Potential of citrus endophyte Bacillus subtilis L1-21 in the control of Candidatus Liberibacter asiaticus in Asian citrus psyllid, Diaphorina citri.

BACKGROUND

Asian citrus psyllid (ACP), also known as Diaphorina citri, is the natural vector of Candidatus Liberibacter asiaticus (CLas), which is responsible for Huanglongbing (HLB), a devastating citrus disease. Previously, the pathogen was successfully excluded from diseased citrus plants by using the indigenous endophyte Bacillus subtilis L1-21. However, the pathogen elimination and colonization potential of B. subtilis L1-21 in the carrier vector ACP, as well as the recruitment of native microbial communities of psyllid in the presence of endophytes, are still unknown.

RESULTS

Initially, we suggested that endophyte L1-21 reduced the CLas copies in ACP from 6.58 × 10 to 5.04 × 10 per insect after 48 h, however, the pathogen copies remained stable in the negative control. The endophyte was stable for 48 h after application. Among the bacterial genera those highlighted in ACP were Candidatus Liberibacter, Pseudomonas, Candidatus Profftella, Methylobacterium-Methylorubrum, Pantoea, Curtobacterium, Wolbachia, Actinomycetospora, and Bacillus. Interestingly, B. subtilis L1-21 easily colonizes the midgut of ACP but cannot be detected in eggs. When ACP with endophyte L1-21 was allowed to feed on new citrus leaves, the highest colonization was observed. We also found that psyllids carrying endophyte L1-21 after feeding on citrus leaves reduced the CLas copies in leaves on the 0, 3rd and 5th day from 8.18 × 10, 2.6 × 10, and 0 pathogen copies/g fresh midvein, respectively.

CONCLUSIONS

We propose that B. subtilis L1-21 is a native endophyte in citrus and psyllid, which efficiently reduces the CLas pathogen in both citrus and psyllids, provides a more protective effect by increasing the number of cultivable endophytes, and successfully colonizes the midgut of ACP. © 2022 Society of Chemical Industry.