AGGRESSIVENESS OF RALSTONIA SOLANACEARUM RACE 1 AND CORRELATION WITH PATHOGEN GENOMIC VARIATION.
Darrasse A1., Trigalet A2., Prior P1.
1
2 Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, CNRS-INRA, BP27, 31326, Castanet-Tolosan Cedex, France.
Bacterial wilt caused by Ralstonia solanacearum is widely distributed in the world. It is recognized as one of the most destructive plant diseases, affecting a wide range of valuable cash crops and key food crops. Recently, there have been comprehensive reports on the disease and its causal agent. For a number of years, many protocols have been applied to classify these bacterial populations including biovar and races typings. More recently, molecular techniques have allowed a better undestanding of these groupings. One of them, the repetitive sequence polymerase chain reaction (rep-PCR) technique, appears to be a rapid, simple, and reliable method to identify and to classify bacterial strains. Genomic fingerprint analysis has already been used to identify biogeographic trends and bacteriocin typing was reported to be correlated with biovars (Frey et al., 1996). For breeding and epidemiological purposes, it is very important to analyse the variability of aggressiveness. Different studies have reported a link between aggressiveness and biovar. Our investigation focussed on the correlation between genomic profiles and agressiveness on tomato within R. solanacearum race 1 strains. Aggressiveness was assessed, in two separate experiments, by using colonization index at midstem site (wilt + latent infections) in each strain x cultivar combination, namely Floradel, Caracoli and Caraïbo which are considered susceptible, moderatly resistant and resistant to bacterial wilt, respectively (Prior et al., 1996).
Two bacterial collections differing in geographical and host origin were surveyed. First, a local population was collected from a single experimental plots (plot 26A) consisting of 24 undistiguishable representatives isolates of biovar 1. These were compared to 11 reference strains (6 West Indian and 5 international). Secondly, a collection of race 1 strains (Bvs 1, 3 and 4) including 15 international strains, 11 West Indian reference strains and 19 of the plot’s 26 A isolates. From the local population, clustering of rep-PCR, which was previously reported to correlated both with biovar and bacteriocin typing (Frey et al., 1996), was also closely correlated with aggressiveness on tomato. None of the rep-PCR profile obtain from reference strains correlate with aggressivenes.
From the international collection, clustering of REP, ERIC and BOX-PCR were reliable as discriminatory method for biovar typing. Factorial correspondence analysis was run with colonization index data resulting from each strain x cultivar x experiment combination. Strains were ranked into five aggressiveness patterns differing in expected behaviour with regard to the susceptibility of tomato genotypes, and environmental conditions during experimentation.
Applying clustering rep-PCR to characterize aggressiveness for epidemiological and breeding goals is discussed at both the local and the international level.
