Abstract
BackgroundPlants can respond to insect attack via defense mechanisms that reduce insect performance. In this study, we examined the effects of several treatments applied to two maize genotypes (one resistant, one susceptible) on the subsequent growth and survival of Sesamia nonagrioides Lef. (Mediterranean corn borer, MCB) larvae. The treatments were infestation with MCB larvae, application of MCB regurgitant upon wounding, wounding alone, or exposure to methyl jasmonate, and they were applied at the V6–V8 stage of maize development. We also monitored changes in the concentrations of compounds known to be involved in constitutive resistance, such as cell wall-bound hydroxycinnamates and benzoxazinoids.ResultsIn both maize genotypes, the leaves of plants pre-infested with MCB larvae were less suitable for larval development than those from untreated plants. Application of MCB regurgitant upon wounding, and wounding itself, resulted in leaf tissues becoming less suitable for larval growth than those of pre-infested plants, suggesting that there could be herbivore-associated effector molecules that suppress some wounding responses. A single application of MCB regurgitant did not seem to mimic feeding by MCB larvae, although the results suggested that regurgitant deposited during feeding may have enhanced ferulates and diferulates synthesis in infested vs. control plants. Jasmonic acid may play a role in mediating the maize response to MCB attack, but it did not trigger hydroxycinnamate accumulation in the leaves to a level comparable to that induced by larval leaf feeding. The EP39 maize genotype showed an increase in leaf cell wall strength by increasing hemicellulose cross-linking in response to MCB attack, while induced defenses in the EP42 plants appeared to reflect a broader array of resistance mechanisms.ConclusionsThe results indicated that leaf feeding by MCB larvae can increase leaf antibiosis against MCB in two maize genotypes with contrasting levels of resistance against this borer. Also, the larval regurgitant played a positive role in eliciting a defense response. We determined the effects of the plant response on larval growth, and detected defense compounds related to borer resistance.
Highlights
Plants can respond to insect attack via defense mechanisms that reduce insect performance
Non-choice feeding bioassays To evaluate if feeding by Mediterranean corn borer (MCB) increases the antibiosis of leaves in young plants, MCB larvae were reared for 13 days on excised leaves from maize plants previously infested with MCB larvae (48 h before), and larval weight and survival were monitored
The antibiosis levels of the leaves from EP39 and EP42 plants pre-infested with MCB larvae were similar at the early stage of the bioassay (6 days after treatment; dat) (p > 0.05) (Table 1)
Summary
Plants can respond to insect attack via defense mechanisms that reduce insect performance. Several studies have focused on identifying constitutive chemical compounds in maize that are involved in resistance to MCB, and the best candidates are benzoxazinoids and hydroxycinnamates [4, 5]. The constitutive contents of pCAs and FAs in cell walls and the degree of hemicellulose cross-linking by DFA bridges has been suggested to be a structural defense mechanism against insect damage. Several studies reported that differences in cell wall-bound hydroxycinnamate contents in grains, leaves, or stem tissues between resistant and susceptible genotypes of maize, wheat, and tall fescue were associated with contrasting levels of resistance to folivores and stem borers and with reduced insect performance [5, 8, 9, 11,12,13]
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