8. The Role of Plant Growth-Promoting Rhizosphere Bacteria in Toxic Metal Extraction by Brassica spp.

Brassicaceae are scattered all over the world, where they exclusively grow on serpentine rocks in Western Australia, New Zealand, South Africa, Japan, Philippines, Brazil, Portugal, Italy, Turkey, Cuba, eastern Canada, and western north America. Although serpentine rocks cover only less than 1% of the earth’s surface their worldwide distribution has recently attracted many researchers in exploring their distinctive potential for phytoremediation plant communities, mainly members of Brassicaceae plant family inhabiting on serpentine rocks of these countries. On the other hand, the majority of Brassicaceae plant family are slow-growing plants producing little biomass and their use for phytoextraction purposes may not be practical, especially when bioavailable metal concentration is high in the contaminated conditions. Therefore, recently emerging practices in the field of phytoremediation have pointed out various focuses such as the utility of high-biomass crops such as maize, peas, oats and Indian mustard and associated soil practices including application of synthetic chelators such as ethylenediaminetetraacetic acid and nitrilotriacetate and elemental sulphur to enhance metal uptake by these plants. These approaches may meet the conditions required for the phytoremediation. However, one of the most critical components of phytoextraction process is the bioavailability of heavy metals meaning the portion of the metals that is available for absorption into living organisms such as plants. It has been known that various plant growth-promoting rhizobacteria (PGPR) associated with plant roots may provide some beneficial effects on plant growth and nutrition through a series of well known mechanisms, namely, nitrogen fixation, production of phytohormones and siderophores, and transformation of nutrients once they are either applied to seeds or incorporated into the soil. Similarly, heavy metal mobility and availability can substantially be driven by PGPR populations through their release of chelating agents, acidification, and phosphate solubilization in rhizosphere. Miscellaneous PGPR were also shown to tolerate heavy metals in different ways including the mechanisms of exclusion, active removal, biosorption, precipitation, and extra- or intracellular bioaccumulation. Since these processes may affect the solubility and the bioavailability of heavy metals to the plant and hence modifying their toxic effects, interactions between hyperaccumulator plants such as Brassicaceae spp., and metal tolerant or resistant PGPR are considered to have an increasing biotechnological potential in the remediation of anthropogenically polluted soils. Present chapter/review considers the role of PGPR on soil-heavy metal-plant interactions and more specifically bioaccumulation of toxic metals by Brassicaceae plant family.