Characterization of a novel root-associated diazotrophic rare PGPR taxa, Aquabacter pokkalii sp. nov., isolated from pokkali rice: new insights into the plant-associated lifestyle and brackish adaptation.

Salinity impacts crop growth and productivity and lowers the activities of rhizosphere microbiota. The identification and utilization of habitat-specific salinity-adapted plant growth-promoting rhizobacteria (PGPR) are considered alternative strategies to improve the growth and yields of crops in salinity-affected coastal agricultural fields. In this study, we characterize strain L1I39(T), the first Aquabacter species with PGPR traits isolated from a salt-tolerant pokkali rice cultivated in brackish environments. L1I39(T) is positive for 1-aminocyclopropane-1-carboxylate deaminase activity and nitrogen fixation and can promote pokkali rice growth by supplying fixed nitrogen under a nitrogen-deficient seawater condition. Importantly, enhanced plant growth and efficient root colonization were evident in L1I39(T)-inoculated plants grown under 20% seawater but not in zero-seawater conditions, identifying brackish conditions as a key local environmental factor critical for L1I39(T)-pokkali rice symbiosis. Detailed physiological studies revealed that L1I39(T) is well-adapted to brackish environments. In-depth genome analysis of L1I39(T) identified multiple gene systems contributing to its plant-associated lifestyle and brackish adaptations. The 16S rRNA-based metagenomic study identified L1I39(T) as an important rare PGPR taxon. Based on the polyphasic taxonomy analysis, we established strain L1I39(T) as a novel Aquabacter species and proposed Aquabacter pokkalii sp nov. Overall, this study provides a better understanding of a marine-adapted PGPR strain L1I39(T) that may perform a substantial role in host growth and health in nitrogen-poor brackish environments.