Synergistic interactions between volicitin,jasmonic acid and ethylene mediate insect-induced volatile emission in Zea mays

Plants display differential responses following mechanical damage and insect herbivory. Both caterpillar attack and the application of caterpillar oral secretions (OS) to wounded leaves stimulates volatile emission above mechanical damage alone. Volicitin ( N- 17-hydroxylinolenoyl- l -glutamine), present in beet armyworm (BAW, Spodoptera exigua ) OS, is a powerful elicitor of volatiles in excised maize seedlings ( Zea mays cv. Delprim). We consider some of the mechanistic differences between wounding and insect herbivory in maize by examining the activity of volicitin, changes in jasmonic acid (JA) levels, and volatile emission from both intact plant and excised leaf bioassays. Compared to mechanical damage alone, volicitin stimulated increases in both JA levels and sesquiterpene volatiles when applied to intact plants. In a bioassay comparison, excised leaves were more sensitive and produced far greater volatile responses than intact plants following applications of both volicitin and JA. In the excised leaf bioassay, volicitin applications (10–500 pmol) to wounded leaves resulted in dose dependent JA increases and a direct positive relationship between JA and sesquiterpene volatile emission. Interestingly, volicitin-induced JA levels did not differ between intact and excised bioassays, suggesting a possible interaction of JA with other regulatory signals in excised plants. In addition to JA, insect herbivory is known to stimulate the production of ethylene. Significant increases in ethylene were induced only by BAW herbivory and not by either wounding or volicitin treatments. Using intact plant bioassays, ethylene (at 1 µl l⁻¹ or less) greatly promoted volatile emission induced by volicitin and JA but not mechanical damage alone. For intact plants, wounding, elicitor-induced JA and insect-induced ethylene appear to be important interacting components in the stimulation of insect-induced volatile emission.     

Wound and insect‐induced jasmonate accumulation in carnivorous Drosera capensis: two sides of the same coin

Carnivorous sundew plants catch and digest insect prey for their own nutrition. The sundew species Drosera capensis shows a pronounced leaf bending reaction upon prey capture in order to form an ‘outer stomach’. This formation is triggered by jasmonates, phytohormones typically involved in defence reactions against herbivory and wounding. Whether jasmonates still have this function in D. capensis in addition to mediating the leaf bending reaction was investigated here. Wounded, insect prey‐fed and insect‐derived oral secretion‐treated leaves of D. capensis were analysed for jasmonates (jasmonic acid, JA; jasmonic acid‐isoleucine conjugate, JA‐Ile) using LC‐MS/MS. Prey‐induced jasmonate accumulation in D. capensis leaves was persistent, and showed high levels of JA and JA‐Ile (575 and 55.7 pmol·g·FW^?1, respectively), whereas wounding induced a transient increase of JA (maximum 500 pmol·g·FW^?1) and only low (3.1 pmol·g·FW^?1) accumulation of JA‐Ile. Herbivory, mimicked with a combined treatment of wounding plus oral secretion (W+OS) obtained from Spodoptera littoralis larvae induced both JA (4000 pmol·g·FW^?1) and JA‐Ile (25 pmol·g·FW^?1) accumulation, with kinetics similar to prey treatment. Only prey and W+OS, but not wounding alone or OS, induced leaf bending. The results indicate that both mechanical and chemical stimuli trigger JA and JA‐Ile synthesis. Differences in kinetics and induced jasmonate levels suggest different sensing and signalling events upon injury and insect‐dependent challenge. Thus, in Drosera, jasmonates are still part of the response to wounding. Jasmonates are also employed in insect‐induced reactions, including responses to herbivory and carnivory.     

Quantitative comparison of chemical, biological and mechanical induction of secondary compounds in Pinus pinaster seedlings

Chemical elicitors and mechanical treatments simulating real insect herbivory have been increasingly used to study induced defensive responses in woody plants. However, simultaneous quantitative comparisons of plant chemical defences elicited by real and simulated herbivory have received little attention. In this paper we compared the effects of real herbivory, simulated herbivory using two chemical elicitors, and mechanical damage treatments on the quantitative secondary chemistry of Pinus pinaster juveniles (namely on non-volatile resin in the stem and total phenolics in the needles). The real herbivory involved phloem wounding by Hylobius abietis and defoliation by Brachyderes lusitanicus (two pine weevils); the chemical elicitors to simulate herbivory induction were 40 mM methyl jasmonate (MJ) and 20 μM benzothiadiazole (BTH); and the mechanical treatments included phloem wounding and needle clipping. We also performed an additional experiment for assessing at what extent insect extracts could increase plant responses over mechanical damage. Chemical induction with MJ, mechanical wounding and real phloem herbivory by H. abietis all produced quantitatively similar results, increasing the concentration of resin in the stem and total phenolics in the needles by equivalent magnitudes. Exogenous application of BTH increased the concentration of phenolic compounds in pine needles, but did not show the same effect on stem resin. Contrastingly, we did not find significant changes in the concentration of resin in the stem or phenolics in the needles after needle clipping and B. lusitanicus feeding. Mechanical damage followed by the application of extracts from the insects B. lusitanicus and H. abietis on the injured tissues did not increase the responses in comparison to mechanical damage alone. The fact that strong induced responses elicited by phloem wounding insects are equally elicited by phloem injuries suggests that defences in pine trees are raised with low specificity regarding biotic enemies. Results from this paper support future methodological approaches using chemical elicitors and mechanical damage as simulated herbivory treatments for the experimental induction of conifer defences.     

Herbivore‐induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack

1. Plant responses to herbivore attack may have community‐wide effects on the composition of the plant‐associated insect community. Thereby, plant responses to an early‐season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early‐season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early‐season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf‐chewing and sap‐sucking guilds. 4. Our results show that community‐wide effects of early‐season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.     

