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Coexistence but Independent Biosynthesis of Catechyl and Guaiacyl/Syringyl Lignin Polymers in Seed Coats
Authors:Yuki Tobimatsu  Fang Chen  Jin Nakashima  Luis L Escamilla-Trevi?o  Lisa Jackson  Richard A Dixon  John Ralph
Institution:aDepartment of Biochemistry, University of Wisconsin–Madison, Wisconsin Energy Institute, Madison, Wisconsin 53726;bPlant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73401;cU.S. Department of Energy, BioEnergy Sciences Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831;dU.S. Department of Energy, Great Lakes Bioenergy Research Center, Wisconsin Energy Institute, Madison, Wisconsin 53726
Abstract:Lignins are phenylpropanoid polymers, derived from monolignols, commonly found in terrestrial plant secondary cell walls. We recently reported evidence of an unanticipated catechyl lignin homopolymer (C lignin) derived solely from caffeyl alcohol in the seed coats of several monocot and dicot plants. We previously identified plant seeds that possessed either C lignin or traditional guaiacyl/syringyl (G/S) lignins, but not both. Here, we identified several dicot plants (Euphorbiaceae and Cleomaceae) that produce C lignin together with traditional G/S lignins in their seed coats. Solution-state NMR analyses, along with an in vitro lignin polymerization study, determined that there is, however, no copolymerization detectable (i.e., that the synthesis and polymerization of caffeyl alcohol and conventional monolignols in vivo is spatially and/or temporally separated). In particular, the deposition of G and C lignins in Cleome hassleriana seed coats is developmentally regulated during seed maturation; C lignin appears successively after G lignin within the same testa layers, concurrently with apparent loss of the functionality of O-methyltransferases, which are key enzymes for the conversion of C to G lignin precursors. This study exemplifies the flexible biosynthesis of different types of lignin polymers in plants dictated by substantial, but poorly understood, control of monomer supply by the cells.
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