on the characterization of the TON, FASS proteins and their partners uncovered converging evidences pointing towards evolutionary (and functional ?) conservation of some centrosomal components at the plant cell's cortex. TON and FASS themselves are related to proteins recently shown to localize at the centrosome in animal cells. Moreover, TON interacts with centrin, a key component of eucaryotic MTOCs. Finally, several TON interactants belong to a new family of cytoskeleton-associated proteins (TIMs) which are reminiscent of docking proteins involved in MT anchoring in animal centrosomes. These are the first indications ever for a functional and evolutionary link between animal centrosomes and the cortical cytoskeleton of plants, apart from gamma-tubulin dependent nucleation. This molecular network of interactions and recruitments, in part or totality, may stem back to the roots of the eucaryotic tree, and represent ancient centrosomal functions that were kept through the evolutionary divergence between plants and animals and reallocated from the centrosome to the cortex. The aim of this project is to capitalize on these original findings to address ancient questions about the distributed nature of the plant's centrosome, by focalising on the following points : - the functional characterization of TON, including its precise localization, partners, dimerization and potential phosphorylation with regard to localization and function of the TON, TIM and FASS proteins, and their roles in driving formation of cortical MT arrays (WP1) - the functional characterization of the TIM family of proteins, and of their other interacting partners in the cell from a cytological, molecular genetics and biochemical point of view. Part of the work will focus on TIM1, which was recently shown to target TON to the cytoskeleton, including biochemical and mutational analysis of the TIM1 subfamily. In another task, the subcellular localization of selected members of the TIM family (31 members in Arabidopsis) will be assayed in order to address the functional significance of this diverse protein family (WP2) - finally, a detailed bioanalysis work will take advantage of a series of recent works on the characterization of centrosomal and flagellar proteomes in various species to address whether centrosomal motifs/regions/proteins can be detected in available plant genomes (WP3).