Isolation of Siderophore-Producing Bacteria from Extreme Environments and Their Role in Improving Maize Salinity–Alkalinity Tolerance

Soil salinization represents a significant abiotic constraint to global agricultural sustainability. The potential of extremophile plant growth-promoting bacteria (PGPB) to alleviate such stress in maize was investigated in this study. Siderophore-producing PGPB enhance plant growth and improve the rhizosphere microenvironment by increasing nutrient availability and inducing systemic resistance. Two salt-tolerant, high-siderophore-producing PGPB strains, Bacillus toyonensis TRM58010 and Peribacillus frigoritolerans TRM58009, were isolated and identified from soil samples collected on the Pamir Plateau. In this study, we found that B. toyonensis TRM58010 synthesized catechol-type siderophores, which enhanced iron availability for maize in saline–alkaline conditions, thereby improving iron nutrition and directly promoting root and stem growth under salt stress. P. frigoritolerans TRM58009 produced hydroxamate-type siderophores, which increased maize iron uptake and stimulated antioxidant enzyme activity, mitigating oxidative stress caused by salinity and alkalinity and supporting overall plant health. Both strains demonstrated robust tolerance to extreme alkaline and saline conditions. Hydroponic and pot experiments showed that these strains significantly improved maize germination rate, root and stem development, plant height, leaf growth, antioxidant enzyme activities, and chlorophyll content under saline–alkaline stress. Notably, the application of P. frigoritolerans TRM58009 bacterial suspension increased maize leaf catalase, peroxidase, and superoxide dismutase activities by 15.712%, 11.584%, and 2.820%, respectively (all p < 0.05), while decreasing malondialdehyde (MDA) content by 15.685% (p < 0.05). P. frigoritolerans TRM58009 elevated chlorophyll a content by 23.4% (p < 0.05). These findings demonstrate the potential of extremophile PGPB strains to mitigate the impact of saline–alkaline stress on maize growth. The distinct growth-promoting effects of these strains, isolated from Pamir Plateau meadow soils, present a promising strategy for bioremediation of saline–alkaline lands and the development of efficient microbial fertilizers. By advancing the use of salt-tolerant siderophore-producing bacteria, this study lays the foundation for innovative approaches to enhance crop resilience and productivity in challenging environments.