Multi-site genetic modulation of monolignol biosynthesis suggests new routes for formation of syringyl lignin and wall-bound ferulic acid in alfalfa (Medicago sativa L.)

• Published in: The Plant journal : for cell and molecular biology Vol. 48; no. 1; pp. 113 - 124
• Main Authors: Chen, Fang, Srinivasa Reddy, Marry S, Temple, Stephen, Jackson, Lisa, Shadle, Gail, Dixon, Richard A
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.10.2006; Blackwell Publishing Ltd; Blackwell Science
• Subjects: Biological and medical sciences; Botany; Cell Wall - metabolism; cell wall phenolics; Coumaric Acids - chemistry; Coumaric Acids - metabolism; Down-Regulation; Enzymes; Flavonoids - chemistry; Flavonoids - metabolism; Fundamental and applied biological sciences. Psychology; genetic modification; lignification; Lignin - biosynthesis; Lignin - chemistry; Medicago sativa; Medicago sativa - enzymology; Medicago sativa - genetics; Metabolism; Metabolism. Physicochemical requirements; Methyltransferases - chemistry; Methyltransferases - metabolism; Methyltransferases - physiology; Models, Biological; O‐methylation; Plant physiology and development; Plant Proteins - chemistry; Plant Proteins - metabolism; Plant Proteins - physiology; Plants, Genetically Modified - metabolism; transgenic alfalfa; Transgenic plants; EC 4.3.1.5; Lignin; EC 1.14.13.11; Medicago sativa; Biosynthesis; transgenic alfalfa; Caffeic acid; Cell wall; Lignification; cell wall phenolics; Leguminosae; Gene; 0-methylation; Dicotyledones; Angiospermae; Spermatophyta; genetic modification
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313X.2006.02857.x

Genes encoding seven enzymes of the monolignol pathway were independently downregulated in alfalfa (Medicago sativa) using antisense and/or RNA interference. In each case, total flux into lignin was reduced, with the largest effects arising from the downregulation of earlier enzymes in the pathway. The downregulation of [smallcapital l]-phenylalanine ammonia-lyase, 4-coumarate 3-hydroxylase, hydroxycinnamoyl CoA quinate/shikimate hydroxycinnamoyl transferase, ferulate 5-hydroxylase or caffeic acid 3-O-methyltransferase resulted in compositional changes in lignin and wall-bound hydroxycinnamic acids consistent with the current models of the monolignol pathway. However, downregulating caffeoyl CoA 3-O-methyltransferase neither reduced syringyl (S) lignin units nor wall-bound ferulate, inconsistent with a role for this enzyme in 3-O-methylation ofS monolignol precursors and hydroxycinnamic acids. Paradoxically, lignin composition differed in plants downregulated in either cinnamate 4-hydroxylase or phenylalanine ammonia-lyase. No changes in the levels of acylated flavonoids were observed in the various transgenic lines. The current model for monolignol and ferulate biosynthesis appears to be an over-simplification, at least in alfalfa, and additional enzymes may be needed for the 3-O-methylation reactions of S lignin and ferulate biosynthesis.