Plant associations by bradyrhizobia have been detected not only in leguminous plants, but also in non-leguminous species including rice.
Bradyrhizobium sp. SUTN9-2 was isolated from
Aeschynomene americana L., which is a leguminous weed found in the rice fields of Thailand. This strain promoted the highest total rice (
Oryza sativa L. cultivar Pathum Thani 1) dry weight among the endophytic bradyrhizobial strains tested, and was, thus, employed for the further characterization of rice-
Bradyrhizobium interactions. Some known bacterial genes involved in bacteria-plant interactions were selected. The expression of the type III secretion component (
rhcJ), type IV secretion component (
virD4), and pectinesterase (
peces) genes of the bacterium were up-regulated when the rice root exudate was added to the culture. When SUTN9-2 was inoculated into rice seedlings, the
peces,
rhcJ,
virD4, and exopolysaccharide production (
fliP) genes were strongly expressed in the bacterium 6–24 h after the inoculation. The gene for glutathione-
S-transferase (
gst) was slightly expressed 12 h after the inoculation. In order to determine whether type III secretion system (T3SS) is involved in bradyrhizobial infections in rice plants, wild-type SUTN9-2 and T3SS mutant strains were inoculated into the original host plant (
A. americana) and a rice plant (cultivar Pathum Thani 1). The ability of T3SS mutants to invade rice tissues was weaker than that of the wild-type strain; however, their phenotypes in
A. americana were not changed by T3SS mutations. These results suggest that T3SS is one of the important determinants modulating rice infection; however, type IV secretion system and
peces may also be responsible for the early steps of rice infection.
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