Saliva of (Drepanosiphidae) Activates the SA Plant Hormone Pathway, Inhibits the JA Plant Hormone Pathway, and Improves Aphid Survival Rate.

The spotted alfalfa aphid () is a kind of destructive pest of cultivated alfalfa () that results in significant financial losses for the livestock sector. To understand how navigates the biochemical defenses of its host, we investigated the effects of susceptible and resistant aphid strains on two alfalfa cultivars. was reared for over 50 generations on two cultivars-WL343, which is susceptible to , and Zhongmu No. 1, which is resistant-resulting in the development of a resistant aphid strain (R-aphid) and a susceptible aphid strain (S-aphid). The results showed that the survival rate of R-aphids was significantly higher than that of S-aphids ( = 0.039) on the resistant cultivar Zhongmu No. 1, while there was no significant difference in survival rates between the two aphid strains on WL343 ( = 0.139). This suggests that S- and R-aphids differ in their ability to modulate plant defense mechanisms, influencing their survival rates. The application of saliva from R-aphids reared on Zhongmu No. 1 (R-saliva) reduced the repellency and toxicity of treated plants, improving aphid survival. Furthermore, R-aphid infestation and R-saliva application activated the salicylic acid (SA) signaling pathway while suppressing the jasmonic acid (JA) pathway, enhancing the host plant's suitability for aphid colonization. We propose that R-aphids may use their saliva to activate the SA pathway, which in turn inhibits JA synthesis, weakening the plant's defenses. These findings provide valuable insights into how interacts with host plant defense systems.