Sphingomonas benefaciens sp. nov. and Sphingomonas cellulosilytica sp. nov., two potential plant growth-promoting bacteria isolated from citrus phyllosphere

Abstract

Two yellow-pigmented, Gram-stain-negative, strictly aerobic bacterial strains, designated as XXL09^T and CLY1604^T, were isolated from healthy citrus leaves collected from a well-known production area located in Deqing County, Guangdong Province, PR China. Phylogenetic analyses based on 16S rRNA gene sequences and 92 core genes showed that strains XXL09^T and CLY1604^T belonged to the genus Sphingomonas and were most closely related to the three validly published species Sphingomonas beigongshangi REN5^T, Sphingomonas aquatilis JSS7^T and Sphingomonas melonis DAPP-PG 224^T. The genome-derived average nucleotide identity values between the two strains and their closely related type strains were below 84.66% and 85.21%, respectively, and the corresponding digital DNA-DNA hybridization values were below 25.70% and 27.70%, respectively. The major fatty acids of two novel strains were identified as summed feature 8 (C_{18 : 1} ω7c), C_{14 : 0} 2-OH and C_{16 : 0}. The respiratory quinone was determined to be ubiquinone 10 and the polar lipids comprised diphosphatidylglycerol, phosphatidyldimethylethanolamine, phosphatidylethanolamine, sphingoglycolipid, phosphatidylglycerol, an unidentified phospholipid, an unidentified glycolipid and five unidentified aminolipids. The genomic DNA G+C contents of strains XXL09^T and CLY1604^T were 67.1% and 68.3%, respectively. Phenotypic and physiological characterizations further revealed plant growth-promoting traits: both strains produced siderophores and indole-3-acetic acid, suggesting capabilities in iron acquisition and phytohormone synthesis. Notably, only strain XXL09^T exhibited inorganic phosphate solubilization activity, highlighting its potential role to enhance nutrient availability to host plants. Integrating comprehensive genomic, phylogenetic, chemotaxonomic and phenotypic evidence, strains XXL09^T and CLY1604^T are proposed to represent two novel species of the genus Sphingomonas, for which the names Sphingomonas benefaciens sp. nov. and Sphingomonas cellulosilytica sp. nov. are proposed, with XXL09^T (=GDMCC 1.5297^T=CCM 9448=JCM 37846^T) and CLY1604^T (=GDMCC 1.5301^T=CCM 9452^T=JCM 37845^T) as the type strains, respectively.

Keywords: Sphingomonas; citrus phyllospherel; novel species; polyphasic taxonomy.