ABSTRACT
This chapter describes the main groups of organisms that are closely associated with plant roots and their effect on plant growth and survival. It reviews the biological and chemical interactions that occur in the rhizosphere and how this changes the outcome for the associated plant. Understanding the implications for ecosystem function of soil biodiversity and processes is the last frontier in terrestrial ecology. The rhizosphere is a hot spot of soil biodiversity driven primarily by plant roots. Nitrogen is the most limiting nutrient for plant growth in terrestrial ecosystems. The specificity of the association between legumes and their bacterial symbionts depends on a very fine molecular communication between the plants and the rhizobia. A mycorrhiza is a symbiotic, nonpathogenic, permanent association between a plant root and a specialized fungus, both in the natural environment and in cultivation. The research about soil fungi that have deleterious effect on plant growth has historically focused on agricultural systems for obvious economic reasons.
