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Toll-like receptors (TLRs) are pathogen recognition receptors (PRRs), which play key roles in helping the host immune system fight pathogen invasions. Systematic information on TLRs at the genome-wide level and expression profiling in response to endophytic colonization is very important to understand their functions but is currently lacking in this field. Here, a total of two TLR genes were identified and characterized in Diaphorina citri. The TLR genes of D. citri were clustered into five families according to the phylogenetic analysis of different species’ TLRs. The domain organization analyses suggested that the TLRs were constituted of three important parts: a leucine-rich repeat (LRR) domain, a transmembrane region (TR) and a Toll/interleukin-1 receptor (TIR) domain. The mRNA expression levels of the two TLR genes (DcTOLL and DcTLR7) were highly regulated in both nymphs and adults of D. citri. These results elucidated the potentiated TLR gene expression in response to endophytically colonized plants. Furthermore, the 3D structures of the TIR domain were highly conserved during evolution. Collectively, these findings elucidate the crucial roles of TLRs in the immune response of D. citri to entomopathogens systematically established as endophytes, and provide fundamental knowledge for further understanding of the innate immunity of D. citri.

Diaphorina citri Kuwayama (Hemiptera: Liviidae) commonly known as Asian citrus psyllid is one of the most ubiquitous of phloemfeeding pests of citrus in the major citrus growing areas across the world. It serves as the main vector of the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas), causing citrus greening disease also known as Huanglongbing (HLB;Hall & Rohrig, 2015).Currently, HLB is a disease responsible for huge economic losses in the citrus producing industry (Chavez et al., 2017;. Imidacloprid and other neonicotinoids, bifenthrin, chlorpyrifos and chlorfenapyr are the most frequently used, and sometimes overused broad-spectrum insecticides for D. citri management (Gatineau et al., 2010;Naeem et al., 2016). However, the persistence or frequency of application has made them effective only for a short period of time and their overuse has led to increased D. citri resistance, pesticide residues, loss of biodiversity and resurgence problems (Oliveira et al., 2013); besides, their applications pose hazards to beneficial insects, human beings and the environment in general (Gharalari et al.,

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