Investigation of citrus HLB symptom variations associated with “Candidatus Liberibacter asiaticus” strains harboring different phages in southern China

Authors: BAO, M - South China Agricultural Univerisity; ZHENG, Z - South China Agricultural Univerisity; LI, C - South China Agricultural Univerisity; Chen, Jianchi; DENG, X - South China Agricultural Univerisity

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2021
Publication Date: 11/9/2021
Citation: Bao, M., Zheng, Z., Li, C., Chen, J., Deng, X. 2021. Investigation of citrus HLB symptom variations associated with “Candidatus Liberibacter asiaticus” strains harboring different phages in southern China. Agronomy. 11(11). Article 2262. https://doi.org/10.3390/agronomy11112262.
DOI: https://doi.org/10.3390/agronomy11112262

Interpretive Summary: Huanglongbing (HLB) is a devastating disease in citrus production worldwide. The disease pathogen is a bacterium. Recent research found that the HLB pathogen also harbored viruses or phages. It is believed that the bacterial viruses may be involved in HLB symptom development. In this study, two HLB pathogen strains carrying different viruses were inoculated into three citrus cultivars. Six months after inoculation, all citrus cultivars showed HLB symptoms. One cultivar showed significant symptom differences from the two HLB pathogens carrying different bacterial viruses. This discovery is important because it will provide the baseline information for further research on the roles of bacterial viruses in HLB symptom development, leading to a better understanding of HLB and formulation of effective control strategies.

Technical Abstract: Huanglongbing (HLB) is a devastating disease in citrus production worldwide. The disease is associated with an unculturable alfa-proteobacterium, “Candidatus Liberibacter asiaticus” (CLas). Phages/prophages of CLas have recently been identified through intensive genomic research. The phage information has facilitated research on CLas virulence and strain diversity. However, little is known about the roles of CLas phage strains in HLB symptom development. Such research is challenging due to the unculturable nature of CLas and the lack of singly phage-infected CLas strains. In this study, CLas strains singly carrying Type 1 phage (CLas-Type 1) and Type 2 phages (CLas-Type 2) were identified in southern China and characterized through next generation sequencing (NGS). The two CLas strains were inoculated into seedlings of three different citrus cultivars/species through graft transmission in a screenhouse in Guangdong, China. Symptom developments were recorded for six months. All CLas-infected cultivars showed HLB symptoms. In cultivar Nianju (Citrus reticulata), strain CLas-Type 1 caused more pronounce yellowing and severe defoliation, and strain CLas-Type 2 caused more dark-greening of leaf veins. In contrast, cultivars Shatianyou (C. maxima), and Eureka lemon (Citrus limon) showed little symptom variations between the two CLas phage strain infections. Results from this study provide baseline information for future research on the roles of CLas phages in HLB symptom development.