Mode of action of the GALA type III effectors that enable Ralstonia solanacearum to subvert Arabidopsis UBIquitin/proteasome system to promote bacterial wilt – GALA-RUBI

In Gram negative plant pathogenic bacteria hrp genes are essential for the establishment of plant disease. These genes encode a protein secretion system that governs injection of bacterial proteins into plant cells. These proteins, known as effectors, are the primary determinants for plant diseases. Analysis of the genome sequence of R. solanacearum allowed us to identify a repertoire of over 80 effectors in this organism. The present project aims to the characterisation of the mode of action as well as the identification of the corresponding target proteins in Arabidopsis of a particular family of effectors: the GALA proteins. There are seven GALA genes in R. solanacearum. We have shown that among the seven GALAs, GALA6 and GALA7 are required for R. solanacearum to promote disease on Arabidopsis. GALAs have an F-box domain and LRR, a structure which is reminiscent of the plant F-box proteins. In Arabidopsis, these proteins together with other subunits form the SCF-type E3 ubiquitin ligase. This type of enzyme is a key step in the ubiquitinate/proteasome specific protein degradation since this E3-ligase will specifically add ubiquitin moieties to target proteins. We propose to study the targets recognized and potentially ubiquitinated and degraded in the plant cell by GALA6 and GALA7. A candidate target, potentially involved in plant innate immunity, identified by yeast-two hybrid, will be tested for GALA6/7-dependent in vitro ubiquitination. By identifying the ubiquitinated targets, we will understand how GALA6 and GALA7 participate in the control of R. solanacearum's pathogenicity. We aim at providing both the knowledge for future new crop plant resistance strategies and a framework for the understanding of the mode of action of a type effector present in a diverse range of pathogenic bacteria pathogenic to plants or to humans ...