Seed endophyte Kosakonia cowanii ZX1-2 enhances drought tolerance of Cenchrus pauciflorus via exopolysaccharide-mediated antioxidant and osmotic regulation

BACKGROUND: Cenchrus pauciflorus Benth. (field sandbur) is a highly invasive weed with strong drought tolerance. However, the contribution of seed-associated endophytes to its stress adaptation remains unclear. This study investigated whether seed endophytes enhance drought tolerance and growth in C. pauciflorus, with particular emphasis on microbial exopolysaccharide (EPS)-mediated mechanisms. RESULTS: A dominant seed endophyte, Kosakonia cowanii ZX1-2, was isolated and characterized for drought tolerance, indole-3-acetic acid (IAA) production, and EPS synthesis. Its effects on seed germination, seedling growth, and drought responses of C. pauciflorus were evaluated using Petri dish and pot experiments under PEG-6000-induced osmotic stress. Antioxidant enzyme activities and EPS sugar composition were also analyzed. Strain ZX1-2 maintained stable growth under 14% PEG-6000 stress and produced 22.23 mg/L IAA. Under drought stress, ZX1-2 inoculation increased germination energy, root-to-shoot ratio, root length, and fresh weight by 72%, 46%, 62%, and 67%, respectively. In pot experiments under drought stress, ZX1-2 increased plant height, stem diameter, root length, and biomass by 34%, 47%, 40%, and 183%, respectively. EPS treatments further promoted growth. In the seed experiment, two-fold-diluted EPS increased germination rate, germination energy, root length, and shoot length by 75%, 61%, 188%, and 113%, respectively. In the pot experiment, two-fold-diluted EPS increased plant height, stem diameter, root length, and biomass by 40%, 201%, 54%, and 70%, respectively. EPS application also enhanced CAT, POD, and SOD activities and increased soluble protein and proline accumulation. Targeted sugar profiling detected 19 EPS-associated sugars, among which inositol, D-( +)-trehalose, maltose, and D-(-)-fructose were dominant. CONCLUSION: These findings suggest that K. cowanii ZX1-2 may enhance drought tolerance and growth in C. pauciflorus, and that EPS-associated changes in antioxidant defense and osmotic-related metabolites may contribute to this effect. This study provides evidence for a seed endophyte-driven mechanism that may contribute to the drought adaptation and invasion success of C. pauciflorus under increasingly arid conditions.