A nostoxanthin-producing bacterium, Sphingomonas nostoxanthinifaciens sp. nov., alleviates the salt stress of Arabidopsis seedlings by scavenging of reactive oxygen species

A novel, nostoxanthin-producing, endophytic bacterium, designated as AK-PDB1-5(T), was isolated from the needle-like leaves of the Korean fir (Abies koreana Wilson) collected from Mt. Halla in Jeju, South Korea. A 16S rRNA sequence comparison indicated that the closest phylogenetic neighbors were Sphingomonas crusticola MIMD3(T) (95.6%) and Sphingomonas jatrophae S5-249(T) (95.3%) of the family Sphingomonadaceae. Strain AK-PDB1-5(T) had a genome size of 4,298,284 bp with a 67.8% G + C content, and digital DNA–DNA hybridization and OrthoANI values with the most closely related species of only 19.5–21% and 75.1–76.8%, respectively. Cells of the strain AK-PDB1-5(T) were Gram-negative, short rods, oxidase- and catalase-positive. Growth occurred at pH 5.0–9.0 (optimum pH 8.0) in the absence of NaCl at 4–37°C (optimum 25–30°C). Strain AK-PDB1-5(T) contained C(14:0) 2OH(,) C(16:0) and summed feature 8 as the major cellular fatty acids (> 10%), while sphingoglycolipid, phosphatidylethanolamine, phosphatidylglycerol, phospholipids and lipids were found to be the major polar lipids. The strain produces a yellow carotenoid pigment; natural products prediction via AntiSMASH tool found zeaxanthin biosynthesis clusters in the entire genome. Biophysical characterization by ultraviolet–visible absorption spectroscopy and ESI-MS studies confirmed the yellow pigment was nostoxanthin. In addition, strain AK-PDB1-5(T) was found significantly promote Arabidopsis seedling growth under salt conditions by reducing reactive oxygen species (ROS). Based on the polyphasic taxonomic analysis results, strain AK-PDB1-5(T) was determined to be a novel species in the genus Sphingomonas with the proposed name Sphingomonas nostoxanthinifaciens sp. nov. The type strain is AK-PDB1-5(T) (= KCTC 82822(T) = CCTCC AB 2021150(T)).