Effects of Symbiotic Legumes and Rhizobia on Plant and Microbial Biodiversity in Natural and Agricultural Ecosystems

Abstract

The biodiversity of plant and microbial species in natural and agricultural ecosystems can .be altered by symbiotic legumes and rhizobia via their effects on ecosystem processes and function. Legumes do not only use their seed/root exudate molecules to control plant and microbial growth in soil, but also to selectively attract and promote germination and growth in specific species including Striga. With. legumes and Nz-fixing diazotrophs, the ability to meet their N requirements from Nz fixation naturally puts them at an advantage over non-fixing systems in occupying various ecological niches. The steady supply of fixed-N in an environment can change plant diversity through the emergence and dominance of nitrophilous species, just as their ability.to secrete fixed-N and increase mineral nutrient supply via the effects of exudates can advantage associated species of plants and microbes and increase their numbers. Through the effect of lumichrome released by rhizobia, some plant species are able to improve their water relations, enhance nutrient uptake and tolerate drought via the molecule's control of stomatal function. That way, such species can colonize harsh ecological niches better than others. Furthermore, the rhizobial microsymbiont is capable of altering the biodiversity of soil-borne plant pathogens either directly by antibiosis, or indirectly via their elicitation of isoflavonoid phytoalexin biosynthesis in host legume plants that decreases the population of pathogenic microbes.