Loosening the Grip on Flowering and Vegetative Growth Control: MiR156 Is the Key to Enhancing Biomass Yield in Medicago sativa.

Plant stress and the control of flowering onset have been a long-term focus of Medicago research, as the breeding community struggles to find alfalfa germplasm with enhanced biomass yield under moderate or extreme growing conditions. Strong improvements in yield would continue to secure alfalfa’s position as the world’s most valued livestock forage and would advance it into the realm of being a competitive low-input biofuel crop. Here, we present the miR156 system as a means to substantially control flowering and biomass yield in forage legumes. Initially, we will show details of miR156 characterization in Arabidopsis, where we found an activation-tagged mutant with enhanced carotenoid levels and shoot branching. Expression of the AtmiR156 in oilseed rape (Brassica napus) enhanced shoot branching and delayed flowering. In L. japonicus, LimiR156 targets two genes (SPL1 and SPL2) for cleavage-mediated silencing, and its over-expression causes a strong delay in flowering time, increased biomass production, and reduced nodulation. Alfalfa plants over-expressing miR156 had reduced internode length, more nodes per stem, and substantially enhanced shoot branching, increased plant height, and higher trichome density, but no effect on root length and nodulation. Over-expression of MsmiR156 also affected the alfalfa floral transition stage by delaying flowering up to five days. Thus, miR156 and its ability to reduce the expression of SPL genes may be utilized to improve quality and yield of alfalfa as both a livestock forage and a potential low-input bioenergy crop.