Alginate as a feature of osmotolerance differentiation among soil bacteria isolated from wild legumes growing in Portugal

Abstract

Plants are naturally colonized by bacteria that can exert beneficial effects on growth and stress tolerance. These bacteria can be used as inoculants to boost crop productivity and plants resilience, and can be especially interesting if they are able to survive to abiotic stresses, such as drought. Herein we report the mechanisms that soil bacteria resort to tolerate drought and we also explore the influence of each mechanism to the level of drought tolerance exhibited, in order to test the hypothesis that different levels of tolerance displayed by bacteria are linked to differential efficiency of osmotolerance mechanisms. For this, the biochemical and physiological responses of bacterial strains of different genera and displaying different levels of tolerance to osmotic stress (sensitive, moderately tolerant and tolerant) induced by polyethylene glycol-6000 (PEG) were studied. Betaine, trehalose and alginate content increased in the majority of the strains exposed to PEG. Betaine was the osmolyte with higher increases, evidencing the important role of this compound in the tolerance of bacteria to drought. However, betaine and trehalose levels were not significantly different among bacteria with different osmotolerance levels. Several biochemical endpoints (protein content, superoxide dismutase, catalase, glutathione-S-transferases) related to oxidative stress were assessed, since oxidative damage has been reported in drought conditions, but little information is available. The oxidative stress parameters were not sufficient to explain differences in the osmotolerance observed for the tested strains. In contrast, alginate showed significant differences among the three levels of osmotolerance, linking the level of osmotolerance with the ability of soil bacteria to synthesize and accumulate alginate intracellularly for the first time. Moreover, our results show that this ability is present in different bacteria genera. Thus, evaluating the ability to synthesize alginate might be an important cue when considering bacterial inoculants for osmotically stressful conditions.