MicroRNAs in the rhizobia legume symbiosis.

Legumes are agronomically valuable crops for food and fodder production worldwide because they are rich in protein, oil, fiber, and micronutrients. In addition, legumes require less chemical fertilizer than other major crop plants since they can assimilate some nutrients through symbiotic interactions with soil microbes. These relationships are mutually beneficial for the partners because the plant provides carbon-based energy to the microbe in exchange for essential nutrients. In permissive environmental conditions, legumes can establish symbiotic interactions with rhizobial bacteria and with arbuscular mycorrhizal (AM) fungi. The rhizobia-legume symbiosis leads to formation of root nodules, the site of bacterial nitrogen fixation and nitrogen uptake by the host plant, whereas the AM symbiosis results in the formation of arbuscules, the site for phosphorous nutrient exchange. Notably, a wide range of plant hosts can form a mycorrhizal symbioses, as opposed to the rhizobial symbiosis which is nearly-exclusive to legumes. Formation of a successful symbiosis is contingent upon a nutritional insufficiency of the plant. In the case of the rhizobia-legume symbiosis, the plants form symbiotic root nodules when grown in nitrogen-limiting conditions, but do not initiate the symbiosis when nitrogen levels are adequate. Nitrogen-stressed plants begin a molecular dialogue with rhizosphere microbes initiating the early stages of symbiosis. Ultimately, changes in plant and bacterial gene expression facilitate microbial infection of the plant root, the development of symbiotic root structures, and the maintenance of nitrogenfixing root nodules. Plants secrete flavonoids and related compounds from the actively growing region of the root. These compounds promote the expression of “nod” genes in compatible rhizobial species, and nod-gene products synthesize “nod factor,” bacterial lipochitooligosaccharide signaling molecules. Plant perception of nod factors by LysM receptor-like kinase potentiates immediate subcellular changes in the root epidermis and later changes in the root cortex. Root epidermal cells respond by depolarization of the root hair plasma membrane, and cytoplasmic Ca2+ spiking in the root hair initiates a signaling pathway involving a calcium and calmodulin dependant protein kinase (CaMK), two GRAS family proteins NSP1 and NSP2, and an ERF transcription factor, ERN. Cortical cells respond to nod factor signals by reinitiating the cell cycle, forming a nodule meristem. Concurrent to plant responses to nod factor, bacteria Plant Physiology Preview. Published on September 29, 2009, as DOI:10.1104/pp.109.144345