Genetic Engineering of Lignin Biosynthesis in Trees: Compromise between Wood Properties and Plant Viability

Lignin is the second most common terrestrial biopolymer. It provides mechanical strength to plants, confers waterproof properties to the vascular system, and plays an important role in protection against biotic and abiotic stresses. The chemical resistance of lignin impedes the conversion of plant biomass into cellulose and biofuels; this circumstance led to intense research on lignin biosynthesis. For a long time, it was believed that lignin consists almost exclusively of three monolignols. However, about thirty more minor monomers of diverse chemical nature have been discovered to date. Using genetic engineering methods, a number of transgenic trees with altered expression of lignin biosynthesis genes and the transcription factor genes that regulate this process have been obtained. Changes in the content and/or composition of lignin allowed researchers to significantly raise the efficiency of delignification and enzymatic hydrolysis of woody biomass, but these changes often led to retarded growth and distorted plant development. In search of a balance between the industrial needs and plant viability, new strategies have been proposed that are based on the inclusion of new monolignols in lignin as well as on the use of lignin-deficient natural tree forms. New physicochemical properties of lignin are expected to increase its extractability. At the same time, growth, development, and stress resistance of such transgenic plants should be studied under field conditions. The review presents the current state of research on properties and modification of lignin in woody plants. In addition, the relations between these modifications and plant viability, as well as the prospects for their commercial use, are discussed.