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

A novel, nostoxanthin-producing, endophytic bacterium, designated as AK-PDB1-5, was isolated from the needle-like leaves of the Korean fir ( Wilson) collected from Mt. Halla in Jeju, South Korea. A 16S rRNA sequence comparison indicated that the closest phylogenetic neighbors were MIMD3 (95.6%) and S5-249 (95.3%) of the family . Strain AK-PDB1-5 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 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 contained C 2OH C 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 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 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 was determined to be a novel species in the genus with the proposed name sp. nov. The type strain is AK-PDB1-5 (= KCTC 82822 = CCTCC AB 2021150).