Characterization of Antimicrobial-Producing Beneficial Bacteria Isolated from Huanglongbing Escape Citrus Trees.

The microbiome associated with crop plants has a strong impact on their health and productivity. Liberibacter asiaticus (Las), the bacterial pathogen responsible for Huanglongbing (HLB) disease, lives inside the phloem of citrus plants including the root system. It has been suggested that Las negatively affects citrus microbiome. On the other hand, members of citrus microbiome also influence the interaction between Las and citrus. Here, we report the isolation and characterization of multiple putative beneficial bacteria from healthy citrus rhizosphere. Firstly, six bacterial strains showing antibacterial activity against two bacteria closely related to Las: and were selected. Among them, strain A53 and strain A63 are within the β-proteobacteria class, whereas strain 100 and strain 95 are within the γ-proteobacteria class. Additionally, two gram-positive bacteria strain 108 and strain 104 were also identified. Secondly, antimicrobial activity against three fungal pathogens: , , , and two oomycetes: and Four bacterial strains A63, A53, 95, and 104 were shown to have antagonistic activity against the citrus root pathogen based on dual culture antagonist assays and compartmentalized petri dish assays. The four selected bacteria were sequenced. Genes involved in phosphate solubilization, siderophore production and iron acquisition, volatile organic compound production, osmoprotection and osmotic tolerance, phytohormone production, antagonism, and nutrient competition were predicted and discussed related to the beneficial traits.