Role of type III secretion system and adhesins in the fitness of Xanthomonas fuscans subsp. fuscans in bean phyllosphere and in transmission to seeds
Résumé
Deciphering the mechanisms enabling plant pathogenic bacteria to disperse, colonize and survive on their hosts provides the necessary basis to set up new control methods. We evaluated the role of the type III secretion system (T3SS) and adhesins in two steps of the asymptomatic host colonization process: phyllospheric colonization and transmission to seeds. Xanthomonas fuscans subsp. fuscans is responsible for the common bacterial blight of bean, a seedborne disease. Unlike the wild-type X. fuscans subsp. fuscans, strains with mutations in T3SS regulatory genes were impaired in their phyllospheric growth as was Escherichia coli on bean. Strains with mutations in the hrp structural genes maintained the same constant epiphytic population densities as did X. campestris pv. campestris on bean in a non-host interaction. Among the five adhesins identified in X. fuscans subsp. fuscans, only the non-polar adhesin YapH was required for adhesion on leaves. Transmission to seeds by the vascular pathway was abolished for mutants in T3SS regulatory and structural genes, and remained possible but altered, for mutants in adhesin genes, except for mutant in yapH which behaved as the wild-type strain. Transmission to seeds by floral structures did not require any of the known adhesins and remained possible but with a low efficiency for hrp mutants and was repeatedly recorded for a non-host pathogen (X. campestris pv. campestris). E. coli did no transmit to bean seed. In conclusion, we showed that T3SS and bacterial adhesins are implicated in the various processes leading to host phyllosphere colonization and systemic transmission to seeds in the absence of symptoms in compatible interactions.