Microbial Siderophores in the Plant Rhizosphere

Abstract

Siderophores are iron chelating agents that are produced by almost all microorganisms in response to iron deficiency. Due to the requirement of iron for cell growth and metabolism, siderophore mediated acquisition of iron plays a central role in determining the ability of different microorganisms to colonize plant roots and contributes to microbial interactions in the plant rhizosphere. There are now approximately 500 known siderophores, some of which are widely used by a variety of microorganisms, whereas others are used only by the same microbial species and strains that produce them. Siderophores also have been examined for their potential importance in plant iron nutrition and for their ability to mobilize heavy metals. There has been considerable debate over their relative importance for plant iron nutrition that has centered around their ability to release iron by means of chelate reductases that function in Strategy I plants and to exchange iron with phytosiderophores in Strategy II grasses. A key question concerning the relevance of siderophores in plant nutrition has been whether siderophores are produced in sufficient quantities by microorganisms to supply physiologically relevant quantities of iron to plants. Siderophores are generally detected in low concentrations in soil extracts. Molecular methods using reporter genes further show that siderophore production is limited to sites of high microbial activity. There is now general recognition that siderophores have a fundamental role in determining the bioavailability of iron to microorganisms that colonize the rhizosphere and are required for rhizosphere competence in microorganisms that are being developed for biocontrol of plant diseases and plant growth promotion.