Lignification and cinnamyl alcohol dehydrogenase activity in developing stems of tomato and poplar: a spatial and kinetic study through tissue printing

Cinnamyl alcohol dehydrogenase (CAD) is an enzyme involved in the synthesis of lignin monomers. Tissue prints were used to localize CAD activity in tomato and poplar tissues. In parallel, lignin deposition was estimated by classical histochemical tests. Image analysis was also used to quantify the amount of CAD activity and the extent of lignification throughout the development of stems. Tomato stems showed a limited lignification. CAD activity, restricted to the xylem, reached a maximum during the early stages of development and showed a patched pattern on the xylem ring. In contrast, CAD activity was much stronger in poplar stems and closely associated with all lignified areas: starting with localized areas within the xylem and finally completely covering the xylem ring. The enzyme showed a significant activity in primary and secondary xylem, and both lignin deposition and CAD activity increased as the plant matured. These results show that CAD activity, which is important for lignification, can easily be detected in plant tissues by tissue prints. They also reveal that tomato (herbaceous stems) and poplar (woody stems) dramatically differ in the developmental pattern of CAD within their lignified tissues.