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.