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Appl. Environ. Microbiol. doi:10.1128/AEM.02906-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Type III secretion system of Xanthomonas fuscans subsp. fuscans is involved in phyllosphere colonization process and in transmission to seeds of susceptible bean

UMR077 PaVé, INRA, 42, rue George Morel, F-49071 Beaucouzé, France

^* To whom correspondence should be addressed. Email: Marie-Agnes.Jacques{at}angers.inra.fr.

	Abstract

Understanding the survival, multiplication and transmission to seeds of plant-pathogenic bacteria is central to study their pathogenesis. We hypothesized that the type III secretion system (T3SS), encoded by hrp genes, could have a role in host colonization by plant-pathogenic bacteria. The seed-borne pathogen Xanthomonas fuscans subsp. fuscans causes common bacterial blight of bean (Phaseolus vulgaris). Directed-mutagenesis in strain CFBP4834-R of X. fuscans subsp. fuscans and bacterial population densities monitoring on bean leaves showed that strains with mutations in the hrp regulatory genes, hrpG and hrpX, were impaired in their phyllospheric growth as in the null interaction with Escherichia coli C600 and bean. In the compatible interaction, CFBP4834-R reached high phyllospheric population densities and was transmitted to seeds at high frequencies with high densities. Strains with mutations in structural hrp genes maintained the same constant epiphytic population densities (1x10⁵ CFU g^-1 of fresh weight) as in the incompatible interaction with X. campestris pv. campestris ATCC33913 and the bean. Low frequencies of transmission to seeds and low bacterial concentrations were recorded for CFBP4834-R hrp mutants and for ATCC33913, whereas E. coli C600 was not transmitted. Moreover, unlike the wild-type strain, strains with mutations in hrp genes did not transmit to seeds by vascular pathway. Transmission to seeds by floral structures remained possible for both. This study revealed the involvement of X. fuscans subsp. fuscans T3SS in phyllospheric multiplication and systemic colonization of bean leading to transmission to seeds. Our findings suggest a major contribution for hrp regulatory genes in host colonization processes.

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