Methyl jasmonate (MeJA) as an elicitor enhances tomato resistance through ROS and digestive enzyme inhibition against Helicoverpa armigera (Lepidoptera: Noctuidae)

Methyl jasmonate (MeJA) is a central regulator of plant defense, yet the physiological consequences of MeJA-induced defenses for herbivorous insects remain incompletely resolved across multiple levels of herbivore performance. Here, we investigated how exogenous MeJA treatment of tomato ( Solanum lycopersicum ) modulates plant oxidative defenses and the subsequent digestive, antioxidant, and nutritional physiology of Helicoverpa armigera larvae. MeJA elicitation induced a significant, dose-dependent H₂O₂-associated response in tomato leaves, with DAB-derived staining extending across approximately 98% of the leaf area at 1 mM MeJA, accompanied by a 45% increase in peroxidase (POD) activity. Feeding third-instar larvae on MeJA-elicited foliage increased mortality from 13% in controls to > 60% at 1 mM MeJA and reduced larval length by up to 68%. These effects coincided with pronounced alterations in midgut digestive physiology, with trypsin, chymotrypsin, α-amylase, and lipase activities decreasing by approximately 45–65%. Larval antioxidant responses were likewise markedly altered, with catalase (CAT) activity declining by approximately 99.7% and POD activity by approximately 42–47.9%, consistent with a substantially reduced enzymatic antioxidant response. These physiological changes were accompanied by marked alterations in whole-organism performance, with relative growth rate (RGR) declining from 1.56 to 0.00017 mg/g/day, corresponding to an approximately 99.99% reduction, while efficiency of conversion of ingested food (ECI) decreased from 3.09 to 0.28% (approximately 91% reduction). Although approximate digestibility (AD) increased at higher MeJA concentrations, this increase was not accompanied by improved growth or food-conversion efficiency, indicating that greater apparent digestibility did not translate into more effective nutrient utilization. Collectively, these findings show that MeJA-induced tomato defenses are associated with coordinated alterations in the digestive, antioxidant, and nutritional physiology of H. armigera , coinciding with impaired larval growth and survival. By integrating plant oxidative responses with multiple physiological dimensions of herbivore performance, this study provides a broader physiological perspective on MeJA-elicited tomato resistance and highlights interconnected physiological responses that warrant further mechanistic and ecological investigation.