N-(18-hydroxylinolenoyl)-L-glutamine: a newly discovered analog of volicitin in Manduca sexta and its elicitor activity in plants.

Plants attacked by insect herbivores release a blend of volatile organic compounds (VOCs) that serve as chemical cues for host location by parasitic wasps, natural enemies of the herbivores. Volicitin, N-(17-hydroxylinolenoyl)-L-glutamine, is one of the most active VOC elicitors found in herbivore regurgitants. Our previous study revealed that hydroxylation on the 17th position of the linolenic acid moiety of N-linolenoyl-L-glutamine increases by more than three times the elicitor activity in corn plants. Here, we identified N-(18-hydroxylinolenoyl)-L-glutamine (18OH-volicitin) from larval gut contents of tobacco hornworm (THW), Manduca sexta. Eggplant and tobacco, two solanaceous host plants of THW larvae, and corn, a non-host plant, responded differently to this new elicitor. Eggplant and tobacco seedlings emitted twice the amount of VOCs when 18OH-volicitin was applied to damaged leaf surfaces compared to N-linolenoyl-L-glutamine, while both these fatty acid amino acid conjugates (FACs) elicited a similar response in corn seedlings. In both solanaceous plants, there was no significant difference in the elicitor activity of 17OH- and 18OH-volicitin. Interestingly, other lepidopteran species that have 17OH-type volicitin also attack solanaceous plants. These data suggest that plants have developed herbivory-detection systems customized to their herbivorous enemies.

Maize developmental stage affects indirect and direct defense expression.

Herbivores elicit a complex indirect and direct defense response in plants, but little is known about how defenses are expressed throughout a plant's life cycle. Here we report the characterization of direct and indirect defense profiles for two vegetative stages of maize, v1 and v3. Indirect defense expression was evaluated by measuring plant headspace volatiles in response to artificial herbivory and caterpillar feeding. To assess direct defense expression, transcripts of four proteinase inhibitors (PI) and larval performance were measured. Artificial herbivory significantly elevated total induced volatiles in v1 and v3 compared with controls, but this induction was marginal in v1 when compared with v3. Both maize stages produced similar total amounts of volatiles, accounting for fresh weight, of different compositions during caterpillar feeding. However, on a per-plant basis, v1 produced less volatiles compared with v3. In contrast, jasmonic acid levels, an indicator of an antiherbivore defense response, increased similarly in both stages. During caterpillar feeding, both developmental stages contained higher levels of PI transcripts compared with controls. However, v1 plants contained more cystatin-like PI transcripts and total larval mass was reduced compared with v3 plants. These results suggest that in maize, direct and indirect defenses against insect herbivory differ between seedling and juvenile plants.