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POSTERS: Biological control

Biological control agent, Rhizobium vitis strain ARK-1 suppresses expression of the essential and non-essential vir genes of tumorigenic R. vitis
Akira Kawaguchi - National Agricultural and Food Research Organization. Mizuho Nita- Virginia Polytechnic Institute and State University, Yoshiteru Noutoshi- Okayama University, Megumi Watanabe- Okayama University, Tomoya Ishii- Okayama University

Crown gall of grapevine caused by tumorigenic (Ti) Rhizobium (=Agrobacterium) vitis

is the most important bacterial disease of grapevine throughout the world. Recently, a nonpathogenic R. vitis strain ARK-1 was identified as a potential biological control agent. By soaking grapevine seedling roots with a cell suspension of ARK-1 prior to planting in the field, ARK-1 treatment significantly (P < 0.05) reduced the number of plants with crown gall symptoms. To gain insights into the virulence suppression mechanism of ARK-1, we co-inoculated potted grapevine shoots with ARK-1 and a Ti strain of R. vitis at a 1:1 ratio, and examined the expression of four essential virulence genes (virA, virD2, virE2, and virG) and one non-essential gene (virD3) of the Ti strain at the wound site. The expression level of the five tested virulence genes in grapevines treated with ARK-1 and a Ti at one day after inoculation was significantly lower (P < 0.05) than that of Ti-only treatment. We also demonstrated the lack of ability by ARK-1 to catabolize a plant-derived metabolite acetosyringone, which induces the entire vir regulon in Ti strains. These results suggest that the reduction in symptom development by ARK-1 treatment may be due to a direct interaction between ARK-1 and Ti strains that affects the expression of a broad range of vir genes.