A role for jasmonate in pathogen defense of Arabidopsis

Abstract

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 wild-type 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.

Figure 1

Disease development and jasmonate-induced resistance in mutant plants inoculated with Pythium mastophorum: (A) Wild-type and fad3–2 fad7–2 fad8 plants were grown on pasteurized soil for 15 days and then transferred to pots containing soil infested with P. mastophorum. Starting 2 days before transfer, the roots of each plant were drenched each day with 1 ml of methyl jasmonate solution or an equal volume of water. The number of disease-free plants from 32 wild-type (•), 28 mutant plants treated with water (○), and 36 mutant plants treated with 45 μM methyl jasmonate (□) were scored daily. Disease development in wild-type plants treated with methyl jasmonate was the same as in the wild-type controls. (B and C) Photographs of plants taken on day 12. (B) Wild-type controls (Left) and mutant plants (Right) treated with water. (C) Wild-type (Left) and mutant plants (Right) treated with 45 μM methyl jasmonate.

Figure 2

Extreme susceptibility of fad3–2 fad7–2 fad8 plants to infection by Pythium mastophorum. Wild-type (Left) and fad3–2 fad7–2 fad8 plants (Right) were grown axenically on agar medium for 20 days before being inoculated with P. mastophorum. (A) Mock inoculation with sterile agar slab. (B) Inoculation with P. mastophorum. The plants were photographed 7 days after inoculation.

Figure 3

Oospore formation by Pythium mastophorum in infected roots of Arabidopsis as indicators of susceptibility of the fad3–2 fad7–2 fad8 and coi1 mutants and the protection of fad3–2 fad7–2 fad8 by 45 μM jasmonate. Wild-type and mutant plants were treated as described in Fig. 1. Microphotographs of representative root segments were taken on the 12th day after inoculation with the pathogen as described in Materials and Methods. The dark stained bodies are oospores of P. mastophorum. (Bar = 1 mm.)

Figure 4

Pathogen induction of gene expression in wild-type (wt) and fad3–2 fad7–2 fad8 (mutant) Arabidopsis plants. Transcript levels are shown in tissue from uninoculated seedlings (C) and seedlings inoculated (P) with P. mastophorum 48 h earlier (24 h for LOX). Results are shown for genes encoding lipoxygenase 2 (LOX2), a plant defensin gene (PDF1.2), and chalcone synthase gene (CHS).