6. Conclusions and Perspectives
The presence of chromium compounds in soils inhibits seed germination and induces various morphological and physiological defects in many plants, including legumes. Toxicity of chromium in plants is connected with the enhanced ROS formation and oxidative stress development resulting in the intensified protein modification, lipid peroxidation, and DNA damage. In legume-rhizobial symbiosis both host plant and nodule bacteria undergo oxidative stress induced by chromium, with rhizobia being more stressed due to preferential Cr accumulation in root nodules. Available data suggest that inoculation with nodule bacteria can be considered as an effective approach to minimize toxic effects of chromium and other heavy metals on agricultural plants. At the same time, the protective efficacy of nodule bacteria depends on many factors such as type and concentrations of metals, compatibility of partners, virulence, adaptive capacity, and nitrogen-fixing activity of bacteria. Therefore, the effects of heavy metal on legume-rhizobium symbiosis and search of ways to enhance metal resistance of nodule bacteria are perspective potential research direction. It seems that Cr at low concentrations may induce mild oxidative stress [128], which can have beneficial rather than detrimental effects on legume-rhizobium symbiosis and plant metabolic processes (Figure 2). As known, mild oxidative stress may induce adaptive response, which enhances resistance to many lethal stresses [128]. In addition, mild oxidative stress plays an important role in establishment of effective legumerhizobium symbiosis [103]. We propose that using the pretreatment with nodule bacteria at low levels of oxidants can aid bacteria to resist high levels of heavy metals in the environment. The construction of nodule bacterial strains with higher resistance to environmental stresses may be a great opportunity to increase the benefit from their use in bioremediation and cultivation at polluted areas.
