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

Kishor Bhattarai,Richard A Dixon,Maria J Monteros,Shanmugam Rajasekar

doi:10.1007/s12155-018-9911-6

Abstract

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 T0HCT down-regulation events 3a and 30a were used to make crosses with a common male sterile line to generate T1 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.

Highlights

Alfalfa is a highly heterozygous, perennial forage legume
The hydroxycinnamoyl transferase (HCT) down-regulated parents (T0 HCT30a and T0 HCT3a) grown in the greenhouse and the corresponding HCT3A+ and HCT30A+ T1 progenies grown in the field had significantly lower relative expression levels of the HCT transcripts compared to the non-down-regulated progenies HCT3A− and HCT30A− (Table 1)
The HCT down-regulation of alfalfa was maintained in the T1 progenies generated from the original T0 plants crossed with the common female parent 208Ms-1

Summary

Introduction

Alfalfa is a highly heterozygous, perennial forage legume. Legumes are important components of sustainable agricultural systems due to their capacity for symbiotic nitrogen fixationBioenerg. The national alfalfa and forage alliance (NAFA) proposed the development of alfalfa as a dualpurpose crop in which the stems are used as biofuel feedstock and the leaves are used as high-value forage. Driving forces behind this effort include the generation and utilization of biomass-based biofuels to promote national energy independence from foreign-based fossil fuels [4] and to generate more carbon neutral energy resources. The G and S subunits are the two main monomers in alfalfa cell walls [7], and the reduced levels of these monomers result in an overall reduction in alfalfa lignin content. In addition to lignin quantity, the relative abundance of the lignin subunits affects the efficiency by which the sugar molecules are released from the cell wall and the efficiency of biomass-based ethanol conversion [6, 12, 13]

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