The Type VI secretion systemof Stenotrophomonas rhizophilaCFBP13503 limits the transmission ofXanthomonas campestrispv. campestris8004 from radish seeds to seedlings

Abstract

ABSTRACTStenotrophomonas rhizophilaCFBP13503 is a seed-borne commensal bacterial strain, which is efficiently transmitted to seedlings and can outcompete the phytopathogenic bacteriaXanthomonas campestrispv.campestris(Xcc8004). The type VI Secretion System (T6SS), an interference contact-dependent mechanism, is a critical component of interbacterial competition. The involvement of the T6SS ofS. rhizophilaCFBP13503 in the inhibition of Xcc8004 growth and seed-to-seedling transmission was assessed. The T6SS cluster ofS. rhizophilaCFBP13503 and nine putative effectors were identified. Deletion of two T6SS structural genes,hcpandtssB, abolished the competitive advantage ofS. rhizophilaagainst Xcc8004 invitro. The population sizes of these two bacterial species were monitored in seedlings after inoculation of radish seeds with mixtures of Xcc8004 and eitherS. rhizophilawild type (wt) strain or isogenichcpmutant. A significant decrease in the population size of Xcc8004 was observed during confrontation with theS. rhizophilawt in comparison to T6SS- deletion mutants in germinated seeds and seedlings. We found that the T6SS distribution among 835 genomes of theStenotrophomonas genus is scarce. In contrast, in all availableS. rhizophilagenomes, T6SS clusters are widespread and mainly belong to the T6SS group i4. In conclusion, the T6SS of S.rhizophilaCFBP13503 is involved in the antibiosis against Xcc8004 and reduces seedling transmission of Xcc8004 in radish. The distribution of this T6SS cluster in theS. rhizophilacomplex could make it possible to exploit these strains as biocontrol agents againstX. campestrispv.campestris.