Implication of C‐terminal mutation of PopA of Ralstonia solanacearum strain OE1‐1 in suppression of bacterial wilt

Ralstonia solanacearum strain OE1-1 causes bacterial wilt on tobacco plants. The popA-mutant 31b, derived from OE1-1 by insertion of transposon Tn4431, did not cause wilt on tobacco plants inoculated through the roots. However, when 31b was directly inoculated into xylem vessels, the tobacco plants wilted, similarly to those inoculated with OE1-1. 31b retained its exopolysaccharide productivity and its type-III secretion function. Furthermore, 31b grew in intercellular spaces and systemically infected tobacco plants, similarly to OE1-1. popA consists of an operon with popB and popC, and suppression of popB and popC expression resulting from polar mutation by transposon insertion did not affect the virulence of 31b. The mutated popA (popA31b) was composed of 960 nucleotides, including 39 derived from Tn4431. A recombinant mutant from OE1-1, where popA31b was introduced by marker exchange, showed the same phenotype as 31b. PopA31b protein was extracellularly secreted by 31b co-cultured with Arabidopsis thaliana. These results suggest that PopA31b extracellularly secreted by 31b in intercellular spaces may be implicated in suppression of disease development, leading to inability of the bacteria to induce wilt on plants. Taken together, interactions between host plants and R. solanacearum existing in intercellular spaces immediately after invasion may be involved in disease development.