Rice Blast Fungus (Magnaporthe oryzae) Infects Arabidopsis via a Mechanism Distinct from That Required for the Infection of Rice    

Abstract

Magnaporthe oryzae is a hemibiotrophic fungal pathogen that causes rice (Oryza sativa) blast. Although M. oryzae as a whole infects a wide variety of monocotyledonous hosts, no dicotyledonous plant has been reported as a host. We found that two rice pathogenic strains of M. oryzae, KJ201 and 70-15, interacted differentially with 16 ecotypes of Arabidopsis (Arabidopsis thaliana). Strain KJ201 infected all ecotypes with varying degrees of virulence, whereas strain 70-15 caused no symptoms in certain ecotypes. In highly susceptible ecotypes, small chlorotic lesions appeared on infected leaves within 3 d after inoculation and subsequently expanded across the affected leaves. The fungus produced spores in susceptible ecotypes but not in resistant ecotypes. Fungal cultures recovered from necrotic lesions caused the same symptoms in healthy plants, satisfying Koch's postulates. Histochemical analyses showed that infection by the fungus caused an accumulation of reactive oxygen species and eventual cell death. Similar to the infection process in rice, the fungus differentiated to form appressorium and directly penetrated the leaf surface in Arabidopsis. However, the pathogenic mechanism in Arabidopsis appears distinct from that in rice; three fungal genes essential for pathogenicity in rice played only limited roles in causing disease symptoms in Arabidopsis, and the fungus seems to colonize Arabidopsis as a necrotroph through the secretion of phytotoxic compounds, including 9,12-octadecadienoic acid. Expression of PR-1 and PDF1.2 was induced in response to infection by the fungus, suggesting the activation of salicylic acid-and jasmonic acid/ethylene-dependent signaling pathways. However, the roles of these signaling pathways in defense against M. oryzae remain unclear. In combination with the wealth of genetic and genomic resources available for M. oryzae, this newly established pathosystem allows comparison of the molecular and cellular mechanisms underlying pathogenesis and host defense in two well-studied model plants.

Novel Bifunctional Nucleases, OmBBD and AtBBD1, Are Involved in Abscisic Acid-Mediated Callose Deposition in Arabidopsis

“…Although the mechanism of callose deposition in rice remains largely unknown, the results of a number of studies suggest that callose deposition is triggered by the penetration of rice pathogens and that this deposition is important in host resistance. Several research groups have found that plant defense responses are partially conserved between Arabidopsis and rice (Song and Goodman, 2001;Ding et al, 2009;Park et al, 2009). The resistance of Arabidopsis to M. oryzae is derived from the induction of the SA and JA/ET signaling pathways, callose deposition, and accumulation of ROIs, all of which are similar to the defense responses against the necrotrophic fungus B. cinerea (Park et al, 2009).…”

“…Several research groups have found that plant defense responses are partially conserved between Arabidopsis and rice (Song and Goodman, 2001;Ding et al, 2009;Park et al, 2009). The resistance of Arabidopsis to M. oryzae is derived from the induction of the SA and JA/ET signaling pathways, callose deposition, and accumulation of ROIs, all of which are similar to the defense responses against the necrotrophic fungus B. cinerea (Park et al, 2009). We found that the ectopic expression of OmBBD in atbbd1 complemented completely the deficient phenotype in terms of callose deposition and susceptibility to B. cinerea, indicating the conserved roles of OmBBD and AtBBD1 in defense responses of Arabidopsis against B. cinerea.…”

Entry Mode–Dependent Function of an Indole Glucosinolate Pathway in Arabidopsis for Nonhost Resistance against Anthracnose Pathogens

“…Nonetheless, each of three M. oryzae mutant strains, lacking genes required for appressorium formation (Dcpka, Dpmk1, and MG01, in which the molecular lesion is unknown), was able to infect Arabidopsis leaves, but with reduced disease severity in comparison with wild-type M. oryzae. Of note, these three M. oryzae mutants fail to infect and colonize leaves of the natural host, rice (Oryza sativa; Park et al, 2009), suggesting that the fungus can employ the alternative entry route only on the nonhost Arabidopsis. Similarly, the C. orbiculare maf1 and mtk1 strains are essentially nonpathogenic upon conidiospore spray inoculation on leaves of the host plant, cucumber (Kojima et al, 2002;see Supplemental Figure 8 online).…”

“…Plant cell penetration on leaf surfaces without appressorium differentiation has recently been noted also for M. oryzae, although this invasion mode was not further studied at the microscopic level (Park et al, 2009). Thus, quantitative data resulting from this alternative entry mode versus appressorium-dependent penetration are unknown.…”

Proteasome Inhibitors Affect Appressorium Formation and Pathogenicity of the Rice Blast Fungus, Magnaporthe oryzae

“…Then, an infection-related structure, the appressorium, differentiates from the germ tube tip within a few hours (Hoch et al, 1987). This dome-shaped cell could accumulate proteins and enzymes, and generate mechanical force to break down the cell wall structure of the host (Howard et al, 1991;Park et al, 2009). This fungal appressorium formation process is the initial stage in host penetration, which is essential for host infection.…”