Root Exudates Modulate Plant—Microbe Interactions in the Rhizosphere

The chemical, physical, and biological interactions between roots and the surrounding environment of the rhizosphere are some of the most complex experienced by land plants. Over the last few years the field of rhizosphere biology has recognized the biological importance of root exudates in mediating interactions with other plants and microbes (Bais et al. 2004a, b; Walker et al. 2003; Weir et al. 2004). Chiefly, root exudates comprise two different classes of compounds. Low molecular weight compounds include amino acids, organic acids, sugars, phenolics, and various secondary metabolites, whereas high molecular weight exudates primarily include mucilage (high molecular weight polysaccharides) and proteins. Root exudation clearly represents a significant carbon cost to the plant (Uren 2000); however, the molecular mechanisms regulating exudation are still poorly understood. The roots of some plants also release border cells into the rhizosphere but literature discussing this phenomenon will not be covered in this chapter (for information see Hawes et al. 2000; Vicre et al. 2005).