Rice blast caused by Magnaporthe oryzae is a devastating disease of rice. Mechanisms of rice resistance to blast have been studied extensively, and the rice–M. oryzae pathosystem has become a model for plant–microbe interaction studies. However, the mechanisms of non-host resistance (NHR) to rice blast in other plants remain poorly understood. We found that penetration resistance to M. oryzae in multiple mutants, including pen2 NahG pmr5 agb1 and pen2 NahG pmr5 mlo2 plants, was severely compromised and that fungal growth was permitted in penetrated epidermal cells. Furthermore, rice Pi21 enhanced movement of infection hyphae from penetrated Arabidopsis epidermal cells to adjacent mesophyll cells. These results indicate that PEN2, PMR5, AGB1, and MLO2 function in both penetration and post-penetration resistance to M. oryzae in Arabidopsis, and suggest that the absence of rice Pi21 contributed to Arabidopsis NHR to M. oryzae.

The rate of entry of Magnaporthe oryzae into the Arabidopsis pen2 quintuple (pen2 NahG pmr5 agb1 mlo2) mutant was significantly higher than those into the pen2 quadruple (pen2 NahG pmr5 agb1 and pen2 NahG pmr5 mlo2) mutants. The lengths of the infection hyphae in the pen2 quintuple mutant were intermediate between the pen2 quadruple mutants. These results suggest that different genetic networks, consisting of PEN2, PMR5, AGB1, and MLO2, control penetration and post-penetration resistance to M. oryzae in Arabidopsis.Key words: non-host resistance; penetration; post-penetration; Magnaporthe oryzae Rice blast, caused by Magnaporthe oryzae, is a devastating disease of rice. The mechanisms of rice resistance to blast have been studied extensively, and the rice-M. oryzae pathosystem has become a model in plant-microbe interaction studies, 1,2) but the mechanisms of non-host resistance (NHR) to rice blast in other plants remain poorly understood.The Arabidopsis penetration2 (pen2) mutant shows a significantly elevated M. oryzae penetration ratio. PEN2 encodes an atypical myrosinase that is involved in glucosinolate metabolism in defense responses. 3) Broadspectrum resistance to adapted powdery mildews is conferred by loss-of-function mutant alleles of MILDEW RESISTANCE LOCUS O (MLO) genes in barley and Arabidopsis. MLO2 encodes a plant-specific family of integral membrane proteins. 4) In a screening to recover loss of susceptibility to Golovinomyces cichoracearum mutants in Arabidopsis, powdery mildew resistant (pmr) mutants were isolated. PMR5 encodes a member of a large family of plant-specific proteins of unknown function that is probably targeted to the endoplasmic reticulum and the secretory pathway. 5) Heterotrimeric G protein complexes couple extracellular signals to downstream effectors via cell surface receptors. AGB1 encodes a subunit of a heterotrimeric G protein. Recent studies have indicated that it is involved in defense responses against several pathogens. 6-9)Recently we conducted systematic analyses of multiple mutants, including pen2 NahG pmr5 agb1 and pen2 NahG pmr5 mlo2 plants, and found that elevated fungal entry rates were related to fungal growth rates in the penetrated cells. 10) Therefore, penetration and postpenetration resistance in Arabidopsis probably share a common mechanism. PEN2 in Arabidopsis probably operates by poisoning fungal penetration (i) as penetration pegs pass through the cell wall and (ii) when infection hyphae invade the plasma membrane. 3,11) We suggest that PMR5, AGB1, and MLO2 are involved in the invagination process, 10) but it remains to be clarified whether the same genetic network of these factors controls both forms of resistances in Arabidopsis.In this present study of the genetic network, we generated the pen2 NahG pmr5 agb1 mlo2 mutant and performed experiments to compare NHR to M. oryzae among Arabidopsis mutants.Arabidopsis plants were grown under short-day conditions (9:15 L:D) at 22 C in a growth room as previously described. 9,10,12) We used the following mut...