Aphid-resistant alfalfa cultivar minimizes the survival of spotted alfalfa aphid through upregulating plant defense compounds

The spotted alfalfa aphid, Therioaphis trifolii, is one of the most important pests of alfalfa, Medicago sativa. Previous studies have revealed a significant difference in aphid numbers between the aphid-susceptible cultivar WL343 and the aphid-resistant cultivar Zhongmu No.1. To explore the reasons for this phenomenon, we constructed life tables for T. trifolii reared on these two alfalfa cultivars, and analyzed the changes in jasmonic acid (JA), salicylic acid (SA), secondary metabolites (total phenolic compounds, tannic acid, total flavonoids, and saponin), the activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) in both cultivars following the introduction of aphids. In addition, we measured the gastric toxicity of M. sativa secondary metabolites (tannic acid and saponin) against T. trifolii. Results revealed a lower fitness of T. trifolii on the resistant cultivar compared to the susceptible cultivar. The concentrations of JA, phenolic compounds, tannic acid, flavonoids, saponin, and the activities of CAT, SOD, and POD in the resistant cultivar were significantly higher than those in the susceptible cultivar. Subsequently, it was confirmed that tannic acid and saponin have direct gastric toxicity against T. trifolii. This indicated that the aphid-resistant cultivar inhibited aphid growth and fecundity by producing more defense substances, i.e., JA, secondary metabolites, and enhanced activities of protective enzymes including tannic acid and saponin found in alfalfa leaves directly associated with toxicity to T. trifolii, while reducing the content of SA. This study provides important support for breeding insect-resistant varieties while elucidating plant-resistant responses to insect herbivory.