Author
WAGNER, ARIMIN - SCION, NEW ZEALAND | |
Ralph, John | |
AKIYAMA, TAKUYA - UNIV OF TOKYO | |
FLINT, HEATHER - SCION, NEW ZEALAND | |
PHILLIPS, LORELLE - SCION, NEW ZEALAND | |
TORR, KIRK - SCION, NEW ZEALAND | |
NANAYAKKARA, BERNADETTE - SCION, NEW ZEALAND | |
TE KIRI, LANA - SCION, NEW ZEALAND |
Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/5/2007 Publication Date: 7/5/2007 Citation: Wagner, A., Ralph, J., Akiyama, T., Flint, H., Phillips, L., Torr, K., Nanayakkara, B., Te Kiri, L. 2007. Modifying lignin in conifers: the role of HCT during tracheary element formation in Pinus radiata. Proceedings of the National Academy of Sciences. 104(28):11856-11861. Interpretive Summary: Lignin is a polymer that is mainly present in plant cell walls, where it provides strength, defense, and allows for water transport through the plant. There is wide interest in understanding the process of lignin biosynthesis and deposition because of its economic relevance. Plant varieties with altered lignin content and composition can have improved performance as fodder crops or in the production of paper and pulp. By studying natural mutants and some genetically engineered transgenic plants, we have come to appreciate that plants can make their crucial lignins with considerable flexibility. A new step in the biosynthesis of the “monolignol” building blocks from which lignin polymers are made was recently discovered in various angiosperm plants. Here, collaborators at Scion in New Zealand have found that a similar gene operates in gymnosperm (e.g., pine) plants, and that down-regulating this gene also profoundly affects the lignin composition and structure. For example, in normal pine the monolignol p-coumaryl alcohol only contributes typically 1% toward the polymer; when the gene was down-regulated here, the contribution increased to over 30%, as shown by nuclear magnetic resonance spectroscopy. Such studies provide the foundations for understanding digestibility limitations for current research aimed at improving the utilization of our crop and forest plant resources. Technical Abstract: The enzyme hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyltransferase (HCT) is involved in the production of methoxylated monolignols that are precursors to guaiacyl and syringyl lignin in angiosperm species. We identified and cloned a putative HCT gene from Pinus radiata, a coniferous gymnosperm, which does not produce syringyl lignin. This gene was up-regulated during tracheary element (TE) formation in P. radiata cell cultures and showed 72.6% identity to the amino acid sequence of the Nicotiana tabacum HCT isolated earlier. RNAi-mediated silencing of the putative HCT gene had a strong impact on flavonoid production, lignin content, monolignol composition, and inter-unit linkage distribution. AcBr assays revealed an up to 42% reduction in lignin content in TEs. Pyrolysis-GC/MS, thioacidolysis, and NMR detected substantial changes in lignin composition. Most notable was the rise of p-hydroxyphenyl units released by thioacidolysis, which increased from trace amounts in wild-type controls to up to 31% in transgenics. 2D 13C-1H correlative NMR confirmed the increase in p-hydroxyphenyl units in the transgenics and revealed structural differences, including an increase in resinols, a reduction in dibenzodioxocins, and the presence of glycerol endgroups. The observed modifications in silenced transgenics validate the targeted gene as being associated with lignin biosynthesis in P. radiata and thus likely to encode HCT. Therefore, this enzyme represents the metabolic entry point leading to the biosynthesis of methoxylated phenylpropanoids in both gymnosperm and angiosperm species. |