Plant‐mediated interactions between two herbivores differentially affect a subsequently arriving third herbivore in populations of wild cabbage

• Plants are part of biodiverse communities and frequently suffer from attack by multiple herbivorous insects. Plant responses to these herbivores are specific for insect feeding guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown.
• We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae. We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses.
• Performance of both B. brassicae and P. xylostella was reduced when feeding simultaneously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subsequently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations.
• Feeding by multiple herbivores differentially activates plant defences, which has plant‐mediated negative consequences for a subsequently arriving herbivore. Plant population‐specific responses suggest that plant populations adapt to the specific communities of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.

     

Volatile organic compounds released by maize following herbivory or insect extract application and communication between plants

To protect themselves from herbivory, plants have evolved an arsenal of physical and chemical defences and release a variety of volatile organic compounds (VOCs). By releasing these VOCs, a signalling plant can both reduce herbivory, sometimes by more than 90%, and also warn neighbouring plants about an attack. The aim of this study was to assess the influence of herbivory and insect extract application on VOC release by damaged/treated and nearby undamaged/untreated maize plants. We confirmed that European corn borer (Ostrinia nubilalis) larvae attack or larvae extract application induced maize VOC release. Greater amounts of (Z)‐3‐hexenal, (E)‐2‐hexenal, (Z)‐3‐hexen‐1‐ol, (E)‐2‐hexen‐1‐ol, β‐myrcene, (Z)‐3‐hexen‐1‐yl acetate, 1‐hexyl acetate, (Z)‐ocimene, linalool, benzyl acetate, methyl salicylate, indole, methyl anthranilate, geranyl acetate, β‐caryophyllene, (E)‐β‐farnesene and (Z)‐3‐hexenal, (Z)‐3‐hexen‐1‐ol, (Z)‐3‐hexen‐1‐yl acetate, (Z)‐ocimene, linalool, indole, methyl anthranilate, geranyl acetate, β‐caryophyllene and (E)‐β‐farnesene were released as a result of biotic stress after insect attack or insect extract application. The amounts of each VOC released were qualitatively and quantitatively distinct and dependent on time after biotic stress exposure. However, for all biotic stresses, significantly lower VOC induction was measured when leaves were damaged/treated for three days, as compared to seven days. Our work also demonstrated that undamaged/untreated neighbouring plants also release significant amounts of VOCs. This suggests that VOC emission by a damaged/treated plant stimulates VOC induction in nearby undamaged/untreated plants. However, the concentrations of all VOCs released by neighbouring undamaged/untreated maize plants were lower than those from damaged/treated plants and were negatively correlated with distance from a damaged/treated plant. Still, significant VOC induction occurred in undamaged/untreated plants even at 3 m distance from a damaged/infected plant. Our work suggests that maize plant protective defence responses (VOC emission) can be induced via application of European corn borer extracts.     

How to induce defense responses in wild plant populations? Using bilberry (Vaccinium myrtillus) as example

Inducible plant defense is a beneficial strategy for plants, which imply that plants should allocate resources from growth and reproduction to defense when herbivores attack. Plant ecologist has often studied defense responses in wild populations by biomass clipping experiments, whereas laboratory and greenhouse experiments in addition apply chemical elicitors to induce defense responses. To investigate whether field ecologists could benefit from methods used in laboratory and greenhouse studies, we established a randomized block‐design in a pine‐bilberry forest in Western Norway. We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus) by nine different treatments using clipping (leaf tissue or branch removal) with or without chemical treatment by methyljasmonate (MeJA). We subsequently measured consequences of induced defenses through vegetative growth and insect herbivory during one growing season. Our results showed that only MeJA‐treated plants showed consistent defense responses through suppressed vegetative growth and reduced herbivory by leaf‐chewing insects, suggesting an allocation of resources from growth to defense. Leaf tissue removal reduced insect herbivory equal to the effect of the MeJa treatments, but had no negative impact on growth. Branch removal did not reduce insect herbivory or vegetative growth. MeJa treatment and clipping combined did not give an additional defense response. In this study, we investigated how to induce defense responses in wild plant populations under natural field conditions. Our results show that using the chemical elicitor MeJA, with or without biomass clipping, may be a better method to induce defense response in field experiments than clipping of leaves or branches that often has been used in ecological field studies.     

Isopentylamine is a novel defence compound induced by insect feeding in rice

Plants produce a broad variety of defensive metabolites to protect themselves against herbivorous insects. Although polyamines have been implicated in various responses to abiotic and biotic stress, there have been no studies focused on amines in response to insect herbivory. By screening for bioactive amines, we identified isopentylamine as a novel type of herbivory‐induced compound in rice leaves, which was derived from the amino acid leucine in stable isotope labelling experiments. Accumulation of isopentylamine increased during herbivory by the brown planthopper (Nilaparvata lugens, BPH) and the rice‐feeding armyworm (Mythimna loreyi), as well as in response to treatment with the plant hormone, jasmonic acid. Likewise, isopentylamine accumulation was compromised in rice jasmonate biosynthesis mutants, hebiba and Osjar1. In bio‐assays, BPH insects feeding on rice seedlings submerged in 50 mg/L isopentylamine solution had a higher mortality compared with BPH feeding on seedlings submerged in water. Notably, the rice leaves submerged in 50 mg/L solution showed the endogenous concentrations of isopentylamine similar to that induced by BPHs. These results suggest that isopentylamine functions as a new type of plant defence metabolite that is rapidly induced by herbivore attack and deters insect herbivores in rice.