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.).
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 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.
Chen
Fang
F
Plant Biology Division, Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, Oklahoma 73401, USA.
Srinivasa Reddy
Marry S
MS
Temple
Stephen
S
Jackson
Lisa
L
Shadle
Gail
G
Dixon
Richard A
RA
D002473:Cell Wall / Q000378:metabolism*; D003373:Coumaric Acids / Q000737:chemistry / Q000378:metabolism*; D015536:Down-Regulation; D005419:Flavonoids / Q000737:chemistry / Q000378:metabolism; D008031:Lignin / Q000096:biosynthesis* / Q000737:chemistry; D000455:Medicago sativa / Q000201:enzymology* / Q000235:genetics; D008780:Methyltransferases / Q000737:chemistry / Q000378:metabolism / Q000502:physiology; D008954:Models, Biological; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D030821:Plants, Genetically Modified / Q000378:metabolism
D003373:Coumaric Acids; D005419:Flavonoids; D010940:Plant Proteins; D008031:Lignin; C004999:ferulic acid; D008780:Methyltransferases; C059825:caffeoyl-CoA O-methyltransferase