Roles of (Z)-3-hexenol in plant-insect interactions

Abstract

Green leaf C6-volatiles are among the most important herbivore-induced plant volatiles (HIPVs). They play important roles in mediating the behavior of herbivores and their natural enemies, and in triggering the plant-plant communication to prevent further attacks. Recently, wound-induced ubiquitous (Z)-3-hexenol, a C6-alcohol synthesized in the lipoxygenase/HPL pathway, was proved to be the most important info chemical for the herbivore repellence/attraction and natural enemy attraction in tritrophic interactions, as well as for the induction of gene expression in neighboring unattacked plants. In spite of the conflict functions of (Z)-3-hexenol in direct and indirect plant defenses, its positive roles in the indirect defense and the priming effect are consistent. Therefore, this compound can be used to develop novel insect pest control strategies.

To date, nearly 2000 volatile compounds have been identified in plant species from over 90 families.¹ These compounds are released from plant organs above or below the ground, and some are induced by biotic activities. Herbivore feeding stimulates the plants to release green leaf volatiles (GLVs), terpenoids, nitrogen-containing nitriles and oximes, methyl salicylate, etc. Production of these volatiles by plants involves at least three biosynthetic pathways: the fatty acid/lipoxygenase pathway for green leaf volatiles, the isoprenoid pathway for terpenoids, and the shikimic acid pathway for methyl salicylate.² Herbivore-damaged plants emit some of the most common GLVs and terpenoids that play important roles in mediating the behaviors of herbivores and their natural enemies, as well as in triggering the plant-plant communication.¹ Recently, functional studies on green leaf C6-volatiles have received wide attention and made exciting progresses. Especially, accumulating evidences on the C6-volatile (Z)-3-hexenol support its role in mediating indirect defense responses of plant.

Biosynthesis of (Z)-3-hexenol

Many inducible defense responses are activated by oxylipins, which are the oxygenated derivatives of fatty acids generated via the oxylipin branch pathways.¹ Based on the current available knowledge, most healthy plant species do not release GLVs such as C6-aldehydes, -alcohols or -esters, but large amounts of GLVs are emitted when plants are wounded.^3–5 In addition, release of these C6-volatiles occurs not only locally at the wounding sites, but also systemically in the distal leaves.^4,6 Through the oxylipin pathway that is parallel to the biosynthetic pathway for JAs, plant C6-volatiles are synthesized using 13-hydroperoxides of linoleic or linolenic acid (LA) as substrate and using fatty acid hydroperoxide lyase (HPL) as catalytic enzyme.^7,8 In the lipoxygenase/HPL pathway, the plant first produces C6-aldehydes, which are then catalyzed by alcohol dehydrogenase to form the corresponding C₆-alcohols, e.g. (Z)-3-hexenol.⁸ Studies of mutant tomato plants and transgenic Nicotiana plants showed that JA-deficient plants (spr2) and HPL-deficient plants (as-hpl) released significantly reduced level of C6-alcohols than wild-type counterparts, suggesting that these enzymes are key regulators for the hexenol production in oxylipin pathway.^9–11 Therefore, the biosynthesis and release of (Z)-3-hexenol are closely related to HPL pathway, which is one of the two major competing oxylipin-pathway branches that produce stressinducible compounds.¹²

Response of Herbivores to (Z)-3-hexenol

Herbivore- or wound-induced (Z)-3-hexenol can directly affect the physiology and behavior of herbivores through its positive (attractive) and negative (repelling and deterring) properties. Electrophysiological studies revealed that most herbivorous insects exhibited distinct electroantennogram (EAG) responses to (Z)-3-hexenol.¹³ Compared with the weak EAG responses of two Inner Mongolia grasshopper species to hexanol, strong EAG responses were induced by C6-alcohols [(Z)-2-hexenol, (E)-2-hexen-ol and (Z)-3-hexenol], aldehyde [(E)-2-hexenal], ester [(Z)-3-hexenyl acetate] and C7-alcohol (1-hepatanol).¹⁴ In addition, (Z)-3-hexenol stimulated similar EAG responses in the first instar nymphs and the adults of a grasshopper species Melanoplus sanguinipes to other C6-alcohols, aldehydes and esters.^15,16 A agromyzid fly species Liliomyza sativae displayed a higher EAG response to C6-alcohols than to the alcohols with longer or shorter chains, and it also showed higher EAG responses to synthetic C6-alcohols (hexanol, (Z)-3-hexenol and (E)-2-hexenol) than to C6-aldehyde (hexenal) and -acetate [(E)-3-hexenyl acetate]; however, the geometric isomers of 3-hexenol led to similar EAG amplitude of leafminers.¹⁷ Despite these advances in electrophysiological studies, behavioral bioassays in laboratories and in fields are essential for exploring the roles of (Z)-3-hexenol in the host selection of herbivores. It has been suggested for long that (Z)-3-hexenol can significantly reduce the preference and performance of aphids.¹⁸ In contrast, the adult emerald ash borer, Agrilus planipennis, was more attractive to the (Z)-3-hexenol containing purple prism traps than to the unbaited controls; supplement of other C6-volatiles such as (Z)-3-hexenal or (Z)-3-hexenyl acetate in binary or ternary combinations, did not show any synergistic effect.¹⁹ In another study, the mixture of C6-volatiles ((Z)-3-hexenol and (Z)-3-hexenyl acetate) and benzaldehyde in a natural ratio, but not individual compound, attracted more female fruit moth, Cydia molesta.²⁰ Furthermore, the preference analysis of adult flies or moths for JA-deficient and wild-type (WT) plants revealed that C6-volatiles, including (Z)-3-hexenol, stimulate the behavioral preferences of these herbivores for host plants.^10,11 In addition to adult, larval herbivores also respond to (Z)-3-hexenol. The neonate larvae of Asian corn borer Ostrinia furnacalis showed repellent behavior to the synthetic (Z)-3-hexenol,²¹ while the Lepidopteran herbivore's larvae Manduca sexta preferred wild-type (WT) Nicotiana attenuate plants over GLV-deficient mutants, suggesting that C6-volatiles are able to elicit the feeding of these insects.²² Although it is difficult to conclude whether (Z)-3-hexenol is an attractant or a repellent, accumulating evidences suggested that (Z)-3-hexenol is, at least in part, important plant-derived infochemical that modulates the behavior of herbivorous insects, and the release of (Z)-3-hexenol should be the defensive responses of the plants.

