Abstract: The citrus-associated microbial community plays a key ecological role for maintaining plant health and stress resistance. However, infection by Huanglongbing (HLB) and inappropriate agricultural management practices can substantially alter microbial community composition. We conducted experiment with Orah citrus in Guangxi, China. Three treatments were set: healthy trees (CK), HLB-infected trees (HLB), and HLB-infected trees treated with the antibiotic ampicillin sodium salt (ASS). High-throughput sequencing combined with physicochemical analyses was employed to investigate the effects of antibiotic application on microbial communities in leaves, roots, and rhizosphere soil of Orah citrus. The results showed that microbial diversity in leaves, roots, and rhizosphere soil infected with HLB and those treated with antibiotics was lower than that of healthy trees. The number of operational taxonomic units (OTUs) in leaves decreased from 827 in CK to 639 in HLB and 745 in ASS. The number of OTUs in roots decreased from 574 in CK to 432 in HLB and 369 in ASS. For the number of soil OTUs, there was no significant difference between the ASS group (861) and the HLB group (869), but both were significantly lower than that of the CK (1206). At the phylum level, Proteobacteria, Firmicutes, and Actinobacteria were dominant. Proteobacteria accounted for 91.48% of leaf microbial community, significantly higher than that in roots (41.08%) and soil (16.32%). At the genus level, Acetobacter and Belnapia were significantly enriched in the leaves of the ASS group. The genera, such as Sphingomonas, were closely associated with the activities of oxidative stress-related enzymes, including polyphenol oxidase (PPO) and peroxidase (POD). In roots, the relative abundance of Actinobacteria increased by 8.47% in the ASS group compared with the HLB group. In soil, genera such as 1921-2 and Nocardioides showed significant positive correlations with PPO activity (P<0.05), and some genera were correlated with the urease activity and the availability of nitrogen, phosphorus, and potassium. LEfSe analysis further confirmed that antibiotic treatment promoted the enrichment of health-associated taxa such as Dyella but suppressed the ecological niche of the HLB pathogen (Candidatus Liberibacter asiaticus, CLas). Overall, the application of ampicillin sodium salt reshaped microbial community structure, enhanced the ecological competitiveness of the disease-resistant taxa, and improved the rhizosphere immune environment by regulating soil enzyme activities. These findings provide a new reference for microbiome-based integrated management strategies for citrus Huanglongbing.

Key words: Orah citrus, Huanglongbing, ampicillin sodium salt, microbiome