Microbial siderophores in rhizophere interactions in heavy metal-containing environments

Heavy metals may affect microbial and plant physiological processes in two ways: (i) an excess of the metals induces elevated levels of reactive oxygen species (ROS), and ROS-induced oxidative stress, in turn, affects microbial and plant cellular and metabolic processes and hence, their growth. (ii) Metals interfere with iron acquisition, which, together with pH-induced insolubility, enhances iron deficiency symptoms in microbes and plants. Iron being essential, microbes and plants have evolved strategies for its acquisition. Basically, most microbes and graminaceous plants elicit low-molecular-weight, high-affinity, iron-scavenging compounds, siderophores. In contrast, dicotyledonous and non-grass monocotyledonous plants elaborate an increased iron reductase activity, coupled with release of phenolics and extrusion of protons. However, such plants can benefit from siderophore production by associated microbes. Despite their preference for iron, siderophores bind other metals, albeit with reduced affinity. Binding of siderophores to metals dramatically alters free metal concentrations and can, thus, play a bioprotective role in microbes and plants. Therefore, mechanisms that reduce the bioavailability of toxic metals in the environment for microbes, as well as provide plants with improved access to metals determine eco-toxicologically relevant metal concentrations in the soil, and their influence in microbe-assisted phytoremediation. The objective of the study was, therefore, to study the role of siderophores produced by Streptomyces spp in metal-induced microbial and plant rhizosphere processes and to evaluate their application in chelator-assisted phytoremediation of metal pollution