The actin cytoskeleton plays a crucial role in innate immune response at the early stage of fungal penetration (Kobayashi et al., 1997). Actin microfilaments are polarized toward the penetration sites of the biotrophic powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh) in barley mutant lacking the susceptibility factor mildew resistance locus (MLO) (Opalski et al., 2005). The polarization of actin microfilaments is closely associated with successful prevention of penetration of Bgh, in which actin focusing is induced in resistant response, but not in susceptible response. An active mutant of barley, HvRACB, partially inhibits actin reorganization, which results in enhanced susceptibility against Bgh (Schultheiss et al., 2005). Since HvRACB belongs to group II/type I subfamily of ROP proteins that function in the induction of subcellular Ca2+ gradients and polar actin patterns in development (see above), CA-HvRACB may induce diffuse instead of focal Ca2+ influx required for actin reorganization. In addition, CA-HvRACB does not affect HR and mlo-mediated disease resistance (Schultheiss et al., 2005), suggesting that HvRACB is not simple negative regulator for defense response. Suppression of HvRACB by RNAi leads to partial activation of resistance to Bgh. This may result from that HvRACB regulates the polar membrane growth process that is involved in PM invagination in haustorium establishment of Bgh (Schultheiss et al, 2002, 2003).
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