Agronomic Performance and Lignin Content of HCT Down-Regulated Alfalfa (Medicago sativa L.)

Lignin is a cell wall polymer that reduces the enzymatic digestibility and conversion efficiency of lignocellulosic biomass to ethanol. Down-regulation of the hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT) gene reduces lignin content in alfalfa (Medicago sativa L.). The objective of this study was to evaluate HCT down-regulated alfalfa plants for their forage composition and agronomic performance in the greenhouse and under field conditions. Individual plants with two separate T-0 HCT down-regulation events 3a and 30a were used to make crosses with a common male sterile line to generate T-1 progenies (3A and 30A, respectively). The experimental design was a randomized complete block with four replications per line and ten individuals per replication. Two-row plots with full-sib progeny rows with (+) and without (-) the transgene were planted along with commercial cultivars used as checks. Most differential responses in forage quality and agronomic characteristics between full-sib down-regulated and non-down-regulated progenies were event-specific and found mainly between the HCT30A+ and HCT30A- progenies. Variation between HCT- and HCT+ alfalfa plants was observed in the HCT gene transcript levels, acid detergent lignin (ADL), relative feed value (RFV), and saccharification efficiency. Although differential responses in agronomic performance of field-grown HCT down-regulated alfalfa plants were identified for biomass yield and plant height, HCT+ and HCT- progenies had similar spring growth and fall dormancy. The reduction in lignin content of alfalfa plants via modification of HCT transcript levels increased forage quality and efficiency of sugar release of plants grown under greenhouse and field conditions.