Mechanisms of Control and lmplications for the Genetic lmprovement of Plants

Lignin, a complex phenolic polymer, is important for mechanical support, water transport, and defense in vaseular plants. Compressive strength and hydrophobicity of xylem cell walls are imparted by the lignin polymer, which is deposited during the terminal differentiation of tracheids and other cell types. The resistance of xylem to compressive stresses imposed by water transport and by the mass of the plants is important to growth and development. In addition, the insolubility and complexity of the lignin polymer makes it resistant to degradation by most microorganisms. Therefore, lignin serves an important function in plant defense. Variation in lignin content, composition, and location is likely to affect these essential processes. The constraints on the amount, composition, and localization of lignin for normal xylem function and plant defense are not known. Lignin composition, quantity, and distribution also affect the agroindustrial uses of plant material. Digestibility and dietary conversion of herbaceous crops are affected by differences in lignin content and composition (Akin et al., 1986, 1991). Lignin is an undesirable component in the conversion of wood into pulp and paper; remova1 of lignin is a major step in the paper making process. Furthermore, the resistance of lignin to microbial degradation enhances its persistence in soils. Lignin is, therefore, a significant component in the global carbon cycle. The mechanisms of control of lignin composition and quantity have wide implications regarding the adaptation and evolution of land plants and provide a basis for improved genetic manipulation of lignin for agroindustrial end uses. In this Update, we will focus on the levels of control of lignin variation, including (a) metabolic control, (b) regulation of individual enzymes in the biosynthetic pathway, and (c) regulation of gene expression. These levels of regulation affect variation in lignin content, quality, and distribution. Finally, the implications of these regulatory mechanisms for the genetic improvement of lignin for