MicroRNA156 as a Regulatory Hub for Alfalfa Traits

Alfalfa (Medicago sativa) is an important forage crop, but to reach each its full potential major improvements must be made to yield, quality and agronomic performance. MicroRNAs have emerged as potent regulators of plant growth and development. Our analysis of microRNA156 (miR156) revealed that it downregulates a network of at least seven SPL genes to affect a number of traits in alfalfa. We generated alfalfa plants that overexpress miR156 (miR156⁺). These miR156⁺ plants exhibited increased vegetative yield, delayed flowering, and longer roots. MiR156 overexpression also improved forage quality, increased root regeneration capacity, and enhanced nitrogen fixation activity by upregulating expression of nitrogenase-related genes in roots inoculated with Sinorhizobium meliloti. We also tested the role of miR156 in drought response of alfalfa, and observed a significant improvement in drought tolerance of miR156⁺ genotypes. In addition to increased root length and better survival rate, miR156⁺ plants exhibited reduced water loss after drought stress. MiR156⁺ plants also maintained higher stomatal conductance, an enhanced accumulation of compatible solute (proline), and increased levels of abscisic acid (ABA), antioxidants and secondary metabolites compared to wild type control. At the molecular level, several genes known to be involved in drought tolerance were differentially expressed in leaves and roots of miR156⁺ plants. Taken together, these findings highlight the important role that miR156 could play in the biotechnological improvement of alfalfa, and potentially other crops. Improvements imparted by miR156 could include forage yield, flowering time, root development, nitrogen fixation activity, and abiotic stress tolerance.