Deposition of lignin on the cell wall is one of the immune responses. Lignin, which is polymerized through peroxidase activity using hydrogen peroxide in the cell wall, presents an undegradable mechanical barrier to most pathogens (Moerschbacher et al., 1990). Rice cinnamoyl-CoA reductase 1 (OsCCR1), a key enzyme for synthesis of monolignols in lignin biosynthesis, has been identified as an effector of OsRAC1 (Kawasaki et al., 2006). OsCCR1 is specifically expressed during defense responses, indicating that OsCCR1 contributes to lignin biosynthesis in the immune response, but not in development. The interaction of OsCCR1 with OsRAC1 drastically activates OsCCR1 activity in vitro, and transgenic cell cultures expressing CA-OsRAC1 accumulates lignin through enhanced CCR activity (Kawasaki et al., 2006). As mentioned above, OsRAC1 also stimulates NADPH oxidase-dependent ROS production that is required for the polymerization of monolignol on the cell wall. These data suggest that OsRAC1 controls lignin synthesis through the coordinate regulation of both NADPH oxidase and OsCCR1 activities during immune responses in rice (Yang and Fu, 2007). In another case, ROP protein was shown to interact with UDP-glucose transferase involved in callose synthesis at the forming cell plate during cytokinesis (Hong et al., 2001). Although callose is produced at the infection sites of pathogens (Kim et al., 2005), whether ROP GTPase is involved in callose formation during immune responses remain to be investigated. Thus, the ROP proteins play important roles in regulating cell wall biogenesis.
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