Functions of wood cell wall polysaccharides on lignification in vitro [an abstract of entire text]

概要

Wood cell walls are mainly composed of cellulose, hemicellulose, and lignin. The monomer composition of lignin differs in wood species: softwood lignin mainly consists of guaiacyl nuclei, while hardwood lignin consists of mainly guaiacyl (G) and syringyl (S) nuclei. Both lignins also comprise of a trace of p-hydroxyphenyl (H) nuclei. During tree growth, lignin is biosynthesized by radical coupling of monomers catalyzed by peroxidase/H2O2 and/or oxidase/O2 in swollen polysaccharide matrices assembled with cellulose and hemicelluloses. However, the interaction among these components and effects of the preformed cell wall polysaccharides on lignification, such as, the morphology, substructure, and generated amount of resultant lignin, are still under discussion.

The investigations of lignin-carbohydrate complex (LCC) have suggested contradictory results of hemicelluloses effects on lignification: one reported that xylan promoted to form β-O-4’ linkage as a predominant interunitary linkage of lignin (Giummarella et al., 2016), but another study reported that xylan facilitated the formation of condensed substructure (Du et al., 2014). To clarify these contradictory results, dehydrogenation polymer (DHP) was synthesized from CA by horseradish peroxidase (HRP) in the polysaccharide matrix containing xylan and bacterial cellulose (BC) film to mimic the lignification in cell wall, and demonstrated that xylan attributed to the increase of both DHP amount and the frequency of aryl ether linkage (Li et al., 2015). However, functions of other polysaccharides are still unknow.

HRP is the enzyme often used for DHP formation. However, HRP is not a tree enzyme, and thus should not involve lignification in tree. By contrast, a cationic cell wall-bound peroxidase (CWPO-C) discovered in poplar (Populus alba L.) callus culture can oxidize monolignols, both coniferyl alcohol (CA) and sinapyl alcohol (SA), and also polymeric lignin (Aoyama et al., 2002; Sasaki et al., 2004). However, it is very difficult to obtain a large quantity of CWPO-C from callus culture. Therefore, the procedure of recombinant CWPO-C (rCWPO-C) preparation was established (Shigeto et al., 2012). rCWPO-C was used for catalyzation of monolignol polymerization to elucidate the effects of cell wall polysaccharides on lignification in hardwood in this study.

A main objective of this study is to elucidate the effects of wood cell wall polysaccharides on lignification through the fabrication of artificial cell walls mimicking the process of tree cell wall formation. In addition, clarification of the interaction between the real tree peroxidase (rCWPO-C) and polysaccharides is also an important objective. The artificial cell walls were fabricated by dehydrogenative polymerizations of CA and SA with two enzymes, HRP and rCWPO-C, in polysaccharide matrices comprised of cellulose and several kinds of hemicelluloses, water-soluble fraction of beech xylan (WXY), partially acetylated WXY with a degree of substitute (DS) of 0.50 (AcXY), galactoglucomannan (GGM) isolated from Picea jezoensis with hot water, and xyloglucan (XG) from tamarind seeds. XG represents the hemicellulose in the primary wood cell wall of both hardwood and softwood. WXY and GGM are representative hemicelluloses in the secondary cell wall. In hardwood, xylan is partly acetylated with the DS of 0.40−0.75. Thus, AcXY was chemically synthesized as an analog of native xylan in hardwood.