Characterization of Migration and Induced Resistance of Rhizobium Vitis Strain ARK-1, a Biological Control Agent Against Grapevine Crown Gall Disease

Abstract

A nonpathogenic strain ARK-1 of Rhizobium vitis has an antagonistic activity toward tumorigenic (Ti) strain of R. vitis, a causal agent of grapevine crown gall disease, and works as a biocontrol agent. We have demonstrated that the gall formation was fully suppressed when ARK-1 was co-inoculated with Ti into the grapevine stem at a 1:1 ratio. For practical use of ARK-1 in agriculture, understanding the temporal dynamics of the bacterial habitat on host plants and the biocontrol property are needed in order to develop proper application methods. Here we demonstrated that the gall incidence by Ti was reduced to about 50% when ARK-1 was pre-inoculated at both upper and lower positions on the grapevine stem 3 cm away from the Ti-inoculation point 5 days before. The bacterial cell detection assay in the grapevine tissue revealed that ARK-1 could migrate at least 3 cm in 5 days. Inoculations of ARK-1 or Ti induced expression of marker genes for defense-related phytohormones such as salicylic acid, jasmonic acid, and ethylene in grapevine within 3 days but they were diminished by 6 days. Inoculation of Ti 5 days after ARK-1 pre-inoculation induced expression of the marker genes except for the LOX-9 gene in a basically similar way to those without the pre-inoculation, suggesting that ARK-1 did not induce typical acquired systemic resistance or induced systemic resistance in grapevine, while the transcript of LOX-9 was detected at 24 and 48 hours after the Ti inoculation when ARK-1 was pre-inoculated, unlike the un-inoculated condition. ARK-1 primed the induction of certain defense genes and it may take part in its biocontrol activity.