Plant physiology processes associated with "plant-plant growth-promoting rhizobacteria" bioassays for the enhanced heavy metal removal

Rhizoremediation considers the phytoextraction and soil bioaugmentation strategies and optimize the synergistic effect between plants and microorganisms with a physiological basis related only to plants. It is known that plant growth-promoting rhizobacteria (PGPR) affect the plants growth facilitating the uptake of nutrients and protecting them; this interaction has been attractive because the biotechnological potential of microorganisms for metal removal from soils and transport of them to the plants. In the following sections of this chapter, the authors give some analysis of the importance about the establishment of "plant-PGPR bioassays" as tools to compare the relationships between in vitro physiological characteristics of rhizobacteria isolated from plant metal accumulators and the plant's physiology response, as follows: importance of siderophores as plant growth-promoting trait, evaluation of the in vitro production of siderophores by rhizobacteria, utility of the measurement of antioxidant activity in plants as indicator of heavy metal stress, and, finally, the description of cadmium effect by the antioxidant activity in two bioassays, with plant cell cultures and plantlets inoculated with a siderophore-producing bacteria (SPB).