Jasmonate is essential for insect defense in Arabidopsis

Abstract

The signaling pathways that allow plants to mount defenses against chewing insects are known to be complex. To investigate the role of jasmonate in wound signaling in Arabidopsis and to test whether parallel or redundant pathways exist for insect defense, we have studied a mutant (fad3-2 fad7-2 fad8) that is deficient in the jasmonate precursor linolenic acid. Mutant plants contained negligible levels of jasmonate and showed extremely high mortality ( approximately 80%) from attack by larvae of a common saprophagous fungal gnat, Bradysia impatiens (Diptera: Sciaridae), even though neighboring wild-type plants were largely unaffected. Application of exogenous methyl jasmonate substantially protected the mutant plants and reduced mortality to approximately 12%. These experiments precisely define the role of jasmonate as being essential for the induction of biologically effective defense in this plant-insect interaction. The transcripts of three wound-responsive genes were shown not to be induced by wounding of mutant plants but the same transcripts could be induced by application of methyl jasmonate. By contrast, measurements of transcript levels for a gene encoding glutathione S-transferase demonstrated that wound induction of this gene is independent of jasmonate synthesis. These results indicate that the mutant will be a good genetic model for testing the practical effectiveness of candidate defense genes.

Figure 1

Kinetics of jasmonate accumulation in unwounded (open symbols) and wounded (solid symbols) leaf tissue of wild-type (○, •) and fad3–2 fad7–2 fad8 mutant (□, ▪) Arabidopsis. Data are the mean ± SEM of three determinations.

Figure 2

Death and protection of mutant plants from Bradysia larvae attack. (A) A mixed population of wild-type and fad3–2 fad7–2 fad8 plants were grown in a net enclosure populated with 20–25 adult Bradysia flies. Each day, eight pots were sprayed with 0.8 ml of H₂O and seven pots were sprayed with with 0.8 ml of a dilute aqueous solution of methyl jasmonate. Data from two experiments are shown in which the methyl jasmonate concentration used was either 0.001% (open symbols) or 0.01% (solid symbols). the graph shows the percentage survival of 117 wild-type plants (○ and • both treatments), 73 mutant plants treated with H₂O (□, ▪) and 73 mutant plants treated with methyl jasmonate (Δ, ▴). (B and C) For clarity, wild-type and mutant seeds were sown in pots in two rows but were otherwise treated as described above. The photographed plants correspond to day 50 in A. (B) Compared with wild-type controls (on the left), mutant plants (on the right) sprayed with water show extensive damage 20 days after the introduction of adult Bradysia flies. Some leaves on mutant plants have been almost completely eaten. Wilting of other leaves was attributed to damage to the petiole or to the roots of the plants. (C) Mutant plants sprayed with 0.01% methyl jasmonate (on the right) remained healthy and vigorous within the same environment.

Figure 3

Wound induction of gene expression in wild-type (wt) and fad3–2 fad7–2 fad8 Arabidopsis. Transcript levels are shown in unwounded (C) and wounded (W) tissue after 2-hour incubation (4 hours for AtVSP). The probes used were AtVSP encoding a vacuole-localized acid phosphatase, DHS1 encoding 3-deoxy-d-arabinoheptulosonate-7-phosphate synthase, Pal1 encoding phenylalanine ammonia lyase, and GST encoding glutathione S-transferase.

Figure 4

Jasmonate induction of gene expression in wild-type (wt) and fad3–2 fad7–2 fad8 Arabidopsis. Transcript levels are shown in unwounded tissue that had been sprayed with water (C, control) or with 0.001% methyl jasmonate (J) and harvested after a 1.5-hour incubation. The probes used are those described in Fig. 3.