Response of Natural Enemies to (Z)-3-hexenol

C6-alcohols have long been involved in indirect plant defense in tritrophic interactions. Extensive evidences imply that nearly all the natural enemies can perceive and positively respond to these volatile compounds. A pioneer study revealed that Manduca sexta larvae-induced release of (Z)-3-hexenol, linalool and cis-abergamotene functioned as indirect defense by attracting predators, which resulted in increasing herbivore mortality in the field.²³ Subsequently, with the combined techniques of chemical analysis (GC-MS), electrophysiological detection (GC-EAD) and behavioral bioassays, several studies showed that (Z)-3-hexenol is not only the most common inducible chemical in 10 plant species from seven families when they were wounded by herbivore insects or physical damage, but also the most important infochemical to attract parasitic wasps compared with other inducible chemicals and their blends.^5,24 Moreover, these results proved that the reduced release of (Z)-3-hexenol in JA-deficient tomato mutant (spr2) led to decreased level of attractiveness and parasitism by leafminer's parasitoid compared with WT plants, confirming the distinct role of (Z)-3-hexenol in the host finding of parasitoids.¹¹ A recent study demonstrated that the oral secretion of larval Manduca sexta is able to rapidly convert the Z-isomers of hexenal, hexenol and hexenyl acetate to the corresponding E-isomers, and the (Z)/(E) ratio changes increase the foraging success of the generalist hemipteran predator Geocoris spp. in the field.²⁵ These studies clearly indicated the innate responses of natural enemies to C6-volatiles. In fact, the odor learning by natural enemies is also an important mechanism for host location. The synchronization of biological rhythms in the tritrophic interactions of Lima bean, leafminer and parasitoid showed that the parasitoid emergence coincided with (Z)-3-hexen-ol emission, and the naive wasps displayed strong preference for this chemical. In contrast, the oviposition-experienced wasps preferred both (Z)-3-hexenol- and terpene-dominated volatiles released in the peak time of leafminer larvae feeding under the photoperiod of light:dark regimes, which optimized their parasitizations.²⁶ The results suggested that the feeding of leafminer, the defense chemical biosynthesis of plant, and the activities of parasitoid in this system can achieve rhythmic synchronization under natural light-dark photoperiod, but not under constant lightness or darkness.

(Z)-3-hexenol in Plant-plant Communication

In addition to the defensive role in tritrophic interaction, airborne (Z)-3-hexenol from wounded plants is also proposed to trigger the pre-defense responses of neighboring unattacked plants; this phenomenon is called plant-plant communication or priming effect of volatiles. A novel study showed that after mechanical damage and caterpillar regurgitation, corn seedlings pre-exposed to synthetic (Z)-3-hexen-ol produced significantly higher level of JA and volatile sesquiterpenes than the seedlings without previous exposure.²⁷ Another study demonstrated that simultaneous exposure to (Z)-3-hexenol and ethylene increased the herbivore-induced volatile emissions in intact maize plants.²⁸ Recently, our group performed a transcriptional analysis of Arabidopsis thaliana defensive response to priming volatiles, tand the results showed that (Z)-3-hexenol can activate the gene expression in the defense related pathways of Arabidopsis (Zhang S, Wei J and Kang L, unpublished data). However, the exact role of (Z)-3-hexenol in priming process requires further investigation.

Perspective

Here, we reviewed the current knowledge about the defensive roles of (Z)-3-hexenol in tritrophic interactions and its priming effects in plant-plant communication. Benefit from the recent progress on plant-insect interactions using advanced chemical analysis and plant materials genetically modified in single trait of signaling pathways,^9–11,25 the functions of airborne (Z)-3-hexenol in plant defenses and communications become more and more clear. However, due to the limited number of available model systems (Arabidopsis, Nicotiana and Lima bean plants) and the relatively simple interactions among plants, herbivores and natural enemies, the exact mechanisms of volatile production, the olfactory responses of herbivores and natural enemies and the signal transduction and underlying molecular mechanisms of boarder communities and more complex systems still remain elusive. Therefore, other model systems need to be developed to understand the exact functions of plant-derived ubiquitous compounds in plant-insect interactions. In addition, a conflict in (Z)-3-hexenol activity is expected because it is potentially used by both herbivores and natural enemies to locate the host and host prey in multitrophic interactions under natural conditions. However, we believe that the major role of this compound is to mediate indirect defense and to prime neighboring plants. Thus, (Z)-3-hexenol can be used to develop insect pest control strategies. Further investigation on its quantitative and qualitative administration under natural condition is needed to elucidate the underlying mechanisms and to develop novel control strategies.