Lignins and lignification: Selected issues

Lignin deposition in plant cell walls is one of the mechanisms which allowed the development of upright plants adapted to a terrestrial habitat. At the present time, lignins and lignification are the subject of very active research which has recently moved from chemical and biochemical aspects to more biological and developmental problems. In this review, three different topics will be addressed. (a) A first section will deal with recent advances related to the biosynthesis of lignins. It will be shown that a complex array of O-methyltransferases may control the production of differentially methylated monolignols, the precursors of lignins, but that the downstream enzymes in the synthesis of monolignols are probably nor: encoded by multigene families which would provide additional possibilities for fine-tuning the monomeric composition of lignins. In addition, recent results obtained on laccases will illustrate the difficulty in identifying the true nature of oxidases involved in the production of phenoxy radicals, the oxidation products of monolignols. (b) A second set of data will highlight the potential interest of Arabidopsis mutants for understanding lignin synthesis, deposition and function. Indeed, different classes of lignification mutants with modifications in lignin content or composition and alterations of vascular differentiation or global vascular pattern have already been characterized. The identification of the corresponding genes will undoubtedly give rise to new insights on key steps and regulation mechanisms in the lignification process. (c) The last section is dedicated to the future of lignin genetic engineering. It will be emphasized that, after a first period which has demonstrated the potential of the approach, it is necessary to consider in greater detail the unexpected side effects and compensation mechanisms associated with induced lignin modifications. New targets for future lignin genetic engineering experiments will be identified and the extension of the technology to new woody species, the advantages for the pulp industry and the problems associated with public perception of these new products will be envisaged. Lignification is a tightly regulated and dynamic process subject to modulation at different levels during normal development and in response to different stresses. Understanding these subtle mechanisms which also involve the other polymers of the cell wall is an important challenge facing plant biology as we enter the next century. (C) 2000 Editions scientifiques et medicales Elsevier SAS.