Arbuscular Mycorrhizal Fungi Enhance Growth and Defense Against Spodoptera litura in Lactuca sativa

Arbuscular mycorrhizal (AM) fungi are common root-associated endophytic fungi that enhance host plant growth and induce resistance against various stresses. However, their role in mediating insect resistance in vegetable crops remains poorly understood. In this study, the effects of AM fungi inoculation on growth and insect resistance against Spodoptera litura in four representative vegetable species (tomato, pepper, cucumber, and lettuce) were investigated. Lettuce (Lactuca sativa), which exhibited the strongest mycorrhiza-induced resistance, was subsequently selected for detailed mechanistic investigation through physiological and biochemical measurements, phytohormone profiling, gene expression analysis, and targeted metabolomics. AM fungi-inoculated lettuce exhibits both elevated growth and pronounced insect resistance. During the early stages of herbivory, AM fungi rapidly activates the jasmonic acid (JA) signaling pathway, leading to increased levels of 12-oxo-phytodienoic acid (OPDA), JA, and bioactive jasmonyl-isoleucine (JA-Ile), as well as the upregulation of key JA biosynthetic gene LsOPR3 and the defense-related gene LsPI. Mycorrhizal inoculation also mitigates lipid peroxidation induced by insect feeding and enhances the activities of antioxidant enzymes (superoxide dismutase and catalase) in leaves. Targeted metabolomic analysis reveales that AM fungi significantly altered secondary metabolism in lettuce, particularly promoting the accumulation of L-tryptophan (L-Trp) and its derivatives (such as methyl indole-3-acetate and indole-3-carboxaldehyde) under insect stress, alongside notable increases in phenylpropanoid and flavonoid pathway metabolites. Exogenous application assays further confirmed that JA treatment strongly induced the expression of insect defense-related polyphenol oxidase genes (LsPPO3, LsPPO4) and enhanced resistance but suppressed plant growth. In contrast, L-Trp treatment elevated LsPPO3 and LsPPO4 expression while maintaining biomass accumulation. These results show that AM fungi inoculation promotes lettuce growth and insect resistance through elevating JA signaling and L-tryptophan accumulation. Moreover, this study emphasizes the potential role of L-Trp in improving vegetable crops biomass and insect defense.