Abstract
In rice, the small GTP binding protein OsRac1 is thought to play a key role in defense signaling by regulating NADPH oxidase, which generates reactive oxygen species (ROS) during oxidative burst. The expression of a metallothionein, OsMT4, an ROS scavenger, was reduced in rice cultured cells that overexpress the constitutive active form of OsRac1. Transgenic plants that overexpress OsMT4 showed increased susceptibility to bacterial blight and blast fungus. Furthermore, OsMT4-overexpressing cells showed reduced elicitor-induced hydrogen peroxide (H2O2) production. In contrast, Tos17::OsMT4 insertional mutant and RNAi-OsMT4 transgenic cells, which were deficient in OsMT4 expression, showed increased elicitor-induced H2O2 production compared to the wild type cells. Furthermore, in vitro assay showed that recombinant OsMT4 protein possesses superoxide scavenging activity. OsMT4-GFP fusion protein was localized at the cytosol indicating, OsMT4 functions primarily in the cytosol. The results suggest that OsRac1 downregulates OsMT4 expression, thereby potentiating ROS accumulation and disease resistance.
