Screening candidate effectors from the salivary gland transcriptomes of brown citrus aphid, Aphis citricidus

The brown citrus aphid, Aphis citricidus, stands out as an important citrus pest that is an efficient vector for Citrus tristeza virus (CTV), the causal agent of important economic losses in citrus. Evidence suggests that aphids deliver salivary effector proteins inside their host cells to modulate plant physiology, suppress defense responses, and consequently favor the establishment and infestation. This study employed deep sequencing of RNA libraries to create a transcriptome of the salivary gland. Screening the transcriptome identified 115 unigenes encoding putatively secreted effector proteins. Eleven A. citricidus effectors exhibiting relatively low sequence identities were selected for gene expression analysis. Among them, six effectors (i.e., AcE1, AcE2, AcE3, AcE5, AcE8, and AcE9) displayed remarkably high expression levels in the head with salivary glands; AcE4 was highly expressed in both head and gut tissue. Further transient overexpression revealed that AcE4 could effectively inhibit INF1/BAX-induced leaf chlorosis in Nicotiana benthamiana leaves, implying its potential role in inhibiting plant defense mechanisms against aphid feeding. The findings of this study demonstrate the in silico identification of effector proteins from A. citricidus. Further investigation and analysis of these effectors, like as AcE4, will provide valuable knowledge regarding the molecular mechanisms that govern the interaction between aphids and plants.