A role for jasmonate in pathogen defense of Arabidopsis (jasmonic acidyroot rotydefense signalingyPythium)

To investigate the role of jasmonate in the defense of plants against fungal pathogens, we have studied a mutant of Arabidopsis, fad3–2 fad7–2 fad8, that cannot accumulate jasmonate. Mutant plants were extremely susceptible to root rot caused by the fungal root pathogen Pythium mastophorum (Drechs.), even though neighboring wild-type plants were largely unaffected by this fungus. Application of exogenous methyl jasmonate substantially protected mutant plants, reducing the incidence of disease to a level close to that of wild-type controls. A similar treatment with methyl jasmonate did not protect the jasmonate-insensitive mutant coi1 from infection, showing that protective action of applied jasmonate against P. mastophorum was mediated by the induction of plant defense mechanisms rather than by a direct antifungal action. Transcripts of three jasmonate-responsive defense genes are induced by Pythium challenge in the wildtype but not in the jasmonate-deficient mutant. Pythium species are ubiquitous in soil and root habitats world-wide, but most (including P. mastophorum) are considered to be minor pathogens. Our results indicate that jasmonate is essential for plant defense against Pythium and, because of the high exposure of plant roots to Pythium inoculum in soil, may well be fundamental to survival of plants in nature. Our results further indicate that the fad3–2 fad7–2 fad8 mutant is an appropriate genetic model for studying the role of this important signaling molecule in pathogen defense. Plants defend themselves against fungi and other microbial pathogens by the induction of both localized and systemic responses. Typically, a pathogen interacting with a resistant host plant triggers a localized hypersensitive response, the intensity and spread of which are regulated by complex molecular mechanisms (1, 2). At the same time, long-distance signals initiated at the infection site lead to the induction of specific pathogenesisrelated (PR) genes in uninfected parts of the plant—a process termed ‘‘systemic acquired resistance’’ or SAR (3). Signaling molecules such as salicylic acid, methyl jasmonate, ethylene, hydrogen peroxide, and superoxide radicals have been proposed to be involved in the induction and coordination of these plant responses (3, 4). However, salicylic acid has been ascribed a central role in both localized responses and SAR (1–3, 5, 6). The complexities of the relationship between the hypersensitive response, salicylic acid, and SAR are beginning to be appreciated and understood through the characterization of several families of mutants (2–4, 7) and the availability of salicylic acid-deficient plants expressing the bacterial NahG gene encoding the salicylate hydroxylase enzyme (6). In particular, the increased susceptibility of NahG plants to a range of fungal and bacterial pathogens establishes the biological relevance of salicylic acid (6). It has been known for some time that some fungal elicitors, such as oligogalacturonides and chitosan, also activate woundresponse genes (8, 9). This point suggests that there is cross-talk from the pathogen response pathway into the wound-signaling pathway, which uses jasmonic acid as the requisite localized effector of defense responses aimed at chewing insects (10, 11). More recent evidence suggests that jasmonic acid is involved in the induction of genes that act primarily in defense against pathogens rather than insects. For example, fungal elicitors induce transient accumulation of jasmonic acid as well as the synthesis of several classes of phytoalexins in suspension cell cultures of a number of plant species (12). Exogenous application of jasmonates induces the same antimicrobial compounds, apparently by transcriptional activation of genes that encode the biosynthetic enzymes involved (12). The defensin gene PDF1.2 of Arabidopsis, which encodes a protein with demonstrated antifungal activity, is induced strongly by either pathogen challenge or methyl jasmonate but not by salicylic acid (13). The same is true of the thionin encoded by the Arabidopsis Thi2.1 gene (14, 15). Finally, jasmonate and ethylene are synergistic in inducing members of the PR1 and PR5 gene families, which encode pathogenesis-related proteins (16). Interpreting the significance of these observations is made difficult by the complexities inherent in plant–pathogen interactions. For example, other defensin and thionin genes closely related to PDF1.2 and Thi2.1 are not induced by jasmonate in Arabidopsis (13–15), and thionin genes in barley can be induced by salicylic acid as well as jasmonate (17). Furthermore, the jasmonate-induced genes also might be induced by pathogen attack through parallel pathways that do not involve jasmonate. Only in the case of the Arabidopsis PDF1.2 gene has a dependence on jasmonate been shown through the inability of the fungal pathogen Alternaria brassicola to induce expression of PDF1.2 in the coi1 mutant, which is deficient in jasmonate signaling (13, 18). These caveats notwithstanding, there appears to be a strong case for a second, jasmonatedependent pathway that mediates some plant responses to pathogens. Until now, it has not been possible to demonstrate the biological relevance of jasmonate signaling in a host–pathogen interaction. For this reason, the practical importance of jasmonate signaling in pathogen defense has remained unclear. For example, the jasmonate-induced genes might have a relatively minor role in supplementing the hypersensitive response andyor SAR. Here, we use a mutant of Arabidopsis that is deficient in jasmonate synthesis to demonstrate that jasmonate-signaling is essential for protection against the soil-borne pathogenic fungus Pythium mastophorum (Drechs.). MATERIALS AND METHODS Plant Materials. Arabidopsis thaliana plants used were descended from the Columbia wild type in which mutations The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. © 1998 by The National Academy of Sciences 0027-8424y98y957209-6$2.00y0 PNAS is available online at http:yywww.pnas.org. Abbreviation: SAR, systemic acquired resistance. §To whom reprint requests should be addressed. e-mail: [email protected].