Low lignin GM trees and forage crops.

• Author: Cummins, Joe
• Position: Plight of Forests

The plant cell is protected by a cell wall that has a structure analogous to reinforced concrete. The cellulose fibrils play the role of steel reinforcing rods, while concrete is represented by lignin. Lignin determines the rigidity, strength and resistance of a plant structure.

When wood fiber is processed to make paper or composite products, lignin must be removed using polluting chemicals and a great deal of energy. Also, the digestibility of animal feed is influenced by lignin content--the greater the lignin content, the poorer the food source. Genetic engineering is now being used to fundamentally modify the lignin of forest trees and animal feed.

Reducing lignin content of fiber and forage leads to greatly reduced costs of preparing fiber and improved digestibility of fodder and forage. However, the advantages of reduced lignin are offset by the disadvantage of plants with reduced lignin, which are more readily attacked by predators such as insects, fungi and bacteria. Indeed, increasing lignin content has been promoted as a defense against pests.

The importance of lignin in disease resistance has been known for well over twenty years. For example, lignification was crucial in reducing predation by spruce bark beetles, and lignin in the roots of the date palm played a key role in defense against the fungus Fusarium. It has been suggested that a guaiacyl (a type of lignin subunit) rich lignin was produced as "defence" lignin when Eucalyptus was wounded by a predator. Lignin content of larch species determined the level of heartwood brown-rot decay. Genetic modification of plants to enhance lignin production is covered in United States Patent 5,728,570.

However, Arabidopsis plants modified in the metabolic pathway leading to lignin formation produced abnormal lignin that was associated with severe fungal attacks. Tobacco plants modified to limit production of lignin subunits were susceptible to virulent fungal pathogens, but it was suggested that the precursors of lignin and not lignin itself protected plants from pathogens. Genetic modifications for reduced lignin level nevertheless resulted in reduced fitness including increased winter mortality and decreased biomass.

It seems clear that plant genetic modification leading to reduced lignin, as proposed for use in pulp and paper or in livestock production, must be fully evaluated for fitness in the environment.

Multiple genetic transformations of forest trees have been used to limit...