Therioaphis trifolii saliva: a key factor in aphid survival and plant-defense

Abstract This study aimed to investigate how Therioaphis trifolii (Monell), a major aphid pest of Medicago sativa, copes with its host’s biochemical defenses. The research involved comparing susceptible and resistant aphid strains on two M. sativa cultivars. Aphid populations were reared separately for several generations on a resistant cultivar (cv. Zhongmu No.1) and a susceptible cultivar (cv. WL343), resulting in R- and S-aphid lines, respectively. The study found that the survival rate of R-aphids on the aphid-resistant Zhongmu No.1 cultivar was significantly higher than that of S-aphids (P=0.039). However, there was no significant difference in the survival of both aphid lines on WL343 (P=0.139). This suggests that when S-aphids and R-aphids infest the host plant, their salivary regulatory capacity on plant defense differs, leading to differences in viability. Further investigation found that treating leaves with saliva collected from aphids reared on cv. Zhongmu No.1 (R-saliva) decreased the repellency and toxicity to aphids subsequently feeding on these plants. In addition, compared with S-aphid infecting and S-saliva leaf infiltration treatment, R-aphid infecting and R-saliva infiltration treatment can activate the SA (Salicylic Acid) signal pathway but inhibit the JA (Jasmonic Acid) signal pathway. This improves the suitability of aphids to the host plant. It is speculated that R-aphids can activate the host plant’s SA pathway and inhibit the JA pathway by secreting S-saliva, thereby reducing the host plant’s defense ability and improving the plant’s fitness. Conversely, S-aphids would activate the JA pathway and inhibit the SA pathway. Overall, these results provide essential support for understanding the regulation of plant defense mechanisms.