Description

The hemicelluloses have not received adequate attention in studies of wood cell walls because the complexity of their structures does not admit easy interpretation within the paradigms of polymer science. Two-phase composite models of the cell wall have led many to view their primary function as one of coupling cellulose and lignin to enhance the mechanical properties of the walls. But that is a microscopic interpretation based on macroscopic theories of reinforced structures. In contrast, recent studies have shown that hemicelluloses can participate in regulation of the nanoscale architecture of cell wall constituents. They influence the aggregation of celluloses, and they can also influence the pattern of inter-unit linkages in lignin analogs polymerized in their presence. It is clear that the hemicelluloses have higher functions than as mechanical coupling agents in a two-phase composite. The hemicelluloses are species and tissue specific, and they have also been shown to vary across the layers of cell walls in patterns that are similar for cells formed contemporaneously in the same tissue. Their structures are clearly genetically encoded. In this report it is proposed that the hemicelluloses have multiple functions in regulation of cell wall consolidation and in determining its properties. The hemicelluloses are viewed as part of a system whereby information encoded in the genes is communicated to regulate the assembly of the cell wall. In addition to their role in regulating the aggregation of cellulose and the formation of inter-unit linkages in lignin, they appear to act in concert with extra-cellular glycosidases, to accomplish gradual dehydration of the polysaccharide matrix within which lignin precursors are polymerized. These proposals are based on viewing the deposition and consolidation of the secondary wall as the result of multiple, simultaneous and sequential processes for synthesis of cell wall constituents wherein the syntheses are distributed in both space and time. The spatially distributed syntheses are considered essential to allow gradual modification of the molecules of the precursors of cell wall constituents. They begin as molecules optimized for solubility within the aqueous environment in the cell and are transformed into ones that can be consolidated within the secondary wall, which is more hydrophobic upon lignification. To add clarity to the analysis, a system theoretic approach to the discussion of the distribution of the biogenetic processes in space is adopted. The overall system within each cell is regarded as three highly coupled subsystems. The nanoscale organization in the native state is then examined as a reflection of the processes of biogenesis of the plant cell walls. Issues associated with aggregation of the constituents are then considered and plausible pathways for the assembly of the matrix that are based on distributed synthesis of the major polymeric constituents are suggested. The molecular organization is regarded as the first stationary level of expression of phenotypic form and the implications of the similarity of this expression within contemporaneous cells in an annual ring of secondary growth are examined. It is concluded that all aspects of molecular organization must be governed by intracellular processes that can be orchestrated at levels well beyond those of the individual cells. It is proposed that the primary molecular carriers of organizing information, that is the mediators of the orchestration, are the hemicelluloses, and the manner of their action in the formation of the structure of lignin is suggested. The results of the analysis of hierarchic organization have a number of additional implications with respect to structure and its formation that are beyond the scope of the present report. One that is worthy of note at this time is that the organization of the deposition of the secondary wall, and its systematic

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Citation

Atalla, Rajai H. 2005. The role of the hemicelluloses in the nanobiology of wood cell walls : a systems theoretic perspective. Proceedings of the Hemicelluloses Workshop 2005 : 2005 January 10-12, The Wood Technology Research Centre, University of Canterbury, Christchurch, NZ. Christchurch, NZ : Wood Technology Research Centre, University of Canterbury, 2005: Pages 37-57

Keywords

Hemicellulose, biotechnology, molecular aspects, biosynthesis, molecular structure, system theory, hemicellulose, lignin, plant cell walls, nanotechnology

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