Jasmonic acid-induced resistance to fall armyworm in soybeans: Variation among genotypes and tradeoffs with constitutive resistance

Abstract

A negative correlation between constitutive and inducible resistance across plant populations is expected for a variety of reasons; however, empirical evidence for such trade-offs remain ambiguous, particularly for crop plants. The current study investigated the relationship between constitutive and inducible resistance in vegetative-stage soybeans (V5-V6) against larvae of the generalist defoliator, Spodoptera frugiperda. Eighteen soybean genotypes differing in their constitutive resistance to coleopteran or lepidopteran defoliators were used over four experiments. Exogenous jasmonic acid (JA, 2 mM) was used to induce plants. Constitutive resistance of each genotype was estimated by measuring weight gains, foliar consumption, and foliar conversion efficiency in short-term feeding assays on excised leaf disks of terminal trifoliate leaves of plants not treated with JA. JA was applied to plants immediately after removing leaf material for assays of constitutive resistance, and induced resistance was estimated 48 h after application of JA using leaf disks from the remaining leaf tissue of the same trifoliate used for measuring constitutive resistance. Larval weight gains before JA treatments revealed genotypic variability in constitutive resistance. . Overall, reductions in weight gain (28.7% to 76.7%), foliar consumption (3.7% - 65%) and conversion efficiency (10.9% - 42.2%) were found in JA treatments. Significant (P < 0.05) or marginally significant (P < 0.10) negative correlations between constitutive resistance (larval weight gains on non-induced plants) and induced resistance (differences in weight gains before and after induction) were found in all four experiments, suggesting tradeoff between the two modes of resistance does exist in soybean for this herbivore. Additional evidence for tradeoffs between constitutive and inducible resitance was also found in the analysis of consumption data. Comparisons of consumption and conversion efficiencies suggest that similar antibiotic and antixenotic factors are involved in constitutive and inducible resistance to fall armyworm in soybean.