Factors Controlling the Synthesis of Natural and Induced Lignins in Phleum and Elodea

Abstract

Lignin is peculiar to the cell walls of vascular plants, and is produced only in specific cells at certain developmental periods. It is not detectable in aquatic vascular plants such as Elodea, but tisstues of these plants can be induced to form lignin-like products if suitable precursors are added (25). Since the products induced in Elodca were similar to those found naturally in other monocots such as Phleum, a comparison of the metabolism of phenolic compounds in these 2 organisms might aid in an understanding of the factors controlling lignification in plants. It is generally agreed that the phenolic monomers which are polymerized into lignin products arise from carbohydrates via the shikimic acid pathway (fig 1) (3). Some of the enzymes involved have been demonstrated in cell free extracts of higher plants (5). Mechanisms controlling lignification can be effective at any point of this synthetic pathway. In addition, since lignin is probably attached to specific sites in the wall, the number and type of these sites could be controlling factors. Possible regulatory agents of the terminal step of lignification have been studied in celery and peas by Siegel et al. (17,18) and in bamboo by Higuchi (10). The agents, indoleacetic acid (IAA), ascorbic acid and glutathione (GSH), were postulated to act as antioxidants. Possible regulatory mechanisms operating at an earlier level as well as at the peroxidation stage were investigated in this study of the metabolism of phenolic compounds associated with lignin biosynthesis in Elodea and Phleurn. In order to eliminate effects of treatments on growth bv elongation of the cell wall, nonelongating but still incompletely lignified tissues from these plants were chosen.