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PARTICIPATION OF CELLULASES IN THE MORPHOGENESIS OF ANATOMICAL ELEMENTS OF HIGHER PLANTS

Evgenii Novozhilov

Abstract

Morphogenesis of structural elements of secondary xylem is associated with the biosynthesis of a massive secondary cell wall consisting of three layers, differing in mass, composition and structural organization. Cellulosic microfibrils are basic structural elements fixing cell walls. The participation of cellulases in cellulose biosynthesis is shown. Overexpression of cellulases (EGase) increased cell expansion and elongation and resulted in an increase in plant growth and biomass production. However, the precise role of the EGases in plants remains unknown. The aim of the work is to clarify the understanding of the physiological role of cellulases in plants. In the cellulase complex of plants, the presence of only endo-1,4-beta-glucanases (EC 3.2.1.4) and (1>4)-beta-D-glucan glucohydrolase (EC 3.2.1.74) are established. The ?ellobiohydrolases (EC 3.2.1.91) genes are absent. The presence of beta-glucosidase (3.2.1.21) needs to be clarified. In higher plants, endo-1,4-beta-glucanases belong to the glycosyl hydrolase family 9. Several hypotheses have been proposed that plant EGases are associated with cellulose biosynthesis and cell wall remodelling. It is established that endo-1,4-beta-glucanases are transported in the plant exosomes. In the secondary phloem septa destruction with the participation of endo-1,4-beta-glucanases leads to the structure formation of sieve elements participating in assimilate transport. The local enzymatic destruction of the cell walls eases the expansion and elongation of the phloem fibres. It was established experimentally the activity of (1>4)-beta-D-glucan glucohydrolase in conducting and non-conducting phloem, however, the role of these enzymes in the plant physiology remains to be clarified. In secondary xylem endo-1,4-beta-glucanases delivered by exosomes into the sites of pit formation participated in the local destruction of the S1 layer in differentiating xylem. As a result, cavities of simple and boarded pits were formed in cell walls. The microfibrils of the primary wall in pit membrane are protected from cellulase action by xyloglucan. The carbohydrates for the biosynthesis of the S2 layer come from the phloem along the rays and enter fibres through pits. The biosynthesis of the S1 layer and the S2 layer is separated by a time interval required to form the pits in the S1 layer.

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DOI resolver: doi.org/10.5593/sgem2019/6.1/s25.108

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