Biological control agent Rhizobium (=Agrobacterium) vitis strain ARK-1 suppresses expression of the essential and non-essential vir genes of tumorigenic R. vitis

To gain insights into the virulence suppressive mechanism of a nonpathogenic strain of Rhizobium vitis ARK-1, we co-inoculated ARK-1 with a tumorigenic (Ti) strain of R. vitis to examine the expression of two essential virulence genes (virA and virG) and one non-essential gene (virD3) of the Ti strain at the wound site of grapevine. Co-inoculation of ARK-1 with a Ti strain VAT03-9 at a 1:1 cell ratio into grapevine shoots resulted in significantly lower expression of the virulence genes virA, virD3, and virG of VAT03-9 at 1 day after inoculation compared with those when shoots were inoculated only with VAT03-9. ARK-1 was not able to catabolize acetosyringone, which is the plant-derived metabolites inducing the entire vir regulon in Ti strains, suggesting the direct effect of ARK-1 on the induction of broad range of vir genes of R. vitis Ti strains.