Génétique d’une voie limitant la croissance racinaire d’Arabidopsis en réponse à la carence phosphatée – CHEMIGENA

Because of the current and predicted environmental problems, the agronomic conditions for crop production are going to change in a near future. In particular, the yields of future crops must satisfy human needs, but with reduced supplies of fertilizers. Thus, the yields will have to be maintained although the adverse agronomic conditions resulting from reduced inputs of fertilizers. Based on our work with Arabidopsis, our view on the consequences of mineral starvation on plant growth has dramatically changed (Nature Genetics, 2007). The sensing of a low-mineral medium by the root tip seems to trigger a signaling pathway ending with an arrest of the primary root growth, before reduction of the metabolic the activity. Our knowledge about this pathway is very weak and only few mutants have been isolated so far. Until today, the power of classical genetics has been modestly exploited whereas chemical genetics has not been used yet for dissecting this biological process. We propose to dissect this transduction pathway by combining classical and chemical genetics. The effect of low-Pi on root architecture is known in crops since decades and in Arabidopsis since few years. In our lab, the working system since the last five years is the Arabidopsis thaliana primary root response to low-Pi. Our project contains two complementary and synergetic parts: the screening and study of mutants altered in this primary root response to low-Pi, and the screening of drugs (as well as their protein targets) interfering with this response. Part1) After an EMS (ethylmethane sulfonate) or gamma rays mutagenesis of WT seeds, we will isolate mutants insensitive to the primary root growth inhibition induced by low-Pi in vitro. The genetic and phenotypic analysis of these mutants will allow us to define complementation groups and to order the corresponding functions. The drugs identified in Part2 will help characterizing these mutants and functions. Two locus particularly interesting (i.e. mutations altering specifically the low-Pi response, several alleles, response altered by a drug, etc) will be chosen in order to identify the corresponding genes by positional cloning. The biological function of these genes will be analyzed. Part2) We will screen a library of several thousands of small organic and bioactive molecules (already available in our lab in 96-wells plates) in order to find those that trigger the low-Pi root response when seedlings are grown in high-Pi (screen 1) and those that repress the low-Pi root response (screen 2). These two primary screens are based on two criterions: a root-specific reporter gene of Pi starvation and the root growth. We have already started the screens and have identified several drugs (unpublished results). After these primary screens, we will test the drugs specificity toward the low-Pi response with additional assays. One crucial test for analyse the drugs effects will be their interference with mutations isolated in Part1. All these experiments will help to order functions altered by the mutations and/or by drugs (epistasis, suppression, etc). They will allow us also to identify hypersensitive or resistant mutants to these drugs. In case no such mutants are identified, we will screen specifically for them in our Arabidopsis EMS-mutagenesis. As in Part1, these last mutants will be analyzed in order to define complementation and phenotypic groups (in particular their response to low-Pi). Two locus will be chosen for positional cloning of the corresponding genes. Their function will be studied, in particular the effect of the corresponding drugs. This project is largely based on Arabidopsis genetics for which our lab has a long experience. It also relies on chemical genetics, a new methodology not yet used on plants, in France.