We have seen that the major limitation for biotechnology with plant cell walls today is our limited knowledge about the biosynthesis of the cell wall components and their assembly into the complex pericellular matrix. The genomic approaches initiated recently promise rapid progress in identifying the genes potentially involved in these processes, at least for some model plants such as Arabidopsis, maize, rice, barley, poplar, and pine (116-120). Proteomic approaches will complement these studies, and they will help in identifying the enzymes involved in the biosynthesis and assembly of specific cell wall domains elaborated under certain sets of environmental conditions. As seen with the cellulose synthase genes and, even more clearly, with the six classes of cellulose synthase-like genes, extensive functional genomic experiments will be needed to assign specific functions to the genes identified, and these will keep us occupied for years to come. Functional genomics will allow us to analyze which enzymes are involved in biosynthesis and assembly of complex cell walls, but they will not shed light on how these enzymes do their jobs. Methods for functional proteomics may have to be developed to investigate the molecular interactions between proteins (e.g., Ref. 121), polysaccharides, and the other cell wall constituents to understand how the cell wall is assembled and how it functions. This knowlegde, of course, is a prerequisite for targeted genetic engineering of plant cell walls for biotechnological uses.
Functional proteomics will have to make use of the newly developed technical possibilities offered by the nanosciences. Nanoanalytical tools allow the visualization and the determination of surface properties of individual molecules (122). They also permit direct measurement of intermolecular forces holding together the noncovalent networks that make up plant cell walls. Interestingly, nanosciences not only offer analytical tools but also are currently developing manipulating tools with which to handle and influence individual molecules. These tools can be increasingly used in vivo, and this will allow us to learn about the discrete roles that individual molecules play in the complex fabric of the cell wall. Eventually, cell walls may serve as ideal models for complex, versatile, and very stable composite materials, and the molecular machinery for assembly or the principles of self-assembly of the different components to build the complex cell wall may well serve as a model for biomimetic processes to be used in the construction of na-nomachines or nanomaterials using nonnatural or semisynthetic molecules.
It should be clear from the preceding discussion that we are only beginning to see the surface of the incredible biodiversity inherent in plant cell walls—as it is in almost every aspect of living systems. We have seen the flexibility and dynamics in cell wall architecture of a single plant, and we have pointed to the differences in cell wall composition and architecture between dicot plants and the commelinoid monocot plants. However, this treatise has focused on the cell walls of the angiosperms while those of the gymnosperms have not been mentioned. But to be comprehensive, we would even have to include cell walls of the nonflowering ferns and mosses and the green, red, and brown algae. Fungal cells are also surrounded by cell walls that function according to the same principle as plant cell walls— stress-bearing fibers in a pressure-bearing matrix—but this is realized using quite different polysaccharides and proteins, and the diversity within the fungal kingdom is enormous. Furthermore, almost all prokaryotic cells possess cell walls, but these are built in a completely different way, such as the murein sacculus, which is one giant covalently linked molecule. We know little about most of these cell walls—and this is an exciting prospect for future researchers who are not intimidated by the enormous complexity of the pericellular matrices.
Clearly, plant cell walls are an incredibly rich renewable resource for sustainable biotechnologies. However, we still understand little of the cause-and-effect relationships in most of the physiological processes involving plant cell wall components. Manipulating single components of such a complex structure has almost invariably yielded results that were different from what was expected. We should seize the opportunities molecular genetic tools offer, but we should take our time to analyze the effects of genetic engineering of cell wall components, assembly, and architecture. There should be no haste to introduce transgenic plants and their products to the environment and to the market prematurely. Only well-planned, well-done, and well-analyzed transgenic plants with clear environmental and consumer benefits will stand a chance to overcome reasonable and irrational fears of the consumers.
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