Extremophiles as Plant Probiotics to Promote Germination and Alleviate Salt Stress in Soybean

Bacteria isolated from extreme environments have been shown to promote plant growth under challenging conditions. This study aimed to examine the performance of the extremophilic microorganisms under salt stress and their ability to improve the tolerance of soybean plants to this stress. In vitro experiments showed that bacteria belonging to the genera Stenotrophomonas and Exiguobacterium were halophilic and displayed plant growth-promoting activities under salt stress. For instance, these two species enhanced soybean germination rate between 35 and 43% in comparison with non-inoculated seeds. In addition, inoculation allowed soybean roots to double their size, both in length and in dry biomass, under 250 mM NaCl. The plant physiological responses correlated with changes in plant gene expression during developmental and stress responses. The expression of a growth marker gene (Glyma.03G226000) increased in the presence of halophilic bacteria both under salt stress (5- to 24-fold) and under non-stress conditions (17- to 25-fold). Soybean genes responsive to stress, such as Glyma.02G228100, Glyma.04G180400, Glyma.08G189600, and Glyma.17G173200, were highly induced by salt in bacteria-inoculated roots. This work showed that the extremophilic bacteria used in these experiments could be used as potential bio-inoculants to help alleviate salt stress and plant growth.