Bacillus spp.-Mediated Drought Stress Tolerance in Plants: Current and Future Prospects

Drought is a common environmental stress that threatens the sustainability of agricultural productivity in many parts of the world. Numerous morphological, physiological, and molecular changes may occur in plants as a response to this stress, thus affecting their growth and development. Some rhizosphere microorganisms, including Bacillus species, have proved efficiency in increasing drought tolerance to plants growing in regions with water scarcity. These endophytic bacteria can survive in the soil under a wide range of conditions and, when colonizing rhizosphere, may enhance drought stress tolerance of plants by promoting their growth. This can be ensured directly by improving the uptake of water and essential nutrients; the production of phytohormones, siderophores, and enzymes; and indirectly by suppressing phytopathogens and/or activating induced systemic resistance (ISR). To mitigate the impact of drought stress, Bacillus spp. employ various mechanisms including the inhibition of the formation of reactive oxygen species (ROS); the modification of phytohormonal activities; the accumulation of osmolytes, soluble sugars, and other molecules; and the induction of stress-response genes expression. This paper provides an overview of the most remarkable achievements in Bacillus spp. research done in agricultural crop plants. These researches could improve the understanding of the physiological and molecular mechanisms deployed by these bacteria to alleviate drought stress and lead to cope with the impacts of climate change and satisfy food demands, especially when there is scarcity of water or drought.