Phosphorus disequilibrium in the tripartite plant-ectomycorrhiza-plant growth promoting rhizobacterial association

Abstract

Plant roots and rhizospheres are colonized by an extensive and diverse microbial community. These microbes may form mutualistic, commensal, and/or pathogenic relationships and influence agricultural and forest productivity. Symbiotic ectomycorrhizal (EcM) fungi colonize the roots of many tree species, and the literature on these associations extensively describes their influence on plant nutrient relations and response to environmental stress. Similarly, soil bacteria ubiquitously colonize roots and rhizospheres and many of these bacteria may also play roles in influencing tree productivity. In particular, plant growth promoting rhizobacteria (PGPR) positively affect plant growth by altering nutrient availability in soils and inducing changes in plant hormone balance, plant stress resistance, and immunity pathways. In nature, EcM fungi and soil PGPR co-exist and the interaction and composition of this multi-tiered rhizosphere community aids in the acquisition of nutrient resources from soils as well as host plant response to environmental stress. The assembly of EcM communities is influenced by tree species and environmental conditions, and the tree and EcM species further influence PGPR community structure. Functionally, these symbiotic associations exhibit unique expression profiles and ecophysiological activities within the tripartite association. EcM and PGPR mediate production of complex arrays of exudates, including organic acids, siderophores, enzymes, and other organic compounds, which alter nutrient equilibria in soils, leading to increased access to phosphorus (P) and other macro- and micronutrients. As a metaorganism, the tripartite ectomycorrhizas increase the ecological breadth of host trees and influence the structure and function of forested ecosystems.