Description of Microbacterium dauci sp. nov., a plant growth hormone indoleacetic acid-producing and nitrogen-fixing bacterium isolated from carrot rhizosphere soil

A plant growth hormone indoleacetic acid-producing strain LX3-4 T was isolated from a carrot rhizosphere soil sample collected in Shandong Province, China. It is Gram-stain-positive, non-motile, and has irregular short rod-shaped cells. LX3-4 T shared high 16S rRNA gene sequence identity with Microbacterium oleivorans DSM 16091 T (99.4%), M. testaceum NBRC 12675 T (98.6%), M. marinum DSM 24947 T (98.5%), M. resistens NBRC 103078 T (98.4%), and M. paraoxydans NBRC 103076 T (98.3%). Phylogenetic analysis based on the concatenated gene sequences of 16S rRNA gene, housekeeping genes gryB and rpoB also showed the distinction between strain LX3-4 T and other Microbacterium species. Furthermore, analysis of the average nucleotide identities (ANI), the average amino acid identity (AAI), and the digital DNA-DNA hybridization (dDDH) values between strain LX3-4 T and its relatives revealed that strain LX3-4 T represents a distinct species. The genomic DNA G + C content of the strain is 69.5%. It can grow at 25–37 °C (optimum 37 °C), pH 5.0–10.0 (optimum pH 6.0–8.0), and the range of NaCl concentration is 0–7% (w/v) (optimum 1–5%). The colonies on agar plates are smooth, translucent, and pale yellow. The main cellular fatty acids of strain LX3-4 T are anteiso-C 15:0 , anteiso-C 17:0 , and iso-C 16:0 . The predominant respiratory quinones are MK-12 and MK-11. Diphosphatidylglycerol, phosphatidylglycerol, an unidentified glycolipid, and an unidentified phosphoglycolipid are major polar lipids. The cell-wall sugar of strain LX3-4 T is glucose. The cell-wall peptidoglycan contains glycine, alanine, lysine, and glutamic acid. In addition, this strain carries nitrogen fixation genes and can grow in nitrogen-free medium. Based on the polyphasic data, strain LX3-4 T represents a novel species of the genus Microbacterium , for which the name Microbacterium dauci sp. nov. is proposed with strain LX3-4 T (= CCTCC AB 2023103 T = LMG 33159 T ) designated as the type strain.