Enzymatic basis for C‐lignin monomer biosynthesis in the seed coat of Cleome hassleriana

Summary C‐lignin is a linear polymer of caffeyl alcohol, found in the seed coats of several exotic plant species, with promising properties for generation of carbon fibers and high value chemicals. In the ornamental plant Cleome hassleriana, guaiacyl (G) lignin is deposited in the seed coat for the...

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Published in:The Plant journal : for cell and molecular biology 2019-08, Vol.99 (3), p.506-520
Main Authors: Zhuo, Chunliu, Rao, Xiaolan, Azad, Rajeev, Pandey, Ravi, Xiao, Xirong, Harkelroad, Aaron, Wang, Xiaoqiang, Chen, Fang, Dixon, Richard A.
Format: Article
Language:English
Subjects:
5-Methyltetrahydrofolate-homocysteine S-methyltransferase
Alcohol
Alcohol dehydrogenase
Alcohols
Amino acids
Biosynthesis
Caffeic acid
Carbon fibers
Cinnamyl-alcohol dehydrogenase
Cleome
Cleome hassleriana
Coating
C‐lignin
E coli
enzyme specificity
Gene sequencing
Introduced species
Lignin
Methionine
Methylation
monolignol biosynthesis
Monomers
Organic chemistry
Ornamental plants
O‐methyltransferase
Pollination
Ribonucleic acid
RNA
Seed coats
Substrates
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Summary:Summary C‐lignin is a linear polymer of caffeyl alcohol, found in the seed coats of several exotic plant species, with promising properties for generation of carbon fibers and high value chemicals. In the ornamental plant Cleome hassleriana, guaiacyl (G) lignin is deposited in the seed coat for the first 6–12 days after pollination, after which G‐lignin deposition ceases and C‐lignin accumulates, providing an excellent model system to study C‐lignin biosynthesis. We performed RNA sequencing of seed coats harvested at 2‐day intervals throughout development. Bioinformatic analysis identified a complete set of lignin biosynthesis genes for Cleome. Transcript analysis coupled with kinetic analysis of recombinant enzymes in Escherichia coli revealed that the switch to C‐lignin formation was accompanied by down‐regulation of transcripts encoding functional caffeoyl CoA‐ and caffeic acid 3‐O‐methyltransferases (CCoAOMT and COMT) and a form of cinnamyl alcohol dehydrogenase (ChCAD4) with preference for coniferaldehyde as substrate, and up‐regulation of a form of CAD (ChCAD5) with preference for caffealdehyde. Based on these analyses, blockage of lignin monomer methylation by down‐regulation of both O‐methyltransferases (OMTs) and methionine synthase (for provision of C1 units) appears to be the major factor in diversion of flux to C‐lignin in the Cleome seed coat, although the change in CAD specificity also contributes based on the reduction of C‐lignin levels in transgenic Cleome with down‐regulation of ChCAD5. Structure modeling and mutational analysis identified amino acid residues important for the preference of ChCAD5 for caffealdehyde. Significance Statement C‐lignin is a potentially valuable polymer currently only known to exist in the seed coats of exotic species. We here describe the enzymatic basis for the biosynthesis of C‐lignin in a model species, with implications for engineering of the polymer
ISSN:0960-7412
1365-313X
DOI:10.1111/tpj.14340