The rate of entry of Magnaporthe oryzae into pen2 mpk6 plants was higher than that into pen2 plants. The infection hyphae in the pen2 mpk6 plants were longer than those in the pen2 plants. The proportion of branched hyphae development in the pen2 mpk6 plants was higher than that in the pen2 plants. These results suggest that MPK6 functions in both penetration and post-penetration resistance to M. oryzae in Arabidopsis thaliana.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.Arabidopsis mutants with altered non-host interactions following Blumeria graminis hordei (Bgh) infection were described recently, and three genes were identified: PENETRATION 1 (PEN1), PEN2, and PEN3.3-6) PEN1 encodes a plasma membrane-anchored syntaxin with a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) domain. 3)PEN2 encodes an atypical myrosinase involved in glucosinolate metabolism in defense responses. 4,7,8) PEN3 encodes a pleiotropic drug resistance (PDR) ATP-binding cassette (ABC) transporter. 5,6) Collectively, these studies demonstrate that Arabidopsis NHR to nonadapted biotrophic powdery mildews has two successive multicomponent defense layers: penetration and postpenetration resistance. We have found that PEN2, PMR5, AGB1, and MLO2 are involved in both penetration and post-penetration resistance to M. oryzae in A. thaliana. 9-11)Plants have evolved effective basal defense systems to detect and limit the growth of pathogens. Pathogens can be recognized by the host via the perception of conserved microbial structures called pathogen-associated molecular patterns (PAMPs). PAMPs are recognized by transmembrane pattern recognition receptors (PRRs) that bind specific PAMPs and initiate intracellular immune responses.12) These PAMP-triggered immunity (PTI) responses include the generation of reactive oxygen species (ROS) and protein phosphorylation with associated gene regulation, which ultimately restricts the growth of the pathogens. 13)Mitogen-activated protein (MAP) kinase cascades play important roles in plant immunity. A MAP kinase cascade consists of a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MKK), and a MAP kinase (MPK).14) Signals from upstream receptors are transduced and amplified through the MAP kinase cascade. The best-characterized MPKs are MPK3, MPK4, and MPK6, each of which is activated by a diversity of stimuli, including abiotic stresses, pathogens, and oxidative stress in A. thaliana. While MPK4 negatively regulates biotic stress signaling, 15) MPK3 and MPK6 act as positive mediators of defense responses. 16)However, it remains to be clarified whether MPK4 and MPK3/MPK6 are involved in NHR to M...

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...