Résumé
Plants display considerable developmental plasticity in response to changing environmental conditions. The adaptations of the root system to variations of growth conditions are an excellent model to study developmental plasticity. In an effort to identify mutants altered in root perception to salt stress, we screened an activation tagging collection (Weigel et al.2000) and identified the lasso1mutant that displays a strong agravitropic root growth on standard medium. Interestingly, the lasso1mutant phenotype is partially reverted by salt and osmotic stress (sorbitol) application. The aux1mutant, harboring a mutation in the auxin influx transporter AUX1 (Bennett et al.1996), displays a strikingly similar root agravitropic phenotype. We demonstrated that (i) lasso1is not allelic to aux1, (ii) theAUX1protein localization is not altered in lasso1,and (iii) neither salt nor osmotic stress alter aux1agravitropic root growth. We thus make the assumptionthat different molecular mechanisms are responsible for the phenotypic behavior of lasso1and aux1. Auxin transport is at the core of root gravitropic response, so we analyzed the perturbation of the auxin response marker DR5::GFP in the root tips of lasso1. Our results suggest a strong accumulation of auxin in the root columella and lateral root cap and we are now focusing on the auxin efflux transporters from the PIN family. Segregation studies have demonstrated that the lasso1 phenotype is independent of the activation tagging T-DNA and therefore the sequencing oflasso1genome is in progress to search for the origin of the mutation. This study will help us to identify new molecular mechanisms involved in the perception and integration by roots of their environment (gravity, salt, water).