Common Mechanism of Lignification of Compression Wood in Conifers and <i>Buxus</i>

Abstract

Woody plants develop a specialized secondary xylem known as reaction wood to enable formation of an ideal shape. Reaction wood in coniferous species is known as compression wood, and that of woody angiosperms as tension wood. However, the genus Buxus which is classified as an angiosperm, forms compression-wood-like reaction wood. We investigated the mechanism of lignification in coniferous compression wood and Buxus reaction wood: 1) Several lignin synthesis genes were upregulated in differentiating reaction wood of Buxus microphylla ; 2) B. microphylla pos sesses a specific laccase gene that is expressed specifically in differentiating reaction wood ( BmLac 4); 3) laccase activity localization was closely related to lignification of reaction wood, and laccase activity was high in the secondary wall middle layer; 4) in reaction wood cell walls, galactan was present in the outer portion of the secondary wall middle layer, and the level of xylan was reduced. These findings suggest that lignification in B. microphylla reaction wood is identical to that of coniferous compression wood. These may represent general mechanisms of increasing lignin content in various reaction woods.