TANSLEY REVIEW NO-80 - BIOCHEMISTRY AND MOLECULAR-BIOLOGY OF LIGNIFICATION

Lignins, which result from the dehydrogenative polymerization of cinna myl alcohols, are complex heteropolymers deposited in the walls of spe cific cells of higher plants. Lignins have probably been associated to land colonization by plants but several aspects concerning their bios ynthesis, structure and function are still only partially understood. This review focuses on the modern physico-chemical methods of structur al analysis of lignins, and on the new approaches of molecular biology and genetic engineering applied to lignification. The principles, adv antages and limitations of three important analytical tools for studyi ng lignin structure are presented. They include carbon 13 nuclear magn etic resonance, analytical pyrolysis and thioacidolysis. The use of th ese methods is illustrated by several examples concerning the characte rization of grass lignins, 'lignin-like' materials in protection barri ers of plants and lignins produced by cell suspension cultures. Our pr esent limited knowledge of the spatio temporal deposition of lignins d uring cell wall differentiation including the nature of the wall compo nents associated to lignin deposition and of the cross-links between t he different wall polymers is briefly reviewed. Emphasis is placed on the phenylpropanoid pathway enzymes and their corresponding genes whic h are described in relation to their potential roles in the quantitati ve and qualitative control of lignification. Recent findings concernin g the promoter sequence elements responsible for the vascular expressi on of some of these genes are presented. A section is devoted to the e nzymes specifically involved in the synthesis of monolignols: cinnamoy l CoA reductase and cinnamyl alcohol dehydrogenase. The recent charact erization of the corresponding cDNAs/genes offers new possibilities fo r a better understanding of the regulation of lignification. Finally, al the level of the synthesis, the potential involvement of peroxidase s and laccases in the polymerization of monolignols is critically disc ussed. In addition to previously characterized naturally occurring lig nin mutants, induced lignin mutants have been obtained during the last years through genetic engineering. Some examples include plants trans formed by O-methyltransferase and cinnamyl alcohol dehydrogenase antis ense constructs which exhibit modified lignins. Such strategies offer promising perspectives in gaining a better understanding of lignin met abolism and functions and represent a realistic way to improve plant b iomass.