Tong Liu

Document Type

Thesis - University Access Only

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Science


Lignin is associated with nutritive value in alfalfa (Medicago sativa L.). Lignification reduces forage digestibility and limits animal performance. Lignin is an aromatic secondary metabolite. Aromatic amino acids are synthesized via the shikimate pathway, in which 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase (EC 4 .1. 2 .15) is the first regulatory enzyme. A partial cD NA encoding D AHP synthase (designated MsDHSJ) was isolated from alfalfa by RNA reverse transcription and polymerase chain reactions. The degenerate primers were based on the conserved regions of known DAHP synthase sequences from other species. DNA fragments of about 875 bp were subcloned into EcoR I-cleaved pCR II, and sequenced. The MsDHSJ sequence consists of an open reading frame of 291 codons with 46.4% of (C+G) content. The sequence of the deduced 291 amino acids of MsDHSJ was compared with those of two Arabidopsis thaliana (AtDHSJ, AtDHS2), two tomato (LeDHSJ, LeDHS2), two potato (StshKA, StshKB), and one tobacco (NtDAHPS-1) genes. The results demonstrate sequence homologies of 88.7%, 88.0%, 87.3%, and 87.0% with AtDHS2, LeDHS2, NtDAHPS-1, and StshKA, respectively, and relatively lower homologies of 83.9%, 85.9%, and 85.6% with AtDHSJ, LeDHSJ, and StshKB, respectively, implying that MsDHSJ is highly conserved. The Southern analysis of alfalfa genomic DNA suggested that there was possibly only a single MsDHSJ gene in the alfalfa genome. Accumulation of D AHP synthase mRN A was compared among alfalfa morphologically variant lines by Northern blotting. Three high lignin (HL) lines, with low leaf-to-stem ratio, and three low lignin (LL) lines, with high leaf-to-stem ratio, were evaluated. The expression of DAHP synthase gene was five times more abundant in HL lines than in LL lines, although LL lines have been shown to have higher stem lignin concentration than HL lines. These results suggest that elevated transcription of the DAHP synthase gene may be associated with higher accumulation of shikimatederived end products other than lignin in HL lines.

Library of Congress Subject Headings

Lignin -- Synthesis

Alfalfa -- Genetics

Plant enzymes



Number of Pages



South Dakota State University



Rights Statement

In Copyright