Heavy Metal and Drought Stress in Plants: The Role of Microbes—A Review

Plants are composed of complex organisms that include morphological, physiological and biochemical segments. A number of environmental factors such as, heavy metals, drought, salinity, heat, and cold cause stress towards plants and impact on their growth and yield values. Heavy metal contamination and drought are considered to be two significant environmental (abiotic) factors that are receiving increased attention due their effects on plants. Many plant species from previously non-drought prone areas now face drought as a new stress factor and must develop their avoidance mechanisms. Comparing drought-related tolerance strategies, plants have developed a number of strategies to fight against heavy metal pollution due to its wide coverage. These heavy metal tolerance strategies include detoxification, compartmentalization, and storage in cell vacuoles in plants. Few similarities have been found in the mechanisms of both drought and heavy metal strategies. The use of beneficial microbes is considered as one of the most promising methods to improve stress tolerance because plant-associated microbes decrease metal accumulation while enhancing nutrient supply and water under such stress. In this review, we summarize how drought and heavy metals contamination effect on plant growth, with particular focus on stress avoidance and tolerance mechanisms, as well as, the role of microbes to enhance stress tolerance. Recommendations including irrigation, fertilization and molecular techniques to increase the plant’s tolerance mechanisms against heavy metal and drought conditions are also discussed.