Soybean induced resistance to Spodoptera eridania herbivory

BACKGROUND: Constitutive resistance is expressed constantly during plant development, whereas induced resistance can be triggered after herbivory. Evaluating induced resistance in soybean varieties and its correlation with expression of defense compounds and plant growth can aid in genotype screening and the use of induced resistance in integrated pest management. This study used two soybean varieties previously selected based on differing constitutive antibiosis-resistance against Spodoptera eridania to test the hypotheses: prior herbivory by S. eridania induces higher resistance to conspecific larvae; resistance induction is associated with increased activities of defense proteins; induced resistance is related to increased flavonoid levels; and induced resistance trades off with plant growth. RESULTS: Effects of variety and injury were significant for leaf area consumed; injury affected larval survival; and variety × injury interacted for larval weight. The resistant variety showed constitutively higher superoxide dismutase, catalase, ascorbate peroxidase, polyphenol oxidase, and malondialdehyde levels. Herbivory decreased superoxide dismutase and polyphenol oxidase activities in both varieties and increased Kunitz trypsin inhibitors in the resistant variety. Isoquercitrin and daidzin were exclusive to the resistant and susceptible soybeans, respectively. Herbivory did not change flavonoid concentrations. Plant growth was not affected by herbivory, and susceptible plants grew faster than resistant soybeans. CONCLUSIONS: Evaluated enzymes and Kunitz trypsin inhibitors play a role in soybean constitutive resistance. Hydrogen peroxide is involved in injury signaling and malondialdehyde correlated with constitutive and induced resistance. Kunitz trypsin inhibitors increased after herbivory only in the resistant variety. Isoquercitrin is involved in constitutive resistance. There was no plant growth trade-off with induced resistance.