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.