Polymorphism of lignification enzymes in plants: Functional importance and applied aspects

The synthesis of lignin and its deposition in secondary cell walls leads to the formation of sclerenchyma, a tissue having mechanical strength. This biochemical mechanism arose about 400–420 million years ago, in the Early Silurian period, in the first vascular plants (Tracheophyta order). The appearance of sclerenchyma is related to the emergence of plants on land and formation of the terrestrial flora. Lignified tissues of plants also perform conducting and protective functions, in addition to its mechanical (supporting) one. The polymorphic options of the enzymes involved into biosynthesis of lignin and its accompanying substances (PAL (phenylalanine ammonia-lyase), C4H (cinnamate 4-hydroxylase), C3H (p-coumarate 3-hydroxylase), F5H (ferulate 5-hydroxylase), COMT (caffeic acid O-methyltransferase), CCoAOMT (caffeoyl-CoA Omethyltransferase), 4CL (4-coumarate:CoA ligase), CCR (cinnamoyl-CoA reductase), CAD (cinnamyl alcohol dehydrogenase), and SAD (sinapyl alcohol dehydrogenase) were considered with the use of a number of plants: corn, millet, sorghum, rice, wheat, fescue, elephant grass, brachypodium (purple false brome), tobacco, alfalfa, arabidopsis, poplar, eucalyptus, pine, spruce, and ginkgo. Mutant variants of enzymes can be found in natural or breeding populations; they may be also obtained with the use of mutagens. In recent decades, T-DNA-mutagenesis is widely used. Mutations can cause changes in the expression of the corresponding genes and (or) disturb the structure of protein molecules. As a result, numerous changes in the phenotype can occur. Among them are changes in the structure and chemical composition of tissues, as well as in growth processes and plant development. The appearance of colored (brown midrib, reddish brown, pink, red-brown, red-wine, etc.) forms of plants was described in many species. Some publications noted an interrelation between the biosynthesis of monolignols and that of flavonoid pigments. Mutant forms are widely used at present to produce forage cultivars and bioethanol, to improve pulp properties (production of paper and viscose fiber), and to sequester atmospheric carbon. The synthesis of mutants from this group of enzymes and their use are considered promising trends of modern plant biotechnology.