Update on Root Exudation and Rhizosphere Biology Root Exudation and Rhizosphere Biology

Our understanding of the biology, biochemistry, and genetic development of roots has considerably improved during the last decade (Smith and Fedoroff, 1995; Flores et al., 1999; Benfey and Scheres, 2000). In contrast, the processes mediated by roots in the rhizosphere such as the secretion of root border cells and root exudates are not yet well understood (Hawes et al., 2000). In addition to the classical roles of providing mechanical support and allowing water/nutrient uptake, roots also perform certain specialized roles, including the ability to synthesize, accumulate, and secrete a diverse array of compounds (Flores et al., 1999). Given the complexity and biodiversity of the underground world, roots are clearly not passive targets for soil organisms. Rather, the compounds secreted by plant roots serve important roles as chemical attractants and repellants in the rhizosphere, the narrow zone of soil immediately surrounding the root system (Estabrook and Yoder, 1998; Bais et al., 2001). The chemicals secreted into the soil by roots are broadly referred to as root exudates. Through the exudation of a wide variety of compounds, roots may regulate the soil microbial community in their immediate vicinity, cope with herbivores, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species (Nardi et al., 2000; Fig. 1A). The ability to secrete a vast array of compounds into the rhizosphere is one of the most remarkable metabolic features of plant roots, with nearly 5% to 21% of all photosynthetically fixed carbon being transferred to the rhizosphere through root exudates (Marschner, 1995). Although root exudation clearly represents a significant carbon cost to the plant, the mechanisms and regulatory processes controlling root secretion are just now beginning to be examined. Root exudates have traditionally been grouped into lowand high-Mr compounds. However, a systematic study to determine the complexity and chemical composition of root exudates from diverse plant species has not been undertaken. Low-Mr compounds such as amino acids, organic acids, sugars, phenolics, and various other secondary metabolites are believed to comprise the majority of root exudates, whereas high-Mr exudates primarily include mucilage (high-Mr polysaccharides) and proteins. The rhizosphere is a densely populated area in which the roots must compete with the invading root systems of neighboring plant species for space, water, and mineral nutrients, and with soil-borne microorganisms, including bacteria, fungi, and insects feeding on an abundant source of organic material (Ryan and Delhaize, 2001). Thus, root-root, rootmicrobe, and root-insect communications are likely continuous occurrences in this biologically active soil zone, but due to the underground nature of roots, these intriguing interactions have largely been overlooked. Root-root and root-microbe communication can either be positive (symbiotic) to the plant, such as the association of epiphytes, mycorrhizal fungi, and nitrogen-fixing bacteria with roots; or negative to the plant, including interactions with parasitic plants, pathogenic bacteria, fungi, and insects. Thus, if plant roots are in constant communication with symbiotic and pathogenic organisms, how do roots effectively carry out this communication process within the rhizosphere? A large body of knowledge suggests that root exudates may act as messengers that communicate and initiate biological and physical interactions between roots and soil organisms. This update will focus on recent advancements in root exudation and rhizosphere biology.