What is Lignin: Evolution of Views (A Review)

The review considers changes in ideas about the biosynthesis, structure, and functions of lignin in plants, starting from early references to the present day. Alternative viewpoints on the process of lignin biosynthesis, lignin spatial and supramolecular structure, and the nature of lignin-carbohydrate bonds were presented. A special place in the review belongs to the results of studies of lignin with the aim of reducing its content and changing the composition of the monomer units using genetic engineering methods. Comparison of the structures of the lignins isolated from transgenic and mutant trees and herbaceous plants showed that lignin biosynthesis is plastic and may involve, alongside canonical monolignols (coniferyl, sinapyl, and coumaryl alcohols), other phenolic compounds. Studies of so-called “stress lignins” demonstrated an important role played by lignin in plant protection against adverse environmental influences: mechanical damage, drought, low temperatures, pathogens, etc. The results of studies on gene modification of lignin allowed outlining a strategy of directed modification of the lignin biosynthesis process with the aim to obtain designer lignins, i.e., lignins with desired properties. This group includes so-called zip-lignins; they are characterized by the presence of ester linkages between the phenylpropane units. It is noted that, when conducting research into gene engineering of lignin, it is necessary to find a compromise between improving the processability of plant raw materials through lignin modification and the viability of transgenic plants.