Exopolysaccharides and Biofilms in Mitigating Salinity Stress: The Biotechnological Potential of Halophilic and Soil-Inhabiting PGPR Microorganisms

Abstract

Soil salinity is a key environmental factor restraining the productivity of soil and crop plants. In different parts of the world, agricultural productivity is decreasing mostly because of drought and salinity increase. The situation may become worse for global warming in the future. A wide range of adaptation and mitigation strategies have been adopted to cope up with these circumstances. Among the measures to control soil salinity, those that involve water management and changes in micro relief can be time-consuming and cost-intensive. In this scenario, biological methods of salinity stress management appear as a good alternative, and halotolerant bacteria may play an important role in mitigating salinity stress. The natural ability of these microorganisms to cope with harsh environmental conditions has been attributed to their high genetic diversity and physiological adaptations, such as the synthesis of compatible solutes, growth in biofilm consortia and production of exopolysaccharides (EPSs) and halotolerant enzymes. Furthermore, they are able to produce plant growth-promoting hormones. Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can play an important role in alleviating soil salinity stress during plant growth, and bacterial EPS can help to mitigate salinity stress by reducing the content of Na+ available for plant uptake. To compensate the stress imposed by salinity, biofilm formation and EPS production are significant strategies of salt-tolerant bacteria to assist metabolism. Halophilic microorganisms can form biofilm and accumulate EPS at increasing salt stress. EPSs promote bacterial colonization of plant roots and soil particles, and they can eventually be added to soil to improve its structure and, consequently, to improve plant growth. In a long-term vision, alleviation of derelict soil, improved crop yield and restoration of mangrove diversity are possible by mechanistic use of EPS-producing halotolerant microorganisms.