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 ⁶ to 5.04 × 10 ⁴ 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, ⁴ 2.6 × 10, ³ 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.
(© 2022 Society of Chemical Industry.)