Limited infection by ‘Candidatus Liberibacter asiaticus’ in ‘Valencia’ sweet orange trees in the presence of citrus tristeza virus

Authors: FU, SHIMIN - Southwest University; GOUIN, CRISTINA - Animal And Plant Health Inspection Service (APHIS); ZHOU, CHANGYONG - Southwest University; Hartung, John

Submitted to: Journal of Integrative Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2018
Publication Date: 1/21/2019
Citation: Fu, S., Gouin, C., Zhou, C., Hartung, J.S. 2019. Limited infection by ‘Candidatus Liberibacter asiaticus’ in ‘Valencia’ sweet orange trees in the presence of citrus tristeza virus. Journal of Integrative Agriculture. 18:2-11.

Interpretive Summary: Huanglongbing (HLB) is the most destructive disease of citrus and is associated with ‘Candidatus Liberibacter asiaticus’ (CLas), a bacterium. Citrus tristeza virus (CTV) is another pathogen of citrus with very great historic as well as current importance. Both CLas and CTV live in the same internal portions of citrus trees and although both pathogens can be found in the same trees there is little information on possible interactions between the two pathogens. CTV-B2 causes essentially no symptoms in citrus. We have inoculated sweet orange trees with both CTV and CLas to study their interaction. When CTV-B2 and CLas were inoculated together, CTV-B2, but not CLas, became established in trees. However, when CLas was inoculated into citrus before CTV-B2, there was no effect on the establishment of CTV-B2. Trees that were inoculated with CTV-B2 before CLas was inoculated and established in the trees survived longer than trees that were inoculated with CLas before CTV-B2. The apparent protective effect of CTV-B2 against CLas is likely due to enhanced expression of plant defense responses. Further research is needed in this area. These results may be of interest to the citrus industry to reduce losses to citrus greening or HLB disease.

Technical Abstract: Huanglongbing (HLB) is the most destructive disease of citrus and is associated with ‘Candidatus Liberibacter asiaticus’ (CLas), a member of the a-proteobacteria. Citrus tristeza virus (CTV) is another pathogen of citrus with very great historic as well as current importance. Both CLas and CTV are phloem-restricted pathogens. A severe CTV isolate, CTV-B6, and CLas-B232 induce a group of symptoms of phloem dysfunction that overlap, but the mild CTV-B2 does not cause any loss to commercial trees. In this study, we confirmed that CLas established in the rootlets earlier and with higher concentration than in leaves. Prior inoculation and establishment of CLas-B232 did not affect subsequent establishment of either CTV-B2 or CTV-B6, while super infection by CLas-B232 was reduced by prior establishment of CTV-B2 and to a lesser extent by prior infection with CTV-B6. Sweet orange seedlings co-infected with CTV-B2 and CLas-B232 also survived longer than seedlings co-infected by either of the two pathogens with CTV-B6. Trees co-infected with CTV-B6 and CLas-B232 developed more severe symptoms, typical of CTV-B6, than either of the two pathogens co-infected with CTV-B2. The distribution of CLas in the plant infected by CLas alone and with CTV-B2 fits a previously proposed model but CLas was more sporadically distributed in a plant co-infected by CLas and CTV-B2 than in a plant infected by CLas alone. These biological phenomena are aligned with previously analyzed transcriptome data and the study provides a novel idea that mild CTV strains may provide some protection against CLas by limiting its multiplication and spread.