Effects of a biocontrol bacterium on growth and defence of transgenic rice plants expressing a bacterial type-III effector

Expression of HpaG(Xoo), a bacterial type-III effector protein, in transgenic plants induces disease resistance. Resistance also can be elicited by biocontrol bacteria. We studied effects of the biocontrol Bacillus subtilis strain B-916 on the rice variety R109 and the the hpaG(Xoo)- expressing rice line HER1. Colonisation of roots by B-916 caused 12.5 +/- 1.3% and 0.5 +/- 0.05% increases, in contrast to controls, in root growth of R109 and HER1. Growth of R109 leaves and stems was increased by 0.5 0.05% but that of HER1 was inhibited. When B-916 colonisation was subsequent to plant inoculation with Rhizoctonia solani, a pathogen that causes sheath blight, the disease was less severe than controls in both R109 and HER1; HER1, nevertheless, was more resistant, suggesting that B-916 and HpaG(Xoo) cooperate in inducing disease resistance. In R109 roots, the OsARF1 gene, which regulates plant growth, was expressed in consistence with growth promotion by B-916. Inversely, the depression of OsARF1 expression was coincident with inhibition in growth of HER1 leaves and stems. In both plants, the expression of OsEXP1, which encodes an expansin protein involved in plant growth, was concomitant with growth promotion in leaves and roots responding to B-916. We also studied OsMAPK5b encoding a mitogen-activated protein kinase involved in multiple defence responses in rice. In response to B-916, early expression of OsMAPK5b was coincident with R109 resistance to the disease, while HER1 expressed the gene similarly whether B-916 was present or not. Evidently, B916 and HER1 interact differently in rice growth and resistance. The combinative effects should stimulate agricultural use and further studies on mechanisms that underlie the interaction.