Can Herbivore-Induced Volatiles Protect Plants by Increasing the Herbivores' Susceptibility to Natural Pathogens?

Abstract

In response to insect herbivory, plants mobilize various defenses. Defense responses include the release of herbivore-induced plant volatiles (HIPVs) that can serve as signals to alert undamaged tissues and to attract natural enemies of the herbivores. Some HIPVs can have a direct negative impact on herbivore survival, but it is not well understood by what mechanisms. Here, we tested the hypothesis that exposure to HIPVs renders insects more susceptible to natural pathogens. Exposure of the caterpillars of the noctuid Spodoptera exigua to indole and linalool, but not exposure to (Z)-3-hexenyl acetate, increased the susceptibility to Spodoptera exiguamultiple nucleopolyhedrovirus (SeMNPV). We also found that exposure to indole, but not exposure to linalool or (Z)-3-hexenyl acetate, increased the pathogenicity of Bacillus thuringiensis Additional experiments revealed significant changes in microbiota composition after forty-eight hours of larval exposure to indole. Overall, these results provide evidence that certain HIPVs can strongly enhance the susceptibility of caterpillars to pathogens, possibly through effects on the insect gut microbiota. These findings suggest a novel mechanism by which HIPVs can protect plants from herbivorous insects.IMPORTANCE Multitrophic interactions involving insect pests, their natural enemies, microorganisms, and plant hosts are increasingly being recognized as relevant factors in pest management. In response to herbivory attacks, plants activate a wide range of defenses that aim to mitigate the damage. Attacked plants release herbivore-induced plant volatiles (HIPVs), which can act as priming signals for other plants and attract natural enemies of herbivores, and which may have a direct negative impact on herbivore survival. In the present work, we show that exposure of the insects to the induced volatiles could increase the insects' susceptibility to the entomopathogens naturally occurring in the plant environment. These findings suggest a novel role for plant volatiles by influencing insect interactions with natural pathogens, probably mediated by alterations in the insect microbiota composition. In addition, this work provides evidence for selectable plant traits (production of secondary metabolites) that can have an influence on the ecology of the pests and could be relevant in the improvement of pest management strategies using natural entomopathogens.

Keywords: Bacillus thuringiensis; baculovirus; entomopathogen; indole; linalool; plant volatiles; plant-microbe interactions.

FIG 1

Effect of the tested HIPVs on S. exigua susceptibility to SeMNPV infection (10² occlusion bodies [OBs]/larvae). Ind (indole 4 mg), Lin (linalool 10%), and Hac (hexenyl acetate 10%). (A) Percentage of larval mortality for the different combinations. Observed mortality (O) and expected mortality (E), assuming the additive model. Statistical analyses were performed using one-way ANOVA with a Newman-Keuls posttest to compare the mortalities, and the chi-square test was used to check whether there is a synergistic or additive effect between the different treatments. (B) Mean time to death produced by baculovirus in the presence/absence of the corresponding HIPV. Values were statistically compared using Student's t test.

FIG 2

Effect of the tested HIPVs on S. exigua larvae susceptibility to B. thuringiensis (Xentari). Ind (indole 4 mg), Lin (linalool 0.1%) and Hac (hexenyl acetate 0.1%). Percentage of larval mortality for the different combinations. Observed mortality (O) and expected mortality (E), assuming the additive model. Statistical analyses were performed using one-way ANOVA with a Newman-Keuls posttest to compare the mortalities, and the chi-square test was used to check whether there is a synergistic or additive effect between the different treatments.

FIG 3

Effect of the tested HIPVs on two enzymatic markers of the cellular immunity of S. exigua. (A) Relative phenoloxidase activity in the hemolymph of insects exposed to selected volatiles at 24 and 48 h after exposure. (B) Relative PLA2 activity in the fat bodies of insects exposed to selected volatiles at 24 and 48 h after exposure. For both markers, the activity is normalized according to the activity obtained for the nonexposed insects.

FIG 4

Effect of the exposure to indole on the gut microbiota composition of the S. exigua larvae. (A) Canonical correspondence analysis (CCA) showing the relationship between gut microbiome composition (genus level) in the indole-exposed and nonexposed insects. (B) Bacterial load calculated for the samples from the indole-exposed and nonexposed insects. (C) Relative abundance in percentage of the top genera in samples from the indole-exposed and nonexposed insects. Exposition to the viral infection is indicated as + in the top of the panel. (D) Microbial diversity calculated as the Shannon index in the samples from the indole-exposed and nonexposed insects. (E) LefSE (linear discriminant analysis effect size) results, reporting the more significantly overrepresented taxa for the indole and no-indole groups.