Alterations of lignin biosynthesis have tissue-specific impact on cell wall formation

摘要

With the progress of genetic engineering in lignin biosynthesis, the synthesis of monolignols via the generally admitted shikimic acid pathway has been revisited. Among the several uncertainties, the sequence of events that relate lignin monomer synthesis and their delivery and polymerisation in the extra-cellular carbohydrate matrix of the wood cell wall is far from being understood. However, typical ultrastructural traits can be identified as resulting from the down-regulation of specific enzymes from the monolignol biosynthetic pathway. This is particularly illustrated with the CCR(-) transformants of various plant origins which all showed alterations consisting of an important loosening of their secondary wall. This corresponds to an inability of the cellulose microfibrils to adopt proper orientation and cohesion. Immunocytochemical analysis of lignin deposition reveals that non-condensed sub-units were absent in these loosened areas. The study of other transformed plants down-regulated on different enzymes demonstrated that the micro-morphological and topochemical consequences of the transformation are tissue-specific, and wall regio-specific.