Mycorrhizosphere: Strategies and Functions

Abstract

Introduction: Hiltner recognized the rhizosphere as the volume of soil in the immediate vicinity of the roots, which is predominantly affected by the activity of plants. The rhizosphere differs from the surrounding soil in most of the physico-chemical factors and a wide range of microorganisms colonizes this rhizosphere soil along with the rhizoplane (i. e., the root surface; Phillips et al. 2003). The number of these microorganisms per gram of soil is much larger in the rhizosphere compared to bulk soil. This increased microbial activity in the vicinity of roots can be ascribed to root exudates, sloughed senescent root cells and mucigel, which have been described as rhizodeposition (Mukerji et al. 1997; Bansal et al. 2000). In nature, most of the actively absorbing rootlets form a symbiotic association with mycorrhizal fungi, which are ubiquitous soil inhabitants. The formation of symbiotic associations with mycorrhizae significantly changes the physiology and/or morphology of roots and plants in general, leading to altered root exudation (Bansal and Mukerji 1994). The changes in root exudates affect the microbial communities around the roots, leading to the formation of the “mycorrhizosphere” (Mukerji et al. 1997; Varma et al. 1999). The mycorrhizosphere is the zone of soil where the physical, chemical and microbiological processes are influenced by plant roots and their associated mycorrhizal fungi. A major difference in the rhizosphere around the nonmycorrhizal roots and mycorrhizosphere effect is the presence of extramatrical hyphae of mycorrhizal fungi. These extramatrical hyphae extend well beyond the roots into the bulk soil and are an important source of carbon to the soil organisms (Schreiner and Bethlenfalvay 1995). Themycorrhizal hyphae increase the soil aggregation and in root association increase exudation,which favors the microbial growth (Schreiner and Bethlenfalvay 1995; Bansal and Mukerji 1996). Themycorrhizospheremicrobiota differs qualitatively as well as quantitatively fromthe rhizosphereofnonmycorrhizal plants.Thesoil microfauna influences the mycorrhiza formation as well as the host growth response (Fitter and Garbaye 1994).Many kinds of interactions occur between these microbial communities in the mycorrhizosphere and mycorrhizae. The interactions between the mycorrhizae and soil microorganisms may be mutualistic or competitive and they affect the establishment and functions of mycorrhizal symbionts as well as modify the interactions of the plant with other symbionts or pathogens in soil.