绿叶挥发物产生特征及其生态生理作用研究进展

该文系统综述了植物绿叶挥发物(green leaf volatiles, GLVs)的形成特征、对植物的生态生理作用及调控机制等方面的最新研究进展。GLVs是指植物经十八碳烷酸途径过氧化物酶分支途径产生的含6个碳原子的醛、醇及其酯。除叶片外, 植物的根、茎、果实、种子等部位均可以合成该类化合物, 合成调控存在转录和转录后水平的调控。在特定植物中, GLVs合成及其组分还受到植物生长发育阶段和生长季节等外源环境的影响。昆虫啃食、微生物感染以及有益真菌的定植等生物逆境与缺氮等物理逆境均具有诱导GLVs合成的作用。除了参与植物特有气味形成外, GLVs在植物直接和间接防御应答中发挥着重要的作用。GLVs不仅具有抑制微生物和多种昆虫繁殖的作用, 还具有诱导多种防御化合物合成、预置(prime)有关信号途径的作用。基于GLVs在植物界的广泛存在性和在防御应答中的多层次作用, 该文作者提出了GLVs在道地药材品质形成中的潜在作用及开展相关研究的必要性和紧迫性。     

Green leaf volatiles: biosynthesis,biological functions and their applications in biotechnology

Plants have evolved numerous constitutive and inducible defence mechanisms to cope with biotic and abiotic stresses. These stresses induce the expression of various genes to activate defence‐related pathways that result in the release of defence chemicals. One of these defence mechanisms is the oxylipin pathway, which produces jasmonates, divinylethers and green leaf volatiles (GLVs) through the peroxidation of polyunsaturated fatty acids (PUFAs). GLVs have recently emerged as key players in plant defence, plant–plant interactions and plant–insect interactions. Some GLVs inhibit the growth and propagation of plant pathogens, including bacteria, viruses and fungi. In certain cases, GLVs released from plants under herbivore attack can serve as aerial messengers to neighbouring plants and to attract parasitic or parasitoid enemies of the herbivores. The plants that perceive these volatile signals are primed and can then adapt in preparation for the upcoming challenges. Due to their ‘green note’ odour, GLVs impart aromas and flavours to many natural foods, such as vegetables and fruits, and therefore, they can be exploited in industrial biotechnology. The aim of this study was to review the progress and recent developments in research on the oxylipin pathway, with a specific focus on the biosynthesis and biological functions of GLVs and their applications in industrial biotechnology.     

Effects of heat and drought stress on post‐illumination bursts of volatile organic compounds in isoprene‐emitting and non‐emitting poplar

Over the last decades, post‐illumination bursts (PIBs) of isoprene, acetaldehyde and green leaf volatiles (GLVs) following rapid light‐to‐dark transitions have been reported for a variety of different plant species. However, the mechanisms triggering their release still remain unclear. Here we measured PIBs of isoprene‐emitting (IE) and isoprene non‐emitting (NE) grey poplar plants grown under different climate scenarios (ambient control and three scenarios with elevated CO₂ concentrations: elevated control, periodic heat and temperature stress, chronic heat and temperature stress, followed by recovery periods). PIBs of isoprene were unaffected by elevated CO₂ and heat and drought stress in IE, while they were absent in NE plants. On the other hand, PIBs of acetaldehyde and also GLVs were strongly reduced in stress‐affected plants of all genotypes. After recovery from stress, distinct differences in PIB emissions in both genotypes confirmed different precursor pools for acetaldehyde and GLV emissions. Changes in PIBs of GLVs, almost absent in stressed plants and enhanced after recovery, could be mainly attributed to changes in lipoxygenase activity. Our results indicate that acetaldehyde PIBs, which recovered only partly, derive from a new mechanism in which acetaldehyde is produced from methylerythritol phosphate pathway intermediates, driven by deoxyxylulose phosphate synthase activity.     

