Extremophilic Bacteria Improve Germination, Early Vigor, and Oxidative-Stress Status in Phaseolus vulgaris Under Simulated Water Deficit

Water deficit constrains common bean (Phaseolus vulgaris L.) production in the acidic Andisols of south-central Chile. We evaluated Bacillus haynesii CamB6 (thermotolerant, Chilean hydrothermal spring) and Microbacterium sp. Se63 (psychrotolerant, Antarctic soil), singly and as a two-strain consortium, for in vitro plant-growth-promoting traits and, via seed bacterization, on P. vulgaris cvs. Manteca and Señorita under simulated water deficit (PEG-8000, 0–15% w/v). The strains showed complementary in vitro profiles: Se63 solubilized more phosphate and produced siderophores and lipase, whereas CamB6 produced catalase, protease, HCN, and more indole-3-acetic acid. Responses in planta were stress- and cultivar-dependent. In the susceptible cultivar Manteca, Se63 alone increased shoot length and seedling dry weight under mild-to-moderate stress but did not rescue germination at 15% PEG, where it did not differ from the non-inoculated control; there, CamB6 was the most effective single strain (germination 31.0% to 52.0%). In the tolerant cultivar Señorita, Se63 alone produced the largest root-length gain under severe stress (100.4 to 208.6 cm) and the thickest roots, while the consortium rescued germination (65.0% to 93.0%), shoot length (+291%) and dry weight (+79%), attenuated lipid peroxidation and raised antioxidant capacity. Combining these extremophile strains therefore yields a stress-conditional, cultivar-dependent benefit in P. vulgaris.