What is lignin - the evolution of views (review)

The review considers changes in ideas about the biosynthesis, structure and functions of lignin in plants, starting from early mentions and up to the present. Alternative points of view on the process of lignin biosynthesis, its spatial and supramolecular structure, and the nature of lignincarbohydrate bonds are presented. A special place in the review is occupied by the results of studies of lignin modification with the aim of reducing the content and changing the composition of monomer units by genetic engineering methods. Comparison of the structure of lignins isolated from transgenic and mutant trees, as well as herbaceous plants, showed that lignin biosynthesis is plastic and, in addition to canonical monolignols (coniferyl, synapic, and coumaric alcohols), other phenolic compounds can also participate in it. The study of the so-called "stress lignins" showed that lignin plays an important role in protecting plants from adverse environmental influences: mechanical damage, drought, low temperatures, pathogens, etc. The results of studies of the genetic modification of lignin made it possible to outline a program for directed modification of the biosynthesis process in order to obtain designer lignins, i.e. lignins with desired properties. This group also includes the so-called zip-lignins. They are distinguished by the presence of ester bonds between phenylpropane units. It is noted that, when conducting research in the field of genetic engineering, it is necessary to find a compromise between improving the processing of plant materials due to the modification of lignin and the viability of transgenic plants.