Résultat de la recherche
Soil salinization effects on volatile signals that mediate the induction of chemical defenses in wild cotton
Plants respond to complex blends of above- and below-ground volatile organic compounds (VOCs) emitted by neighboring plants. These responses often involve priming (i.e., preparation) or induction (i.e., increase) of defenses by “receiver” plants upon exposure to VOCs released by herbivore-damaged neighboring “emitters.” However, recent work has shown that induc- tion of VOC emissions by herbivory is modulated by abiotic factors, potentially affecting plant–plant signaling. We tested the effect of soil salinization on the induction of VOC emissions in wild cotton (Gossypium hirsutum) due to leaf damage and its consequences for the induction of defenses in neighboring plants. To this end, we performed a greenhouse factorial experiment where emitter plants were subjected to augmented soil salinity (vs. ambient salinity) and within each group emitter plants were subsequently exposed to simulated caterpillar damage (mechanical leaf damage treated with Spodoptera frugiperda oral secretion) or no damage (control). After 48 h of exposure, we collected VOCs released by emitter plants and then damaged the receivers and collected their leaves to measure levels of chemical defenses (terpenoid aldehydes of known insecticidal effects). We found an interaction between leaf damage and salinization for two groups of VOCs released by emitters (sesquiterpenes and other aromatic compounds), whereby damaged receivers had higher emissions than control plants under ambient but not salinized soil conditions. We also found that, upon being damaged, receiver plants exposed to damaged emitters exhibited a significantly higher concentration of heliocides (but not gossypol) than control plants. However, salinization did not alter this VOC exposure effect on receiver induced responses to damage. Overall, we show that exposure to induced VOC emissions from damaged plants magnifies the induction of chemical defenses due to leaf damage in neigh- boring individuals and that this is not contingent on the level of soil salinity despite the latter's effect on VOC induction.
Experimental Growth Conditions affect Direct and Indirect Defences in two Cotton Species
Cotton has been used as a model plant to study direct and indirect plant defence against herbivorous insects. However, the plant growing conditions could have an important effect on the outcome of such plant defence studies. We examined how common experimental growth conditions influence constitutive and inducible defences in two species of cotton, Gossypium hirsutum and G. herbaceum. We induced plants by applying caterpillar regurgitant to mechanical wounds to compare the induction levels between plants of both species grown in greenhouse or phytotron conditions. For this we measured defence metabolites (gossypol and heliocides) and performance of Spodoptera frugiperda caterpillars on different leaves, the emission of plant volatiles, and their attractiveness to parasitic wasps. Induction increased the levels of defence metabolites, which in turn decreased the performance of S. frugiperda larvae. Constitutive and induced defence levels were the highest in plants grown in the phytotron (compared to greenhouse plants), G. hirsutum and young leaves. Defence induction was more pronounced in plants grown in the phytotron and in young leaves. Also, the differences between growing conditions were more evident for metabolites in the youngest leaves, indicating an interaction with plant ontogeny. The composition of emitted volatiles was different between plants from the two growth conditions, with greenhouse-grown plants showing more variation than phytotron-grown plants. Also, G. hirsutum released higher amounts of volatiles and attracted more parasitic wasps than G. herbaceum. Overall, these results highlight the importance of experimental abiotic factors in plant defence induction and ontogeny of defences. We therefore suggest careful consideration in selecting the appropriate experimental growing conditions for studies on plant defences.
Aphid and caterpillar feeding drive similar patterns of induced defences and resistance to subsequent herbivory in wild cotton
Plant-induced responses to attack often mediate interactions between different species of insect herbivores. These effects are predicted to be contingent on the herbivore’s feeding guild, whereby prior feeding by insects should negatively impact subsequent feeding by insects of the same guild (induced resistance) but may positively influence insects of a differ- ent guild (induced susceptibility) due to interfering crosstalk between plant biochemical pathways specific to each feeding guild. We compared the effects of prior feeding by leaf-chewing caterpillars (Spodoptera frugiperda) vs. sap-sucking aphids (Aphis gossypii) on induced defences in wild cotton (Gossypium hirsutum) and the consequences of these attacks on subse- quently feeding caterpillars (S. frugiperda). To this end, we conducted a greenhouse experiment where cotton plants were either left undamaged or first exposed to caterpillar or aphid feeding, and we subsequently placed caterpillars on the plants to assess their performance. We also collected leaves to assess the induction of chemical defences in response to herbivory. We found that prior feeding by both aphids and caterpillars resulted in reductions in consumed leaf area, caterpillar mass gain, and caterpillar survival compared with control plants. Concomitantly, prior aphid and caterpillar herbivory caused similar increases in phenolic compounds (flavonoids and hydroxycinnamic acids) and defensive terpenoids (hemigossypolone) compared with control plants. Overall, these findings indicate that these insects confer a similar mode and level of induced resistance in wild cotton plants, calling for further work addressing the biochemical mechanisms underpinning these effects.