Plants cannot avoid being attacked by an almost infinite number of microorganisms and insects. Consequently, they arm themselves with molecular weapons against their attackers. Plant defense responses are the result of a complex signaling network, in which the hormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET) are the usual suspects under the magnifying glass when researchers investigate host-pest interactions. However, Green Leaf Volatiles (GLVs), C6 molecules, which are very quickly produced and/or emitted upon herbivory or pathogen infection by almost every green plant, also play an important role in plant defenses. GLVs are semiochemicals used by insects to find their food or their conspecifics. They have also been reported to be fundamental in indirect defenses and to have a direct effect on pests, but these are not the only roles of GLVs. These volatiles, being probably one of the fastest weapons exploited, are also able to directly elicit or prime plant defense responses. Moreover, GLVs, via crosstalk with phytohormones, mostly JA, can influence the outcome of the plant’s defense response against pathogens. For all these reasons GLVs should be considered as co-protagonists in the play between plants and their attackers.

Summary666I.Introduction667II.Biosynthesis667III.Meta‐analysis669IV.The type of stress influences the total amount of GLVs released669V.Herbivores can modulate the wound‐induced release of GLVs669VI.Fungal infection greatly induces GLV production672VII.Monocots and eudicots respond differentially to different types of stress673VIII.The type of stress does not influence the proportion of GLVs per chemical class673IX.The type of stress does influence the isomeric ratio within each chemical class674X.GLVs: from signal perception to signal transduction676XI.GLVs influence the C/N metabolism677XII.Interaction with plant hormones678XIII.General conclusions and unanswered questions678Acknowledgements679References679 Summary Plants respond to stress by releasing biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs), which are abundantly produced across the plant kingdom, comprise an important group within the BVOCs. They can repel or attract herbivores and their natural enemies; and they can induce plant defences or prime plants for enhanced defence against herbivores and pathogens and can have direct toxic effects on bacteria and fungi. Unlike other volatiles, GLVs are released almost instantly upon mechanical damage and (a)biotic stress and could thus function as an immediate and informative signal for many organisms in the plant's environment. We used a meta‐analysis approach in which data from the literature on GLV production during biotic stress responses were compiled and interpreted. We identified that different types of attackers and feeding styles add a degree of complexity to the amount of emitted GLVs, compared with wounding alone. This meta‐analysis illustrates that there is less variation in the GLV profile than we presumed, that pathogens induce more GLVs than insects and wounding, and that there are clear differences in GLV emission between monocots and dicots. Besides the meta‐analysis, this review provides an update on recent insights into the perception and signalling of GLVs in plants.

Plants emit green leaf volatiles (GLVs) in response to herbivore damage, thereby attracting predators of the herbivores as part of an indirect defense. The GLV component of this indirect defense was thought to be a general wound signal lacking herbivore-specific information. We found that Manduca sexta-infested Nicotiana attenuata attract the generalist hemipteran predator Geocoris spp. as the result of an herbivore-induced decrease in the (Z)/(E) ratio of released GLVs, and that these changes in the volatile bouquet triple the foraging efficiency of predators in nature. These (E)-isomers are produced from plant-derived (Z)-isomers but are converted by a heat-labile constituent of herbivore oral secretions. Hence, attacking herbivores initiate the release of an indirect defense a full day before the attacked plants manufacture their own defensive compounds.

Whereas jasmonic acid (JA) and its amino acid conjugates, particularly JA-isoleucine (Ile), are known to play important roles in plant-herbivore interactions, whether other compounds also function as signals independently of JA-Ile and whether conjugates elicit systemic responses are unknown. To answer these questions, we simultaneously silenced JASMONATE-RESISTANT4 (JAR4) and JAR6, two functionally redundant enzymes in Nicotiana attenuata that conjugate JA to amino acids to produce plants (irjar4/6) with low levels of JA-Ile, JA-leucine (Leu), and JA-valine (Val; ,16% of wild type). As expected, irjar4/6 plants are more vulnerable to herbivore attack, but only JA-Ile-not JA-Leu or JA-Val-applications restored the resistance of irjar4/6 plants, suggesting that JA-Leu and JA-Val do not mediate herbivore defense responses. Interestingly, the direct defense traits of irjar4/6 plants are significantly higher than those in LIPOXYGENASE3 (LOX3)-silenced (aslox3) plants, which are impaired in JA biosynthesis, and JA-Ile treatment could not fully restore the resistance of aslox3 plants. We thus conclude that JA, its precursors, or other metabolites complement the function of JA-Ile by eliciting a panoply of induced defenses. Similarly, transcriptional profiling of wild-type, irjar4/6, and aslox3 plants with microarrays demonstrated that JA-Ile and JA play overlapping yet distinct roles in herbivore defense. Analysis of transcripts in distal tissues demonstrated that JAR activity is essential in eliciting systemic responses. However, attempts to recover JA-13 C 6 -Ile in systemic leaves and roots after feeding wounded leaves with 13 C 6 -Ile were unsuccessful, suggesting that JA-Ile is not a long-distance signal, but is rather synthesized after the arrival of an unknown mobile signal to systemic tissues.

