Thierry Gaude

Dr Thierry Gaude established his group at ENS Lyon in 1993. The main scientific interest of his group was to decipher the molecular bases of the self-incompatibility response (SI) in the Brassicaceae (cabbage). SI prevents self-fertilization and hence inbreeding by allowing the pistil to recognize and reject self-pollen grains. The recognition step between the female and male partners relies on a receptor kinase-ligand interaction, where the receptor is localized to the plasma membrane of the stigma epidermal cells (stigma papillae), whereas the peptidic ligand is released from the pollen wall surface. Dr Gaude's team first reported that the receptor kinase could autosphosphorylate in vivo and that it was always associated with other stigma proteins to form a receptor complex (Giranton et al., PNAS 2000). They also showed that self-pollination led to the phosphorylation of the receptor in planta, and that the basal inactive level of the receptor was maintained by the activity of a stigmatic thioredoxin (Cabrillac et al., Nature 2001). This work was the first demonstration that a plant receptor kinase was directly phosphorylated (activated) following perception of its ligand. For all his scientific achievements in the SI field, T. Gaude was awarded laureate of the"Leconte Prize" by the French Academy of Sciences in 2001. T. Gaude then pursued the identification of stigma components involved in the signal transduction pathway leading to SI response. By a yeast two-hybrid screen using the cytosolic kinase domain of the receptor as a bait, they identified the first plant sorting nexin (designated SNX1) as a possible modulator of the SI receptor (Vanoosthuyse et al., Plant Physiology 2003). SNXs are cytosolic and endosomal-associated membrane proteins that are involved in endocytic pathways in yeast and mammals. The putative role of SNX1 in the regulation of plant receptor kinases and the fact that there are only three SNXs in Arabidopsis thaliana led T. Gaude to investigate the functions of SNXs in plants. This new scientific orientation started in 2003 and aims at understanding the role of intracellular trafficking of membrane proteins in plant development. To investigate the function of SNX1 in A. thaliana, the group of Lyon analyzed first several SNX1 loss-of-function mutant alleles. They found these to exhibit a semi-dwarf phenotype and mutant roots to be affected in sensing gravity (agravitropic phenotype). They showed that these defects were linked to an abnormal distribution of the plant hormone auxin, a key hormone in plant growth and development. Transport of auxin from cell-to-cell is facilitated by a variety of plasma membrane proteins, known as auxin carriers. Any deregulation in auxin flux, notably due to mislocalization of auxin carriers, leads to growth or developmental defects. The group of T. Gaude recently identified an endosomal compartment in plant cells, where SNX1 is located. They found that these SNX1 endosomes are involved in the trafficking of one of the auxin carriers, the PIN2 protein (Jaillais et al., Nature 2006). Besides, the analysis of various plasma membrane proteins that pass through SNX1 endosomes during their intracellular trafficking led to the conclusion that this endosomal compartment is at the crossroad between the secretory and endocytic pathways, and hence constitutes the sorting endosome in root cells (Jaillais et al., Plant Journal 2007). Auxin carriers of the PIN family (auxin efflux carriers) are often polarized at one side of the cell. During certain developmental processes, such as embryo development or initiation of new organs, PIN polarity is modified, switching for instance from the basal to lateral side of the cell. This allows redirection of auxin flux. The molecular machinery involved in the repolarization of PIN proteins remains largely unknown to date. T. Gaude's team has recently discovered one of the components of this machinery (Jaillais et al., Cell 2007). This is a protein conserved from yeast to man, which interacts with SNX proteins, and was mainly known hitherto to be involved in protein trafficking between endosomes to the trans-Golgi network (TGN). This protein, named Vacuolar Protein Sorting 29 (VPS29), is required for proper embryo development and formation of lateral roots. Using confocal microscopy and transgenic lines stably expressing fluorescent (GFP)-tagged PIN proteins, they showed that VPS29 is involved in PIN recycling to the plasma membrane and in PIN relocalization during initiation of lateral roots. This work reveals an unexpected function for VPS29 in modulating cell polarity and opens the possibility that a similar function might be conserved for the human form of VPS29. Altogether, the work of T. Gaude's group highlights the major role played by endocytosis and intracellular trafficking in the control of developmental processes in plants.