Orientation of European corn borer first instar larvae to synthetic green leaf volatiles

European corn borer (ECB) neonate larvae are capable of orienting towards maize odours and of avoiding spinach odours. We previously reported that maize odours’ attraction was dependent on the stimulus regime. This led us to propose that maize odours could have a repellent or attractive effect depending on their concentration. In this work, we tested this hypothesis by evaluating attraction or avoidance of neonate ECB larvae to four concentrations of each of six single green leaf volatiles (GLVs); these are commonly found in maize and other plants. We found a dose‐dependent effect for all of these GLVs with the exception of 1‐hexyl acetate, which did not elicit any orientation behaviour over the range of concentrations tested. These five GLVs were repellent at high concentrations, while two of them were attractive at a lower concentration. These observations indicate for the first time that plant odours induce different behaviours in ECB neonate larvae depending not only on their chemical identity but also their concentration.     

Pollinator attraction of the wasp-flower Scrophularia umbrosa (Scrophulariaceae)

Certain species of Scrophularia (Scrophulariaceae), such as S. nodosa and S. umbrosa, are mainly pollinated by social wasps and are consequently described as wasp-flowers. Because plants attract their pollinators with the help of various floral cues, such as floral odour and/or optical cues, we have investigated the role of olfactory and visual floral signals responsible for wasp attraction in S. umbrosa. Using a combination of chemical (GC, GC-MS) and electrophysiological analyses (GC-EAD), we identified ten compounds in the complex floral odour bouquet that are detectable by the wasps' antennae. As in the wasp-flower Epipactis helleborine, we found so-called 'green leaf volatiles' (GLVs) in the floral odour; these GLVs are highly attractive to the wasps. GLVs, mostly six-carbon aldehydes, alcohols and acetates, and other volatile organic compounds (VOCs), are emitted by many plants infested with herbivores, e.g. caterpillars. In contrast to other investigated wasp-flowers, behavioural experiments have demonstrated that, in addition to the floral odour of S. umbrosa, visual cues are involved in pollinator attraction.     

Salicylhydroxamic acid (SHAM) negatively mediates tea herbivore-induced direct and indirect defense against the tea geometrid Ectropis obliqua

We investigated the effect of the SHAM treatment of tea plants on their induced defense on a tea geometrid (TG), Ectropis obliqua Prout. Treatment of tea leaves with SHAM reduced the performance of TG and TG-elicited level of the lipoxygenase gene CsiLOX1 and the putative allene oxide synthase gene CsiAOS1. The release of wound-induced green leaf volatiles (GLVs) and the expression of the hydroperoxide lyase (HPL) gene CsiHPL1 were also reduced by SHAM treatment. The negative effect of SHAM dramatically reduced the total hebivore-induced plant volatiles (HIPVs) and the attractiveness to the parasitoid wasp Apanteles sp. These results indicated that SHAM may negatively mediate tea defense response against TG by modulating the wound-induced emission of GLVs, the expression of genes involved in oxylipin pathway, and the emission of other HIPV compounds that mediate direct and indirect defenses.     

Shared signals -'alarm calls' from plants increase apparency to herbivores and their enemies in nature

The attraction of natural enemies of herbivores by volatile organic compounds as an induced indirect defence has been studied in several plant systems. The evidence for their defensive function originates mainly from laboratory studies with trained parasitoids and predators; the defensive function of these emissions for plants in natural settings has been rarely demonstrated. In native populations and laboratory Y-tube choice experiments with transgenic Nicotiana attenuata plants unable to release particular volatiles, we demonstrate that predatory bugs use terpenoids and green leaf volatiles (GLVs) to locate their prey on herbivore-attacked plants. By attracting predators with volatile signals, this native plant reduces its herbivore load – demonstrating the defensive function of herbivore-induced volatile emissions. However, plants producing GLVs are also damaged more by flea beetles. The implications of these conflicting ecological effects for the evolution of induced volatile emissions and for the development of sustainable agricultural practices are discussed.     

