16 - Mycorrhizas in ecological interactions

Abstract

Mycorrhizal symbioses are prevalent in all major terrestrial biomes. The researchers are now actively addressing the questions that relate to the biology of mycorrhizal plants in natural environments, such as whether there are impacts of mycorrhizal colonization on the fitness of individual plants. Do mycorrhizal fungi influence the outcome of competitive interactions? Investigations have explored the contributions of mycorrhizas to fitness of individuals and species, to outcomes of competitive interactions between species, and to the possibility that mycorrhizal status may change both the ability of plants to coexist and the diversity of plant assemblages. With the wider recognition of structural and functional diversity among different mycorrhizal partnerships, has come the appreciation that the interdependence of plant and fungal partners may lead to changes in fungal, as well as plant communities. This chapter discusses the current research and understanding on the ecological interactions of mycorrhizas. The recognition of stimulatory effects of some classes of bacteria on the processes of ectomycorrhiza (ECM) colonization of roots has heightened awareness of the complexities of microbial interactions in soil. There are many reports of the interactions between mycorrhizal colonization of plants and the incidence and severity of diseases caused by plant pathogens. The effects are variable and influenced by plant nutrition, relative density of the inoculum of the pathogen and mycorrhizal fungus, and whether or not the plants were mycorrhizal before being challenged with propagules of the pathogen. It has long been recognized that hyphae of arbuscular mycorrhizal (AM) fungi are important binding agents in soil. Exudates from hyphae of quite specialized types have been suggested to be important contributors to stabilization of soil structure by AM fungi. The outcomes of multitrophic interactions are variable and mechanisms even less well understood. They are, however, likely to play significant roles in nature.