Development and Resource Exchange Processes in Root Symbioses of Legumes

Abstract

Plants are associated with complex microbiomes, and many of the microorganisms that reside on plant surfaces (epiphytes) or within plant tissues (endophytes) are beneficial for the host plant and improve plant growth or stress resistance by a variety of plant growth-promoting capabilities. The plant microbiome could serve as a tool box to design synthetic microbiomes to enhance plant growth and crop resiliency under stress or to integrate benefits of plant microbiomes as important traits into plant breeding programs. For legumes, the most important members of the plant microbiome are nitrogen (N)-fixing rhizobia and arbuscular mycorrhizal (AM) fungi. Legumes harbor rhizobia in specialized root nodules, in which the bacteria fix gaseous N from the atmosphere and transfer plant available forms of N to host. AM fungi play a key role for the uptake of nutrients such as phosphate and nitrogen and improve the resistance of plants against abiotic (e.g. drought, salinity, and heavy metals) and biotic (herbivores and pathogens) stresses. Both partners compete with these benefits for photosynthetically fixed carbon from the host. In this review, we will summarize our current understanding of these interactions and will also focus on cooperative or competitive interactions between these two root symbionts in tripartite interactions.