Immune response of the Asian citrus psyllid, Diaphorina citri, in response to microbial challenge

The Asian citrus psyllid, Diaphorina citri, is the vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening disease. D. citri harbor CLas in a persistent, propagative manner, thus a heightened immune response as is seen in other insect pathogen vectors in response to infection are expected. Recent annotation of the D. citri genome indicated an absent Imd pathway and few antimicrobial peptides, suggesting that D. citri may not mount an immune response in the presence of gram-negative bacteria. We investigated the capacity of D. citri to eliminate bacteria in a series of challenge assays. D. citri  were injected with live bacteria , then held on sterile artificial diet sachets for 0, 24,  and 48 hours. Serial dilutions of crushed insects plated onto nutrient agar indicated that D. citri was able to effectively clear infections with gram-positive, but not gram-negative, bacteria. Transcriptional responses to bacterial challenges were determined by evaluating –the expression of defensive protein, Toll and JAK/STAT pathway genes in RT-qPCR assays following hemolymph microinjections. Taken together, our results indicate that gram-negative bacteria are able to avoid the D. citri immune system.