Methylobacterium spp. Isolated From Semiarid Soils Promote Growth and Drought Tolerance in Maize in Kenya

Drought significantly impacts crop growth and yield. Advances in agricultural biotechnology have enabled the use of drought-tolerant bacterial strains to mitigate the detrimental effects of drought on crops. This study, conducted during the 2024 dry season in Kiambu County, Kenya, evaluated 100 methylotrophic bacteria isolated from the rhizosphere of maize variety WE-4141 cultivated under severe drought conditions. Isolation was performed via serial dilution using ammonium mineral salt (AMS) medium supplemented with methanol as the sole carbon source, with identification confirmed through 16S rRNA gene sequencing. The isolates were assessed for plant growth-promoting and drought-tolerance properties under varying polyethylene glycol (PEG) 6000-induced osmotic stresses (-0.13, -0.50, -0.75, and -1.3 MPa). Seed priming was used to introduce bacterial inoculants at 10⁸ CFU/mL concentration. Under PEG-induced stress, strains K19 and K2 produced the highest quantities of gibberellic acid (GA) and indole acetic acid (IAA), with K2 also showing superior exopolysaccharide (EPS) production at the highest osmotic level. Enhanced biofilm formation was observed under drought conditions compared to controls. Antioxidant enzyme activity correlated positively with increasing PEG concentrations, with K19 exhibiting the highest catalase activity. In planta, drought stress significantly reduced plant growth, with uninoculated controls showing notable decreases in plant height, root, and shoot dry weights. Inoculation with K2 and K19 enhanced maize performance under drought, with their combination yielding the greatest improvements: shoot dry weight increased by 42.0%, root dry weight by 46.2%, and plant height by 65.8% compared to controls. These findings suggest that strains K2 and K19 hold potential as biostimulants for improving maize resilience and productivity under drought conditions and warrant further evaluation through field trials for potential bioformulation development.