Lignin Modification Leads to Increased Nodule Numbers in Alfalfa

Reduction of lignin levels in the forage legume alfalfa by down-regulation of the monolignol biosynthetic enzyme hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT) results in strongly increased digestibility and processing ability of lignocellulose. However, these modifications are often also associated with dwarfing and other changes in plant growth. Given the importance of nitrogen fixation for legume growth, we evaluated the impact of constitutively targeted lignin modification on the below-ground organs (roots and nodules) of alfalfa plants. HCT down-regulated alfalfa plants exhibit a striking reduction in root growth accompanied by an unexpected increase in nodule numbers when grown in the greenhouse or in the field. This phenotype is associated with increased levels of gibberellins and certain flavonoid compounds in roots. Although HCT down-regulation reduces biomass yields in both the greenhouse and field experiments, the impact on the allocation of nitrogen to shoots or roots is minimal. It is therefore unlikely that the altered growth phenotype of reduced lignin alfalfa is a direct result of changes in nodulation or nitrogen-fixation efficiency. Furthermore, HCT down-regulation has no measurable effect on carbon allocation to roots in either greenhouse or three year field trials.