Dispensing synthetic green leaf volatiles in maize fields increases the release of sesquiterpenes by the plants, but has little effect on the attraction of pest and beneficial insects

Maize plants respond to feeding by arthropod herbivores by producing a number of secondary plant compounds, including volatile organic compounds (VOCs). These herbivore-induced VOCs are not only known to attract natural enemies of the herbivores, but they may also prime inducible defences in neighbouring plants, resulting in stronger and faster defence responses in these VOC-exposed plants. Among the compounds that cause this priming effect, green leaf volatiles (GLVs) have received particular attention, as they are ubiquitous and rapidly emitted upon damage. In this study, we investigated their effects under realistic conditions by applying specially devised dispensers to release four synthetic GLVs at physiologically relevant concentrations in a series of experiments in maize fields. We compared the VOC emission of GLV-exposed maize plants to non-exposed plants and monitored the attraction of herbivores and predators, as well as parasitism of the caterpillar Spodoptera frugiperda, the most common herbivore in the experimental maize fields. We found that maize plants that were exposed to GLVs emitted increased quantities of sesquiterpenes compared to non-exposed plants. In several replicates, herbivorous insects, such as adult Diabrotica beetles and S. frugiperda larvae, were observed more frequently in GLV-treated plots and caused more damage to GLV-exposed plants than to non-exposed plants. Parasitism of S. frugiperda was only weakly affected by GLVs and overall parasitism rates of S. frugiperda were similar in GLV-exposed and non-exposed plots. The effects on insect presence depended on the distance from the GLV-dispensers at which the plants were located. The results are discussed in the context of strategies to improve biological control by enhancing plant-mediated attraction of natural enemies.     

Green leaf volatiles and oxygenated metabolite emission bursts from mesquite branches following light–dark transitions

Green leaf volatiles (GLVs) are a diverse group of fatty acid-derived compounds emitted by all plants and are involved in a wide variety of developmental and stress-related biological functions. Recently, GLV emission bursts from leaves were reported following light–dark transitions and hypothesized to be related to the stress response while acetaldehyde bursts were hypothesized to be due to the ‘pyruvate overflow’ mechanism. In this study, branch emissions of GLVs and a group of oxygenated metabolites (acetaldehyde, ethanol, acetic acid, and acetone) derived from the pyruvate dehydrogenase (PDH) bypass pathway were quantified from mesquite plants following light–dark transitions using a coupled GC–MS, PTR-MS, and photosynthesis system. Within the first minute after darkening following a light period, large emission bursts of both C₅ and C₆ GLVs dominated by (Z)-3-hexen-1-yl acetate together with the PDH bypass metabolites are reported for the first time. We found that branches exposed to CO₂-free air lacked significant GLV and PDH bypass bursts while O₂-free atmospheres eliminated the GLV burst but stimulated the PDH bypass burst. A positive relationship was observed between photosynthetic activity prior to darkening and the magnitude of the GLV and PDH bursts. Photosynthesis under ¹³CO₂ resulted in bursts with extensive labeling of acetaldehyde, ethanol, and the acetate but not the C₆-alcohol moiety of (Z)-3-hexen-1-yl acetate. Our observations are consistent with (1) the “pyruvate overflow” mechanism with a fast turnover time (<1 h) as part of the PDH bypass pathway, which may contribute to the acetyl-CoA used for the acetate moiety of (Z)-3-hexen-1-yl acetate, and (2) a pool of fatty acids with a slow turnover time (>3 h) responsible for the C₆ alcohol moiety of (Z)-3-hexen-1-yl acetate via the 13-lipoxygenase pathway. We conclude that our non-invasive method may provide a new valuable in vivo tool for studies of acetyl-CoA and fatty acid metabolism in plants at a variety of spatial scales.     

13C-labelling patterns of green leaf volatiles indicating different dynamics of precursors in Brassica leaves

