Constitutive and induced resistance to Helicoverpa armigera in grain legumes

Host plant resistance (HPR) to insects is expressed either constitutively or induced in response to plant damage by the insects. Several varieties with moderate to high levels of resistance have been identified with resistance to Helicoverpa armigera in grain legumes. A number of inducible genes have been identified that are up-regulated in response to insect damage or exogenous application of phytohormones such as salicylic acid (SA) and jasmonic acid (JA). Leaf feeding by H. armigera in chickpea resulted in greater amounts of malic acid, in addition to increased number of fingerprints of flavonoids. In groundnut, there was a significant increase in the activities of antioxidative enzymes enzymes [peroxidase (POD), polyphenol oxidase (PPO), phenylalanine ammonia lyase (PAL), superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT)], and the secondary metabolites [total phenols, hydrogen peroxide (H₂O₂), malondialdehyde (MDA)] at 6 days after insect infestation/exogenous application of JA and SA. However, JA showed greater induction than SA. The insect-resistant genotypes showed a stronger response to JA and SA application as compared to the susceptible cultivar. Significantly reduced damage, and low larval survival and larval weights were observed in plants treated with JA than those treated with SA pretreated plants. Chemically induced expression systems or “gene switches” that enable temporal and quantitative control of native or introduced genes have a great potential for boosting HPR to insects for integrated pest management.