Abstract
SummaryThe homoterpenes (3E)‐4,8‐dimethyl‐1,3,7‐nonatriene (DMNT) and (E,E)‐4,8,12‐trimethyl‐1,3,7,11‐tridecatetraene (TMTT) are major herbivore‐induced plant volatiles that can attract predatory or parasitic arthropods to protect injured plants from herbivore attack. In this study, DMNT and TMTT were confirmed to be emitted from cotton (Gossypium hirsutum) plants infested with chewing caterpillars or sucking bugs. Two CYP genes (GhCYP82L1 and GhCYP82L2) involved in homoterpene biosynthesis in G. hirsutum were newly identified and characterized. Yeast recombinant expression and enzyme assays indicated that the two GhCYP82Ls are both responsible for the conversion of (E)‐nerolidol to DMNT and (E,E)‐geranyllinalool to TMTT. The two heterologously expressed proteins without cytochrome P450 reductase fail to convert the substrates to homoterpenes. Quantitative real‐time PCR (qPCR) analysis suggested that the two GhCYP82L genes were significantly up‐regulated in leaves and stems of G. hirsutum after herbivore attack. Subsequently, electroantennogram recordings showed that electroantennal responses of Microplitis mediator and Peristenus spretus to DMNT and TMTT were both dose dependent. Laboratory behavioural bioassays showed that females of both wasp species responded positively to DMNT and males and females of M. mediator could be attracted by TMTT. The results provide a better understanding of homoterpene biosynthesis in G. hirsutum and of the potential influence of homoterpenes on the behaviour of natural enemies, which lay a foundation to study genetically modified homoterpene biosynthesis and its possible application in agricultural pest control.
Highlights
Plants promote self-fitness against herbivore attack by producing toxins and repellents and by emitting volatiles that attract natural enemies of herbivorous insects (Gols, 2014)
gas chromatography– mass spectrometry (GC-MS) analysis of headspace volatile compounds from herbivore-injured cotton plants showed that both DMNT and TMTT were emitted from Helicoverpa armigera- and Apolygus lucorum-damaged cotton plants, while neither DMNT nor TMTT was released from herbivore-free control plants (Figure 1)
The amounts of DMNT and TMTT produced were calculated by comparing the peak area ratio to an internal standard (Table 1)
Summary
Plants promote self-fitness against herbivore attack by producing toxins and repellents and by emitting volatiles that attract natural enemies of herbivorous insects (Gols, 2014). It has been reported that herbivore-induced plant volatiles (HIPV) play an important role in plant communication, functioning as airborne cues to induce defence in adjacent foliage or plants or to prime uninfected plant tissue for potentiated defence responses upon subsequent herbivore attack (Turlings and Ton, 2006). A well-studied example of the role of volatiles in plant defence is the tritrophic interaction among lima bean (Phaseolus limensis, plant), spider mites (Tetranychus urticae, herbivore) and predatory mites (Phytoseiulus persimilis, carnivore). After damage by T. urticae, P. limensis leaves release a complex volatile blend containing homoterpenes that play a crucial role in plant indirect defence to attract the predators of herbivores (de Boer et al, 2004). When the emission of homoterpenes was inhibited by the terpenoid pathway inhibitor fosmidomycin, reduced attraction of the predatory mite P. persimilis was observed (Mumm et al, 2008)
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