To investigate the dynamics of precursor compounds of green leaf volatiles (GLV)s and other biogenic compounds released by mechanically damaged Brassica oleracea leaves, plants were exposed for two consecutive 16h light phases to highly enriched (13)CO(2). Analysis by GC-MS indicated (1) biogenic compounds released upon wounding, (2) a different labelling pattern between and (3) within compounds, and (4) evidence for spatial heterogeneity of the precursor pool extrapolated from points (1)-(3). First, GLVs comprised C(5) and C(6) molecules, with the GLV pentenyl acetate being reported here for the first time from higher plants. Second, the labelling pattern found in most GLVs indicates a low turnover of the precursor alpha-linolenic acid. Moderate labelling of dimethyldisulphide indicates a connection to an active plastidic methyl pool closely connected to CO(2) fixation, and very weak labelling of terpenes indicates a constitutive monoterpene pool. Third, not all GLVs exhibit similarly strong labelling patterns (hexenyl acetate vs. hexyl acetate), indicating different precursors. As the labelling patterns of alcohol and acetate moieties in the esters differ, with only the former being strongly labelled, the precursor of the acetate moiety, acetyl-CoA, is likely to derive from a different cellular pool to that used in chloroplastic fatty acid synthesis, or was rapidly synthesised after the end of labelling. Fourth, the exceptionally high relative abundance of labelled GLV and the low concentration of unlabelled molecules are likely to occur because recently synthesized alpha-linolenic acid is bound in lipids that are organised in distinct areas, or are chemically different from the older lipids. They must be preferentially used as precursors.     

The rice hydroperoxide lyase OsHPL3 functions in defense responses by modulating the oxylipin pathway

As important signal molecules, jasmonates (JAs) and green leaf volatiles (GLVs) play diverse roles in plant defense responses against insect pests and pathogens. However, how plants employ their specific defense responses by modulating the levels of JA and GLVs remains unclear. Here, we describe identification of a role for the rice HPL3 gene, which encodes a hydroperoxide lyase (HPL), OsHPL3/CYP74B2, in mediating plant‐specific defense responses. The loss‐of‐function mutant hpl3‐1 produced disease‐resembling lesions spreading through the whole leaves. A biochemical assay revealed that OsHPL3 possesses intrinsic HPL activity, hydrolyzing hydroperoxylinolenic acid to produce GLVs. The hpl3‐1 plants exhibited enhanced induction of JA, trypsin proteinase inhibitors and other volatiles, but decreased levels of GLVs including (Z)‐3‐hexen‐1‐ol. OsHPL3 positively modulates resistance to the rice brown planthopper [BPH, Nilaparvata lugens (Stål)] but negatively modulates resistance to the rice striped stem borer [SSB, Chilo suppressalis (Walker)]. Moreover, hpl3‐1 plants were more attractive to a BPH egg parasitoid, Anagrus nilaparvatae, than the wild‐type, most likely as a result of increased release of BPH‐induced volatiles. Interestingly, hpl3‐1 plants also showed increased resistance to bacterial blight (Xanthomonas oryzae pv. oryzae). Collectively, these results indicate that OsHPL3, by affecting the levels of JA, GLVs and other volatiles, modulates rice‐specific defense responses against different invaders.     

三种绿叶挥发性物质对云南切梢小蠹寄主定位行为的干扰作用

为了研究环境中非寄主阔叶植物释放出的绿叶挥发性物质（GLVs）对针叶树蛀干害虫云南切梢小蠹Tomicus yunnanesis的影响, 选取了(E)-2-己烯醛、 (E)-2-己烯醇和(Z)-3-己烯醇3种释放量较大的绿叶挥发性物质, 通过室内松梢取食试验测试了单组分及两两混合后对云南切梢小蠹寄主定位行为的干扰作用。结果表明： 源于阔叶植物的3种绿叶挥发性物质及其混合物能够不同程度干扰云南切梢小蠹的寄主定位行为。当虫放入广口瓶12 h后, 3个单组分绿叶挥发性物质处理组［A： (E)-2-己烯醛, P＜0.01; B： (E)-2-己烯醇, P＜0.01; C： (Z)-3-己烯醇, P＜0.01］及2个混合组分［D： (E)-2-己烯醛+(E)-2-己烯醇, P＜0.01）; E： (E)-2-己烯醛+(Z)-3-己烯醇, P＜0.01］中滞留在松梢外部的虫数与对照组相比都有显著性差异, 绿叶挥发性物质的存在显著降低了云南切梢小蠹侵害云南松松梢的概率。但是, 24 h后只有D组（P＜0.01）和E组（P＜0.01）滞留在松梢外部的虫数与对照组相比具有显著性差异, 在48 h后只有D组（P＜0.01）与对照相比仍具有显著性差异。本研究为利用非寄主植物的次生代谢产物防治云南切梢小蠹进行了有益的探索。     

