Both caffeoyl Coenzyme A 3-O-methyltransferase 1 and caffeic acid O-methyltransferase 1 are involved in redundant functions for lignin, flavonoids and sinapoyl malate biosynthesis in Arabidopsis.
Two methylation steps are necessary for the biosynthesis of monolignols, the lignin precursors. Caffeic acid O-methyltransferase (COMT) O-methylates at the C5 position of the phenolic ring. COMT is responsible for the biosynthesis of sinapyl alcohol, the precursor of syringyl lignin units. The O-methylation at the C3 position of the phenolic ring involves the Caffeoyl CoA 3-O-methyltransferase (CCoAOMT). The CCoAOMT 1 gene (At4g34050) is believed to encode the enzyme responsible for the first O-methylation in Arabidopsis thaliana. A CCoAOMT1 promoter-GUS fusion and immunolocalization experiments revealed that this gene is strongly and exclusively expressed in the vascular tissues of stems and roots. An Arabidopsis T-DNA null mutant named ccomt 1 was identified and characterised. The mutant stems are slightly smaller than wild-type stems in short-day growth conditions and has collapsed xylem elements. The lignin content of the stem is low and the S/G ratio is high mainly due to fewer G units. These results suggest that this O-methyltransferase is involved in G-unit biosynthesis but does not act alone to perform this step in monolignol biosynthesis. To determine which O-methyltransferase assists CCoAOMT 1, a comt 1 ccomt1 double mutant was generated and studied. The development of comt 1 ccomt1 is arrested at the plantlet stage in our growth conditions. Lignins of these plantlets are mainly composed of p-hydroxyphenyl units. Moreover, the double mutant does not synthesize sinapoyl malate, a soluble phenolic. These results suggest that CCoAOMT 1 and COMT 1 act together to methylate the C3 position of the phenolic ring of monolignols in Arabidopsis. In addition, they are both involved in the formation of sinapoyl malate and isorhamnetin.
Do
Cao-Trung
CT
Biologie Cellulaire, INRA, 78026, Versailles cedex, France.
Pollet
Brigitte
B
Thévenin
Johanne
J
Sibout
Richard
R
Denoue
Dominique
D
Barrière
Yves
Y
Lapierre
Catherine
C
Jouanin
Lise
L
D017360:Arabidopsis / Q000201:enzymology / Q000378:metabolism*; D001483:Base Sequence; D017931:DNA Primers; D005419:Flavonoids / Q000096:biosynthesis*; D008031:Lignin / Q000096:biosynthesis*; D008293:Malates / Q000378:metabolism*; D008780:Methyltransferases / Q000378:metabolism*; D010666:Phenylpropionates / Q000378:metabolism*; D017550:Spectroscopy, Fourier Transform Infrared
C079164:2-O-sinapoylmalate; D017931:DNA Primers; D005419:Flavonoids; D008293:Malates; D010666:Phenylpropionates; D008031:Lignin; D008780:Methyltransferases; C059825:caffeoyl-CoA O-methyltransferase; C019608:caffeate O-methyltransferase