Lignins and lignification: Recent biochemical and biotechnological developments

As a result of recent advances in molecular biology, the lignin biosynthetic pathway has been critically re-examined and appears more complex than originally assumed. The existence of alternative pathways, on one hand, and the presence of specific isoforms for the ''classical pathway'' on the other, may be implicated in the regulation of the monomeric composition of lignins. Indeed the chemical heterogeneity of lignins that exists between species, tissues (within a given individual), and as a function of various physiological conditions appears to be tinder strict control. Differential partitioning of assimilated photosynthetic carbon into lignins (as reflected in the varying lignin content among species) is likely to involve competition between primary and secondary metabolism. In this context, a coordinated activation of key enzymes of primary: pre-aromatic, and secondary phenolic metabolism must be an important control mechanism. Beyond the characterization of lignification genes, it is now possible to demonstrate, via chimeric gene expression, the tissue specific nature of their promoter activity. The identification of promoter regions conferring this specificity is currently underway Lignins synthesized de novo in response to stress (elicitation, wounding) not only have an altered monomer composition as compared to constitutive lignins bur they also appear to be under strict spatial control. On a fundamental level, lignification often accompanies programmed cell death, a domaine of growing interest in the field of developmental biology. Finally, this review highlights recent advances aimed at modifying lignification by genetic engineering and describes potential biotechnological applications.