Diaphorina citri Kuwayama (D. citri) is an insect vector of phloem-limited ‘Candidatus Liberibacter asiatus’ (CLas), the presumed pathogen of citrus Huanglongbing (HLB). Recently, our lab has preliminarily found it acquired and transmitted Citrus tristeza virus (CTV), which was previously suggested to be vectored by species of aphids. However, the influences of one of the pathogens on the acquisition and transmission efficiency of the other pathogen remain unknown. In this study, CLas and CTV acquisition and transmission by D. citri at different development stages under field and laboratory conditions were determined. CTV could be detected from the nymphs, adults, and honeydew of D. citri but not from the eggs and exuviates of them. CLas in plants might inhibit CTV acquisition by D. citri as lower CTV–positive rates and CTV titers were detected in D. citri collected from HLB-affected trees compared to those from CLas–free trees. D. citri were more likely to obtain CTV than CLas from host plants co-infected with the two pathogens. Intriguingly, CTV in D. citri facilitated the acquisition and transmission of CLas, but CLas carried by D. citri had no significant effect on the transmission of CTV by the same vector. Molecular detection and microscopy methods confirmed the enrichment of CTV in the midgut after a 72-h acquisition access period. Collectively, these results raise essential scientific questions for further research on the molecular mechanism of pathogen transmission by D. citri and provide new ideas for the comprehensive prevention and control of HLB and CTV.

Periwinkle facilitates study of the non-culturable pathogen 'Candidatus Liberibacter asiaticus' (CaLas), the causal agent of Huanglongbing (HLB) in citrus. Isochorismate synthase (ICS) is involved in the defense responses to pathogens through the salicylic acid (SA) signaling pathway in some plants. In this study, crICS from Catharanthus roseus was identified. The fulllength cDNA of crICS is 2206 bp long, with an open reading frame (ORF) encoding a putative protein of 322 amino acids, including a conserved chorismate binding enzyme motif. Phylogenetic analysis revealed the crICS protein to cluster together with those of Rubia cordifolia and Coffea canephora to form a separate clade. Results of a qRT-PCR analysis indicated that crICS was constitutively expressed in leaf, stem, and root tissue of healthy periwinkle plants. During CaLas infection, crICS expression levels increased gradually and reached a peak at 22 days after inoculation (DAI), and then decreased gradually but were still higher than the expression level for the control group (CK) until the end of the experiment. To investigate whether crICS may act to mediate resistance through the SA signaling pathway in periwinkle, we sprayed SA on leaves, and found that SA increased crICS expression levels, but did not decrease the cumulative CaLas-positive rate of infected plants. The results suggest that crICS most likely mediates resistance to CaLas through the SA signaling pathway.

Objective: Diaphorina citri (D. citri) is the vector of “Candidatus Liberibacter asiaticus” (CLas), the most prevalent bacterial species causing citrus Huanglongbing. We recently reported for the first time that the D. citri could acquire the Citrus tristeza virus (CTV), a vitrus previously reported vectored by different aphids, and persist it for at least 15 days. In this study, the CTV and CLas profiles in different organs, development stages, and sexes of D. citri were analyzed. Results: The 3rd instar nymph D. citri is with relatively higher CTV titer, high CTV-positive rates and CLas-positive rates. No significant difference between sexes in their carried CTV and CLas titer. CTV titers in the midguts of adult D. citri were significantly higher than those in salivary glands. Conclusion: Although there is no significant differences between nymphs and adults on the CTV titers, the 3rd instar nymph D. citri is efficient in CTV and CLas acquisition.