BackgroundHerbivore feeding elicits dramatic increases in defenses, most of which require jasmonate (JA) signaling, and against which specialist herbivores are thought to be better adapted than generalist herbivores. Unbiased transcriptional analyses of how neonate larvae cope with these induced plant defenses are lacking.Methodology/Principal FindingsWe created cDNA microarrays for Manduca sexta and Heliothis virescens separately, by spotting normalized midgut-specific cDNA libraries created from larvae that fed for 24 hours on MeJA-elicited wild-type (WT) Nicotiana attenuata plants. These microarrays were hybridized with labeled probes from neonates that fed for 24 hours on WT and isogenic plants progressively silenced in JA-mediated defenses (N: nicotine; N/PI: N and trypsin protease inhibitors; JA: all JA-mediated defenses). H. virescens neonates regulated 16 times more genes than did M. sexta neonates when they fed on plants silenced in JA-mediated defenses, and for both species, the greater the number of defenses silenced in the host plant (JA > N/PI > N), the greater were the number of transcripts regulated in the larvae. M. sexta larvae tended to down-regulate while H. virescens larvae up- and down-regulated transcripts from the same functional categories of genes. M. sexta larvae regulated transcripts in a diet-specific manner, while H. virescens larvae regulated a similar suite of transcripts across all diet types.Conclusions/SignificanceThe observations are consistent with the expectation that specialists are better adapted than generalist herbivores to the defense responses elicited in their host plants by their feeding. While M. sexta larvae appear to be better adapted to N. attenuata's defenses, some of the elicited responses remain effective defenses against both herbivore species. The regulated genes provide novel insights into larval adaptations to N. attenuata's induced defenses, and represent potential targets for plant-mediated RNAi to falsify hypotheses about the process of adaptation.

Lipoxygenases (LOXs) are key enzymes in the biosynthesis of oxylipins, and catalyse the formation of fatty acid hydroperoxides (HPs), which represent the first committed step in the synthesis of metabolites that function as signals and defences in plants. HPs are the initial substrates for different branches of the oxylipin pathway, and some plant species may express different LOX isoforms that supply specific branches. Here, we compare isogenic lines of the wild tobacco Nicotiana attenuata with reduced expression of NaLOX2 (irlox2) or NaLOX3 (irlox3) to determine the role of these different LOX isoforms in supplying substrates for two different pathways: green leaf volatiles (GLVs) and jasmonic acid (JA). Reduced NaLOX2 expression strongly decreased the production of GLVs without influencing the formation of JA and JA-related secondary metabolites. Conversely, reduced NaLOX3 expression strongly decreased JA biosynthesis, without influencing GLV production. The temporal expression of NaLOX2 and NaLOX3 also differed after elicitation; NaLOX3 was rapidly induced, attaining highest transcript levels within 1 h after elicitation, whereas NaLOX2 transcripts reached maximum levels after 14 h. These results demonstrate that N. attenuata channels the flux of HPs through the activities of different LOXs, leading to different direct and indirect defence responses mediating the plant's herbivore resistance.

The ability to decrypt volatile plant signals is essential if herbivorous insects are to optimize their choice of host plants for their offspring. Green leaf volatiles (GLVs) constitute a widespread group of defensive plant volatiles that convey a herbivory-specific message via their isomeric composition: feeding of the tobacco hornworm Manduca sexta converts (Z)-3- to (E)-2-GLVs thereby attracting predatory insects. Here we show that this isomer-coded message is monitored by ovipositing M. sexta females. We detected the isomeric shift in the host plant Datura wrightii and performed functional imaging in the primary olfactory center of M. sexta females with GLV structural isomers. We identified two isomer-specific regions responding to either (Z)-3- or (E)-2-hexenyl acetate. Field experiments demonstrated that ovipositing Manduca moths preferred (Z)-3-perfumed D. wrightii over (E)-2-perfumed plants. These results show that (E)-2-GLVs and/or specific (Z)-3/(E)-2-ratios provide information regarding host plant attack by conspecifics that ovipositing hawkmoths use for host plant selection.DOI: http://dx.doi.org/10.7554/eLife.00421.001

SUMMARYThe role of methyl salicylate (MeSA) production was studied in indirect and direct defence responses of tomato (Solanum lycopersicum) to the spider mite Tetranychus urticae and the root-invading fungus Fusarium oxysporum f. sp. lycopersici, respectively. To this end, we silenced the tomato gene encoding salicylic acid methyl transferase (SAMT). Silencing of SAMT led to a major reduction in SAMT expression and MeSA emission upon herbivory by spider mites, without affecting the induced emission of other volatiles (terpenoids). The predatory mite Phytoseiulus persimilis, which preys on T. urticae, could not discriminate between infested and non-infested SAMT-silenced lines, as it could for wild-type tomato plants. Moreover, when given the choice between infested SAMT-silenced and infested wild-type plants, they preferred the latter. These findings are supportive of a major role for MeSA in this indirect defence response of tomato. SAMT-silenced tomato plants were less susceptible to a virulent strain of F. oxysporum f. sp. lycopersici, indicating that the direct defense responses in the roots are also affected in these plants. Our studies show that the conversion of SA to MeSA can affect both direct and indirect plant defence responses.