Halotolerant Kocuria and Bacillus species enhanced saline-alkaline stress tolerance in foxtail millet

Abstract

Saline-alkaline stress is an increasing threat to agricultural productivity due to climate change and anthropogenic activities. Plant growth-promoting bacteria (PGPB) have emerged as a sustainable approach to enhance crop growth and stress tolerance under adverse environmental conditions. In this study, two halotolerant strains, Kocuria palustris Pp13 (Pp13) and Bacillus aryabhattai Pp16 (Pp16), isolated from the endosphere of Napier grass, were selected to analyze their stress tolerance mechanisms and evaluate their plant growth-promoting traits under saline-alkaline conditions. The stress tolerance and plant growth-promoting traits of Pp13 and Pp16 was evaluated under different salinity and pH conditions. The effects of bacterial inoculation on the growth and stress tolerance of foxtail millet ( Setaria italica L.) were assessed by analyzing plant growth, osmotic and oxidative stress responses, and ion accumulation under saline-alkaline conditions. Both Pp13 and Pp16 formed biofilms and accumulated proline under stress conditions, while their phosphorus-solubilizing activity was only slightly affected by elevated salinity and pH. Notably, Pp13 maintained IAA-like metabolite production and ACC deaminase activity under stress conditions. Inoculation with either strains significantly improved the growth and stress tolerance of foxtail millet under saline-alkaline conditions. Bacterial inoculation increased proline accumulation, elevated ascorbate peroxidase activity, improved photosynthetic performance, and reduced oxidative damage and sodium accumulation, resulting in greater biomass accumulation. These findings demonstrate that Pp13 and Pp16 possess multiple stress-adaptive and plant growth-promoting traits that contribute to improved crop performance under saline-alkaline conditions. The results highlight the potential of these strains as bioinoculants for sustainable crop production in saline-alkaline soils and identify Kocuria species as a relatively unexplored source of beneficial microorganisms for enhancing plant stress tolerance.