Arachidicoccus rhizosphaerae gen. nov., sp. nov., a plant-growth-promoting bacterium in the family Chitinophagaceae isolated from rhizosphere soil

Abstract

Three novel bacterial strains, designated Vu-144T, Vu-7 and Vu-35, were isolated on minimal medium from rhizosphere soil of field-grown cowpea and subjected to a taxonomic study using a polyphasic approach. Cells of the strains were Gram-stain-negative, non-motile, non-spore-forming, coccoid rods, and formed non-pigmented colonies. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain Vu-144T was affiliated with an uncultivated lineage of the phylum Bacteroidetes . Its closest phylogenetic neighbour was the recently described species Niastella populi , a member of the family Chitinophagaceae , with just 90.7 % sequence similarity to the type strain. The only isoprenoid quinone detected was menaquinone 7 (MK-7). The fatty acid profiles showed large amounts of iso-C15 : 0, iso-C17 : 0 3-OH and iso-C15 : 1 G and minor amounts of summed feature 3 (C16 : 1ω7c and/or iso-C15 : 0 2-OH), C16 : 0 and other fatty acids, allowing the differentiation of the strains from other genera. The G+C content of the genomic DNA of the three strains ranged from 43.1 to 44.3 mol%. In addition to phosphatidylethanolamine, the major polar lipids were three unidentified aminophospholipids (APL1–APL3), two unidentified phospholipids (PL1, PL2) and three unidentified lipids (UL1–UL3). Biochemical test patterns also differed from those of Niastella populi and members of other genera. All three isolates showed plant-growth-promoting properties, e.g. the ability to produce indole-3-acetic acid and NH3 and to solubilize phosphate, utilized 1-aminocyclopropane 1-carboxylate (ACC) as a sole source of nitrogen and possessed the ACC deaminase enzyme. The novel isolates readily colonized roots and stimulated growth of tomato and cowpea under glasshouse conditions. Inoculated plants showed a 45–60 % increase in dry matter weight with respect to uninoculated controls. On the basis of the evidence from our polyphasic study, isolate Vu-144T represents a novel genus and species in the family Chitinophagaceae , for which the name Arachidicoccus rhizosphaerae gen. nov., sp. nov. is proposed. The type strain of Arachidicoccus rhizosphaerae is Vu-144T ( = KCTC 22378T = NCIMB 14473T).