The role of seven-transmembrane domain MLO proteins, heterotrimeric G-proteins, and monomeric RAC/ROPs in plant defense

MLO proteins are structurally reminiscent of G-protein-coupled receptors but act independently of heterotrimeric G-proteins as major susceptibility factors to powdery mildew fungi. In barley, monomeric RAC/ROPs, instead of heterotrimeric G-proteins, MLO-dependently modulate susceptibility to powdery mildew, which may involve functions in cytoskeleton remodeling. In contrast to the role of RAC/ROPs in barley susceptibility to powdery mildew, rice OsRAC1 exerts a central function in basal and effector-triggered immunity. In this context, a functional cooperation with the heterotrimeric G-protein subunit, Gα, and additional protein complexes with functions in plant immunity has been discovered. These polypeptides together modulate the oxidative burst and regulate the abundance of defense-associated messenger RNAs and defense proteins. This chapter highlights the interconnection of MLO, RAC/ROP, and heterotrimeric G-proteins in plant immunity.