ROOT INFECTION AND VASCULAR COLOLIZATION PHENOTYPE ON TOMATO PLANTS OF RALSTONIA SOLANACEARUM MUTANTS ALTERED IN DIFFERENT BRP GENES. J VASSE, B BRITO and C. BOUCHER Laboratoire de Biologie Moldculaire des Relations Plantes-Nficroorganismes, -CNRS, BP27, 31326 - Castanet-Tolosan. Cedex. FRANCE. Background and objectives Ralstonia solanacearum is the causal agent of bacterial wilt of solanaceous crops and of many other economically important plants. The phenotype of the wild-type strain GNMI1000 has been described in terms of root infection and of invasion of tomato plants [1]. We have defined this soil borne bacterium as 1/ fully infective if it infects intercellular spaces of both cortical and vascular parenchyma tissues; 2/ as invasive if it penetrates into and colonizes xylem vessels and 3/ pathogenic if it induces wilt symptoms. Like other pathogenic bacteria, R. solanacearum contains a set of conserved pathogenicity genes, the hrp (hypersensitive response and pathogenicity) gene cluster, which code for a type of bacterial protein secretion system, known as the " 111 secretion system. Expression of hrp gene is induced in planta and is genetically controlled by the hrpB gene. This gene positively regulates the expression of the hrp transcription units, as well as certain loci located adjacent to the left handside of the hrp gene cluster [2]. Results and conclusions Recently, the study of the region adjacent to the right handside of thehrp gene cluster has shown that hrp gene regulation is more complex. Diflerent mutations in this region present altered Hrp phenotypes and their regulation is independent of the hrpB gene. Sequence analysis of this region has led to the characterization of two regulatory genes, hrpG and prhJ (plant regulator of hrp genes). These two genes seem necessary for hrpB expression upon co-culture with plant cell suspensions, and might be involved in the transduction of an inducer signal of plant origin (see poster BRITO et al). The tomato root infection phenotype and the corresponding vascular colonization ability of hrp mutant strains altered either in the secretion machinery (hrpO) or in different hrp regulatory genes (hrpB, hrpG and prhJ), have been studied using microscopy techniques. The microscopic observations on the root system of axenic tomato seedlings inoculated with these mutants were correlated with bacterial numeration and wilting symptoms on whole tomato plants. Our results show significant differences between the phenotypes of the mutants studied. References 1. Vasse J, Frey P, Trigalet A, 1995. NTNU 8, 241-5 1. 2. Genin S, Gough CL, Zischek C, Boucher C, 1992. Mol. Nherobiol 6, 3065-76.