Comparing induction at an early and late step in signal transduction mediating indirect defence in Brassica oleracea

Abstract

The induction of plant defences involves a sequence of steps along a signal transduction pathway, varying in time course. In this study, the effects of induction of an early and a later step in plant defence signal transduction on plant volatile emission and parasitoid attraction are compared. Ion channel-forming peptides represent a class of inducers that induce an early step in signal transduction. Alamethicin (ALA) is an ion channel-forming peptide mixture from the fungus Trichoderma viride that can induce volatile emission and increase endogenous levels of jasmonic acid (JA) and salicylic acid in plants. ALA was used to induce an early step in the defence response in Brussels sprouts plants, Brassica oleracea var. gemmifera, and to study the effect on volatile emission and on the behavioural response of parasitoids to volatile emission. The parasitoid Cotesia glomerata was attracted to ALA-treated plants in a dose-dependent manner. JA, produced through the octadecanoid pathway, activates a later step in induced plant defence signal transduction, and JA also induces volatiles that are attractive to parasitoids. Treatment with ALA and JA resulted in distinct volatile blends, and both blends differed from the volatile blends emitted by control plants. Even though JA treatment of Brussels sprouts plants resulted in higher levels of volatile emission, ALA-treated plants were as attractive to C. glomerata as JA-treated plants. This demonstrates that on a molar basis, ALA is a 20 times more potent inducer of indirect plant defence than JA, although this hormone has more commonly been used as a chemical inducer of plant defence.

Fig. 1.

Response of Cotesia glomerata females in dual-choice tests in the windtunnel to control plants, plants sprayed with 10 ml of a 20 μg ml⁻¹ alamethicin (ALA) solution, and plants infested with five Pieris brassicae caterpillars. The numbers to the right of each bar represent the number of parasitoids making a choice, and the total number of parasitoids used in the windtunnel tests is indicated in parentheses (***P <0.001).

Fig. 2.

Effect of the alamethicin (ALA) concentration used for treating Brussels sprouts plants on the attraction of Cotesia glomerata. The numbers to the right of each bar represent the number of parasitoids making a choice, and the total number of parasitoids used in the windtunnel tests is indicated in parentheses (n.s. P >0.05; *P <0.05; ***P <0.001).

Fig. 3.

Effect of combinations of alamethicin and jasmonic acid compared with the effects of either inducer alone on behavioural responses of Cotesia glomerata parasitoids in the windtunnel. The numbers to the right of each bar represent the number of parasitoids making a choice, and the total number of parasitoids used in the windtunnel tests is indicated in parentheses (*P <0.05).

Fig. 4.

Principal component analysis score plot of the volatile pattern of mechanically damaged Brussels sprouts plants sprayed with Tween-20 (Ct), mechanically damaged Brussels sprouts plants sprayed with a 10 ml solution of 20 μg ml⁻¹ alamethicin (ALA), 0.05 mM jasmonic acid (JA), or a mixture of 20 μg ml⁻¹ alamethicin and 0.05 mM jasmonic acid (JA+ALA) (n=6 per treatment). First (PC1) and second (PC2) principal components plotted against each other. Percentage variation explained in parentheses. The ellipse defines the Hotelling's T² confidence region (95%).