Lignin monomer composition is determined by the expression of a cytochrome P450-dependent monooxygenase in Arabidopsis

Abstract

The phenylpropanoid pathway provides precursors for the biosynthesis of soluble secondary metabolites and lignin in plants. Ferulate-5-hydroxylase (F5H) catalyzes an irreversible hydroxylation step in this pathway that diverts ferulic acid away from guaiacyl lignin biosynthesis and toward sinapic acid and syringyl lignin. This fact led us to postulate that F5H was a potential regulatory step in the determination of lignin monomer composition. To test this hypothesis, we have used Arabidopsis to examine the impact of F5H overexpression. Arabidopsis is a useful model system in which to study lignification because in wild-type plants, guaiacyl and syringyl lignins are deposited in a tissue-specific fashion, while the F5H-deficient fah1 mutant accumulates only guaiacyl lignin. Here we show that ectopic overexpression of F5H in Arabidopsis abolishes tissue-specific lignin monomer accumulation. Surprisingly, overexpression of F5H under the control of the lignification-associated cinnamate-4-hydroxylase promoter, but not the commonly employed caulif lower mosaic virus 35S promoter, generates a lignin that is almost entirely comprised of syringylpropane units. These experiments demonstrate that modification of F5H expression may enable engineering of lignin monomer composition in agronomically important plant species.

Expression Pattern of Two Paralogs Encoding Cinnamyl Alcohol Dehydrogenases in Arabidopsis. Isolation and Characterization of the Corresponding Mutants

“…Promoter fusion analysis suggested that corresponding AtCAD-D and -C genes may be expressed in xylem and fibers, although only fibers contain S lignin. The ability to reduce sinapaldehyde to sinapyl alcohol in xylem (a G unitenriched tissue) is consistent with the fact that sinapyl alcohol was synthesized in xylem bundles when ferulate-5-hydroxylase was overexpressed in transgenic Arabidopsis (Meyer et al, 1998;Sibout et al, 2002). These observations demonstrate that the absence of SAD-type protein in vessels of stems and roots likely would not be the limiting step for S-unit biosynthesis in these tissues, at least in Arabidopsis, and that other CADs could be involved in this processes.…”

Downregulation of Caffeic Acid 3-O-Methyltransferase and Caffeoyl CoA 3-O-Methyltransferase in Transgenic Alfalfa: Impacts on Lignin Structure and Implications for the Biosynthesis of G and S Lignin

“…Sewalt and R.A. Dixon, unpublished results). Recently, it was shown that modification of lignin composition by overexpression of F5H in transgenic Arabidopsis was more effective if the transgene was driven by the lignification-associated Arabidopsis cinnamate 4-hydroxylase promoter than by the constitutive 35S promoter (Meyer et al, 1998). We therefore decided to utilize the previously characterized lignification-associated bean phenylalanine ammonia-lyase PAL2 promoter, which is strongly expressed in the vascular tissue of transgenic tobacco (Leyva et al, 1992;Shufflebottom et al, 1993), to drive expression of COMT and CCOMT transgenes in alfalfa.…”

Independent origins of syringyl lignin in vascular plants

“…To test whether SmF5H can rescue the syringyl lignindeficient phenotype of fah1-2, we transformed into the mutant a construct in which the SmF5H gene was under the control of the Arabidopsis C4H promoter, a strategy previously reported to efficiently target transgene expression to vascular tissue (6). Stem cell wall samples were prepared from the 17 independent fah1-2-pAtC4H::SmF5H T1 transgenic lines and were analyzed for lignin composition by using the DFRC method.…”

“…1) (3). C4H and C3ЈH are responsible for phenylpropanoid 4 and 3-hydroxylation (4, 5), respectively, whereas F5H catalyzes the 5-hydroxylation of coniferaldehyde and coniferyl alcohol, leading to the formation of syringyl lignin (6,7). Lignin monomer composition has been found to vary among major phyla of vascular plants (2).…”