Roots of many plant species develop symbioses with mycorrhizal fungi and nitrogen-fixing bacteria. Arbuscular mycorrhization is a mutualistic association between the vast majority of land plants and Glomeromycotan fungi that improves the uptake of phosphorus, nitrogen and water. Nutrients are transferred from the fungus to host plants through intracellular structures known as arbuscules. Plants of the legume family are also able to develop an association with nitrogen-fixing rhizobia that leads to the formation of root nodules. The establishment of such symbioses requires a signal exchange between plants and their microbial partners. Flavonoids and strigolactones are the primary plant signals that initiate the molecular conversations. In turn, rhizobia and mycorrhizal fungi produce signals known as Nod and Myc factors respectively. Genetic and biochemical approaches in model legumes allowed the identification of Nod factor receptors and downstream signaling pathways. Several of these signaling components are required for both nodule development and arbuscular mycorrhization demonstrating the existence of a common symbiotic (Sym) pathway. Recent data also point towards the existence of parallel signaling pathways. Environmental conditions affect the establishment of symbiotic associations and plant hormones often mediate these responses. Plant hormones coordinate the development of new organs, the root nodules, in a process similar but not identical to lateral root formation. Therefore, hormones play key roles in adaptive and developmental processes that regulate plant – microbe symbioses.
Mukherjee, A. and J. M. Ané (2011). Plant hormones and initiation of legume nodulation and arbuscular mycorrhization. Ecological Aspects of Nitrogen Metabolism in Plants. J. a. T. Polacco, C. Hoboken, NJ, Wiley Blackwell Press: 354-396.