An engineered Pseudomonas stutzeri strain overexpressing the sigma factor AlgU promotes maize growth under salt stress

Soil salinization poses a significant threat to global agriculture, necessitating the development of sustainable strategies to increase crop resilience. Plant growth-promoting rhizobacteria (PGPR) offer a promising solution; however, their effectiveness in saline soils is often limited by their own salt sensitivity. The role of the extracytoplasmic function of the sigma factor AlgU in the salt stress adaptation of the nitrogen-fixing PGPR Pseudomonas stutzeri A1501 was investigated in this study. Through the construction of isogenic algU knockout (ΔalgU) and overexpression (OE-algU) strains, combined with phenotypic assays, transcriptomic profiling, and plant experiments, we demonstrate that AlgU acts as a master regulator of salinity tolerance. AlgU increased bacterial survival under acute salt shock, promoted biofilm formation, and, crucially, protected nitrogenase activity from salt inhibition. RNA-seq analysis revealed that AlgU orchestrates a comprehensive transcriptional reprogramming, upregulating the expression of genes involved in exopolysaccharide synthesis, osmoprotection (otsA), and central carbon metabolism (zwf, fumC). This coordinated response ensures an adequate supply of energy and reducing equivalents while maintaining cellular homeostasis. Consequently, inoculation with the OE-algU strain significantly alleviated salt stress in maize, improving seedling growth in pot experiments and outperforming the wild-type strain by increasing grain yield in saline-alkali field trials. Our findings establish AlgU as a key genetic determinant for engineering salt-tolerant PGPR, providing a mechanistic framework for the development of effective microbial inoculants to improve crop productivity in saline soils. KEY POINTS: AlgU orchestrates biofilm formation, osmoprotection, and energy metabolism to confer salt tolerance in P. stutzeri. Engineering an AlgU overexpression strain increases maize yield under saline-alkali field conditions. AlgU safeguards nitrogenase activity under salt stress by reprogramming energy and osmoprotective pathways, thus linking bacterial resilience to improved crop yield.