UDP-glucuronic acid decarboxylase in alfalfa: a target to improve ruminal digestibility of stems

Alfalfa (Medicago sativa) has a high nutritional value, but poor digestibility of the stems limits its value as an energy source in ruminant diets. Xylan and lignin negatively impact cell wall digestibility, whereas pectins have high digestibility in the rumen. In plants, UDP-xylose synthase (UXS) catalyzes the decarboxylation of UDP-glucuronic acid to form UDP-xylose in an irreversible step that is key for xylan synthesis. We functionally characterized two UXS genes in alfalfa (MsaUXS2 and MsaUXS4) and investigated their impact on ruminal digestibility. Both genes are more highly expressed in stems than leaves, and the enzymes have UDP-glucuronic acid decarboxylase activity in vitro. Silencing MsaUXS2/MsaUXS4 via RNAi altered plant growth and resulted in a 40% decrease in xylose, a 115% increase in arabinose, and a 60% increase in galacturonic acid in the polysaccharide matrix as well as a 20% decrease in lignin in the cell wall. Together, our data show a major role for MsaUXS2 and MsaUXS4 in xylan synthesis and secondary cell wall deposition in alfalfa. Additionally, silenced lines had on average 30% increased gas production at 24 h in in vitro rumen digestibility assays, demonstrating the potential of targeting UXS genes to increase stem digestibility.