The salivary glands of the psyllid vector Diaphorina citri may act as a transmission barrier to Candidatus Liberibacter asiaticus associated with citrus huanglongbing disease

Candidatus Liberibacter asiaticus (Las) has been associated with huanglongbing (HLB), or citrus greening, which is currently the most devastating citrus disease worldwide. Las is transmitted by the Asian citrus psyllid Diaphorina citri (Hemiptera, Psyllidae) in a persistent manner, but its interactions with the vector particularly at the organ and cellular levels are poorly understood. We used fluorescent in situ hybridization (FISH) and quantitative PCR (Q-PCR) for the localization of Las in dissected organs of D. citri that fed on HLB-infected citrus plants in the field or laboratory. Las was detected by Q-PCR in the salivary glands and alimentary canals, and by FISH in the hemolymph, salivary glands, filter chamber, midgut, and other organs/tissues of D. citri. The relative titer of Las, compared to psyllid genomic DNA in each sample using Q-PCR, was significantly higher in both the salivary gland and alimentary canal compared to that in the rest of the insect body. Additionally, using either FISH or Q-PCR, the proportion of infected (Las-positive) salivary glands was significantly lower than that in other organs/tissues. These results show the near systemic infection of D. citri by Las. They also suggest that the salivary glands constitute an important infection and/or transmission barrier to Las in the psyllid vector, and that Las may replicate or accumulate in both the alimentary canal and salivary glands of D. citri.