3.4 Salivary protein-encoding genes and other gall formation associated genes
S. chinensis can induce closed galls on host plants. Previous studies have shown that gall induction is highly species-specific and galling insects deliver effectors into plant tissues, resulting in gall formation. The gall midge Mayetiola destructor can inject effector proteins into tissues through its saliva during feeding, resulting in converting a whole wheat seedling into a gall (Wang et al., 2018; Aljbory et al., 2020). A novel family of insect secreted proteins named BICYCLE was identified in Hormaphis cornu , which induces galls on the leaves of witch hazel, Hamamelis virginiana(Korgaonkar et al., 2021). BICYCLE may regulate many aspects of gall development because they are expressed very abundantly in salivary glands specifically in gall aphids.S. chinensis feeds on host leaves where it 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 to salivary proteins from 10 other free-living Hemipterans, the presence of a high proportion of proteins with binding activity was noticeable, including DNA-, protein-, ATP-, and iron-binding proteins, which may all be involved in gall formation. BICYCLE proteins were not identified in salivary glands suggested different mechanisms of gall induction betweenS. chinensis and H. cornu. From the RNA-Seq analysis, transcripts corresponding to 35 genes (Sc.chr03.1184- Sc.chr10.506) encoding salivary gland proteins have shown high expression levels at the gall forming fundatrix stage (Figure S4). These salivary proteins are potentially may be related to interaction between insects and host plants. According to their functions, these genes can be divided into detoxification, signal transduction, secreted protein metabolism, energy metabolism, basic biological processes, movement and function unknow (Table S10). The largest number of genes related to detoxification may be related to host plant defense inhibition. Gene belonging to movement and energy metabolism may be related to the contraction of salivary gland muscle and provide energy to salivate.