The biosynthetic pathways leading to the formation of the cell wall polymer lignin have been extensively studied in recent years. Natural lignins are generally composed of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, that are biosynthesized by polymerization of the three primary monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, respectively; natural angiosperm lignins have only low levels (<~2%) of H-units. Catechyl (C) and 5-hydroxyguaiacyl (5H) units that may derive from polymerization of the corresponding caffeyl and 5-hydroxyconiferyl alcohols are not found in 'normal' lignins. Until recently, these rare units were only found in the lignin of transgenic plants in which the first or second O-methylation reactions had been blocked. We have recently described the presence of a hitherto unsuspected lignin polymer (C-lignin) in the seed coats of vanilla orchid and cacti of the genus Melocactus (Chen et al., PNAS 109: 1772-1777, 2012). C-lignin is a homopolymer formed by endwise �-O-4-coupling of caffeyl alcohol monomers onto the growing polymer, resulting in benzodioxane units. The lignin in the vegetative tissues of Vanilla and cactus plants appears, however, to be of the normal guaiacyl/syringyl (G/S) type. A study of over 130 species of cactus revealed that seed coat lignin composition is still evolving in the Cactaceae, and closely related species with either G/S, C or 5H lignin can be found. Although all cactus species examined never contained both C and G or S lignins in their seed coats, seeds of Jatropha contain both C and G lignins which are, however, independent polymers that are deposited at different times during seed coat development. As a further example of the flexibility of lignin structures, stems of the Medicgao truncatula CAD1 mutant contain lignin that is more than 95% composed of hydroxycinnamaldehyde units. We discuss possible mechanisms underlying the synthesis of novel lignin types in plants.

Novel lignins demonstrate the flexibility of lignin biosynthesis in plants

247th National Meeting (2014)

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