Abstract: Citrus Huanglongbing (HLB) is a significant threat to the healthy and long-term development of citrus industry. Protists are an important member of microbial family and play an important role in maintaining plant health. In this study, we analyzed the diversity and community structure of protists in the rhizosphere soil of healthy and diseased citrus, and explored key soil chemical factors affecting the protist community. Results showed that Ciliophora, Chlorophyta, and Cercosoa were the dominant groups in the rhizosphere soil of the healthy and diseased citrus. The occurrence of Huanglongbing significantly reduced the diversity of protist community and led to a decrease in the contents of total nitrogen, total phosphorus, alkali-hydrolyzable nitrogen, available phosphorus, organic matter, and pH. Results of RDA analysis showed that Protosteliida, Diatomea, Cryptophyla, and Heterolobosea were positively correlated with available phosphorus. Protolveolata was positively correlated with alkali-hydrolyzable nitrogen and pH.  Cercozoa, MAST-12, Apicomplexa, and Schizoplasmodiida showed significant negative correlations with various soil chemical indicators. Candidatus Liberibacter asiatichus was significantly negatively correlated with predatory and phototrophic protists. Overall, the infection of Candidatus Liberibacter asiatichus causes changes in soil chemical properties and significantly altered the diversity and structure of protist community in the rhizosphere. Soil organic matter, total nitrogen, and pH are key factors affecting the structure of protist community. Our results help understand the impacts of Huanglongbing on the structure and diversity of protist community and soil chemical properties, and provide reference for green prevention and control of Huanglongbing in citrus.

Key words: protist, rhizosphere soil, citrus Huanglongbing, high throughput sequencing