3.5 Salivary protein-encoding genes and other gall formation associated genes
S. chinensis can induce the formation of closed galls on host plants. Previous studies have reported that gall induction is highly species-specific, and that galling insects deliver effectors into plant tissues, resulting in gall formation (Yang et al., 2018). The gall midgeMayetiola destructor can inject effector proteins into tissues via its saliva during feeding, leading to the conversion of a whole wheat seedling into a gall (Wang et al., 2018; Aljbory et al., 2020). A novel family of insect secreted proteins named BICYCLE have been identified in Hormaphis cornu , which induce gall formation on the leaves of witch hazel, Hamamelis virginiana (Korgaonkar et al., 2021). BICYCLE may regulate numerous aspects of gall development, due to their abundant expression in salivary glands specifically in gall aphids. S. chinensis feeds on host leaves where it presumably injects saliva into host leaf cells, resulting in gall formation. A total of 141 proteins have been identified from its salivary glands by LC-MS/MS analysis (Yang et al., 2018). In comparison with salivary proteins from 10 other free-living Hemipterans, the presence of a high proportion of proteins with binding activity is noticeable, including DNA-, protein-, ATP-, and iron-binding proteins. These proteins may be involved in gall formation. In this study, we did not identify any BICYCLE protein in the salivary glands ofS. chinensis , suggesting the different mechanisms of gall induction between S. chinensis and H. cornu . As demonstrated by RNA-Seq analysis, transcripts corresponding to 35 genes (Sc.chr03.1184- Sc.chr10.506) that encoded salivary gland proteins exhibited high expression levels in the gall forming fundatrix stage (Figure S5). These salivary proteins were potentially related to the interaction between insects and host plants. According to their predicted functions, these genes can be divided into several categories, including detoxification, signal transduction, secreted protein metabolism, energy metabolism, basic biological processes and movement (Table S10). The largest number of genes related to detoxification may be related to defense inhibition in host plants. On the other hand, gene belonging to movement and energy metabolism categories may be associated with the contraction of salivary gland muscle and the supply of energy for salivation.