Coniferin production in relation to lignin biosynthesis in conifers

Coniferin, the 4-O-β-D-glucopyranoside of trans-coniferyl alcohol, occurs as a dominant metabolite in conifers and has also been reported in woody angiosperms. Coniferin has long been considered as a storage precursor of gymnosperm lignin, and more recently coniferin content of the cambial zone has been suggested as a biochemical indicator for wood formation in conifers. However, coniferin is not present in the cambium except during cambial growth. Moreover, coniferin content rises well before lignification commences in springtime and disappears prior to completion of lignification at the end of the growing season.Appearance and activity of UDP-glucose: coniferyl alcohol glucosyltransferase (CAGT), the enzyme catalysing production of coniferin from coniferyl alcohol, is not specifically linked to lignification but rather specifically associated with the annual cycle of growth and dormancy. Progress in genetic engineering of lignin content and composition has raised questions about the basic concept of lignin biosynthesis, indicating that the metabolic flexibility may be broader than previously thought. The incorporation of labelled coniferin into syringyl units of angiosperm wood as well as interconversions between cinnamyl alcohols and cinnamyl aldehydes have indicated that the turnover of lignin precursors may occur distantly downstream from the phenylpropanoid acid and/ or cinnamyl CoA-ester stages. We found in vitro that CAGT could convert cinnamyl alcohols as well as cinnamaldehydes. Our results support the view that CAGT has a regulatory influence and lignin modulating function exceeding mere substrate supply in support of guaiacyl lignification.