The herbivore-induced plant volatile methyl salicylate negatively affects attraction of the parasitoid Diadegma semiclausum

Abstract

The indirect defense mechanisms of plants comprise the production of herbivore-induced plant volatiles that can attract natural enemies of plant attackers. One of the often emitted compounds after herbivory is methyl salicylate (MeSA). Here, we studied the importance of this caterpillar-induced compound in the attraction of the parasitoid wasp Diadegma semiclausum by using a mutant Arabidopsis line. Pieris rapae infested AtBSMT1-KO mutant Arabidopsis plants, compromised in the biosynthesis of MeSA, were more attractive to parasitoids than infested wild-type plants. This suggests that the presence of MeSA has negative effects on parasitoid host-finding behavior when exposed to wild-type production of herbivore-induced Arabidopsis volatiles. Furthermore, in line with this, we recorded a positive correlation between MeSA dose and repellence of D. semiclausum when supplementing the headspace of caterpillar-infested AtBSMT1-KO plants with synthetic MeSA.

Fig. 1

Illustration of the AtBSMT T-DNA mutant (GARLIC_776_B10 Line) from The Torrey Mesa Institute’s T-DNA insertion mutant collection. The exons are indicated as grey boxes and nucleotide numbers are given below. The position of the approximately 200-nt T-DNA insertion, as well as a deletion of 328-nt extending downstream from the right border of the T-DNA element and including the end part of intron 2 and the beginning part of exon 3 also are shown. Both insertion and deletion were determined by sequencing. Furthermore, we determined that transcripts from the mutant gene were still produced in the plant, and the processed mRNA had a similar size to that of the wild type (WT) mRNA. However, when the mutant transcript was amplified by RT-PCR and sequenced, it was seen that in the absence of the original 3′ end of intron 2, a new 3′ splice site was used within the remains of exon 3. This led to a protein that is much shorter and does not have SAMT activity (See also Fig. S1)

Fig. 2

Emission of volatile compounds of Col-0 wild-type and AtBSMT-KO Arabidopsis plants, either uninfested or infested with 20 Pieris rapae caterpillars, expressed as peak area (arbitrary units; mean ± SE; N = 5) for the identifying ion per g FW. Compounds: 1=1-pentanol (m/z 70); 2= (Z)-2-penten-1-ol (m/z 57); 3= (Z)-3-hexen-1-ol (m/z 67); 4= α-pinene (m/z 93); 5= 1-octen-3-ol (m/z 57); 6= ß-myrcene (m/z 93); 7= (Z)-3-hexen-1-ol acetate (m/z 67); 8= (E)-ß-ocimene (m/z 93); 9= linalool (m/z 93); 10= methyl benzoate (m/z 136) 11= 1-nonanol (m/z 56); 12= methyl salicylate (m/z 120); 13= ethyl salicylate (m/z 120); 14= indole (m/z 117); 15= (E,E)-α-farnesene (m/z 93); 16= (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) (m/z 69). Bars marked with * indicate a treatment for a genotype significantly emitting more volatiles than its opposite uninfested or infested genotype. Bars marked with arrows represent compounds emitted in significantly different amounts by the two genotypes

Fig. 3

Behavioral responses of naïve Diadegma semiclausum females to volatiles of two sets of Arabidopsis thaliana plants (Col-0 vs. AtBSMT1-KO), as assessed in the Y-tube olfactometer. All plants were infested (inf.) with 20 Pieris rapae caterpillars and in some cases the headspace was supplemented with synthetic methyl salicylate (MeSA), added downwind from the plants. Added MeSA-doses (μg) are indicated to the right of the bars in the left bar plot. Each bar represents the percentage of choices for each of the two odor sources as determined in five replicate experiments; on each replicate day 10 parasitoids were tested per odor source (X ² test, P values). Error bars indicate SE. Generalized linear model analysis for experiments B and C, demonstrated that MeSA dosage (P < 0.001) and not the offered genotype (P = 0.167) explained parasitoid behavior. Parasitoid MeSA dosage responses between two tested genotype combinations did not differ (P = 0.270). The right bar plots indicate the percentage of no choice in each experiment; total number of tested parasitoids are given in these bars