Abstract
Upon herbivory, plants emit specific herbivore‐induced plant volatiles (HIPVs) that can attract natural enemies of the herbivore thus serving as indirect plant resistance. Not only insect herbivores, but microorganisms may also affect HIPV emission before or after plant colonisation, which in turn can affect behaviour of natural enemies of the herbivore. Yet, it remains elusive whether volatiles from microorganisms influence HIPV emission and indirect plant resistance. In this study, we investigated whether exposure of Brassica rapa roots to volatiles from soil‐borne fungi influence HIPV emission and the recruitment of natural enemies of Pieris brassicae larvae. Using a two‐compartment pot system, we performed greenhouse and common‐garden experiments, and we profiled plant HIPV emission. We found that exposure of plant roots to fungal volatiles did not affect the number of P. brassicae larvae recollected from the plants, suggesting a neutral effect of the fungal volatiles on natural predation. Likewise, in a greenhouse, similar numbers of larvae were parasitised by Cotesia glomerata wasps on control plants as on fungal volatile‐exposed plants. Additionally, chemical analysis of HIPV profiles revealed no qualitative and quantitative differences between control plants and fungal volatile‐exposed plants that were both infested with P. brassicae larvae. Together, our data indicate that root exposure to fungal volatiles did not affect indirect plant resistance to an insect herbivore. These findings provide new insight into the influence of indirect plant resistance by fungal volatiles that are discussed together with the effects of fungal volatiles on direct plant resistance.
Highlights
Upon attack by an herbivore, plants emit specific volatile organic compounds (VOCs) that can attract natural enemies of the herbivores, acting as an indirect plant resistance mechanism (Schoonhoven et al, 2005)
We explored whether the emission of herbivore-induced plant volatiles (HIPVs) by P. brassicae-infested Brassica rapa plants is influenced by the exposure of roots to volatiles emitted by four soil-borne fungi, and the consequences for the recruitment of parasitoids
We found that the number of P. brassicae larvae recollected from B. rapa plants whose roots had been exposed to volatiles from soil-borne fungi did not differ with that of control plants when subjected to natural predation in the field
Summary
Upon attack by an herbivore, plants emit specific volatile organic compounds (VOCs) that can attract natural enemies of the herbivores, acting as an indirect plant resistance mechanism (Schoonhoven et al, 2005). Green leaf volatiles (GLVs), typically six-carbon compounds, are usually among the first VOCs emitted upon attack. Brassica plants infested with Pieris brassicae eggs, recruitment of parasitoids is important upon hatching of the larvae because Cotesia wasps parasitise only first and second instar larvae, which can lead to up to 40% of larval mortality (Lucas-Barbosa et al, 2014). Recruitment of parasitoids and predators can lead to nearly 100% of P. brassicae mortality (Lucas-Barbosa et al, 2014). Recruitment of natural enemies through HIPV emission is essential to alleviate the herbivore pressure on plants, and represents an important component of plant resistance
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