Wound-induced green leaf volatiles cause the release of acetylated derivatives and a terpenoid in maize

Green leaf volatiles (GLVs), generally occurring C6 alcohols, aldehydes and acetates from plants, play an important role in plant-plant communication. These compounds induce intact plants to produce jasmonic acid, and induce defense-related gene expression and the release of volatile compounds. Here, we address wound-induced GLVs cause the release of acetylated derivatives and a terpenoid, (E)-4,8-dimethylnona-1,3,7-triene (DMNT) in intact maize, which may be a type of plant-plant interaction mediated by airborne GLVs. Upon exposure of intact maize seedlings to wound-induced GLVs, (Z)-3-hexenyl acetate was consistently the most abundant compound released. Exogenous application of individual alcohols and aldehydes mostly resulted in the release of corresponding acetate esters. C6-alcohols with a double bond between the second and third, or the third and fourth carbon atoms, C5- or C6-aldehydes, and (Z)-3-hexenyl acetate triggered the release of DMNT. When (Z)-3-hexenyl acetate and hexyl acetate were used to treat maize seedlings, they were recovered from the plants. These data demonstrated that: (1) apart from direct adsorption and re-release of acetate esters, absorption and conversion of exogenous alcohols and aldehydes into acetate esters occurred, and (2) DMNT was induced by a range of aldehydes and unsaturated alcohols.     

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

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.Key words: green leaf volatiles, (Z)-3-hexenol, direct defense, indirect defense, primingTo 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.     

Differential metabolisms of green leaf volatiles in injured and intact parts of a wounded leaf meet distinct ecophysiological requirements

Almost all terrestrial plants produce green leaf volatiles (GLVs), consisting of six-carbon (C6) aldehydes, alcohols and their esters, after mechanical wounding. C6 aldehydes deter enemies, but C6 alcohols and esters are rather inert. In this study, we address why the ability to produce various GLVs in wounded plant tissues has been conserved in the plant kingdom. The major product in completely disrupted Arabidopsis leaf tissues was (Z)-3-hexenal, while (Z)-3-hexenol and (Z)-3-hexenyl acetate were the main products formed in the intact parts of partially wounded leaves. (13)C-labeled C6 aldehydes placed on the disrupted part of a wounded leaf diffused into neighboring intact tissues and were reduced to C6 alcohols. The reduction of the aldehydes to alcohols was catalyzed by an NADPH-dependent reductase. When NADPH was supplemented to disrupted tissues, C6 aldehydes were reduced to C6 alcohols, indicating that C6 aldehydes accumulated because of insufficient NADPH. When the leaves were exposed to higher doses of C6 aldehydes, however, a substantial fraction of C6 aldehydes persisted in the leaves and damaged them, indicating potential toxicity of C6 aldehydes to the leaf cells. Thus, the production of C6 aldehydes and their differential metabolisms in wounded leaves has dual benefits. In disrupted tissues, C6 aldehydes and their α,β-unsaturated aldehyde derivatives accumulate to deter invaders. In intact cells, the aldehydes are reduced to minimize self-toxicity and allow healthy cells to survive. The metabolism of GLVs is thus efficiently designed to meet ecophysiological requirements of the microenvironments within a wounded leaf.     

Green leaf volatiles protect maize (Zea mays) seedlings against damage from cold stress

Although considerable evidence has accumulated on the defensive activity of plant volatile organic compounds against pathogens and insect herbivores, less is known about the significance of volatile organic compounds emitted by plants under abiotic stress. Here, we report that green leaf volatiles (GLVs), which were previously shown to prime plant defences against insect herbivore attack, also protect plants against cold stress (4 °C). We show that the expression levels of several cold stress‐related genes are significantly up‐regulated in maize (Zea mays) seedlings treated with physiological concentrations of the GLV, (Z)‐3‐hexen‐1‐yl acetate (Z‐3‐HAC), and that seedlings primed with Z‐3‐HAC exhibit increased growth and reduced damage after cold stress relative to unprimed seedlings. Together, these data demonstrate the protective and priming effect of GLVs against cold stress and suggest an activity of GLVs beyond the activation of typical plant defence responses against herbivores and pathogens.