Understanding of plant cell wall biosynthesis for utilization of lignocellulosic biomass resources

How do plants make cell walls? How can the cell wall biomass be turned into fuel energy? These questions are still unsolved puzzles that have attracted a great deal of interest in scientific community as well as in industrial and political worlds. Different from animal cells, plant cells develop characteristic structures of cell walls, which play crucial roles in planting growth and response to environments. As cell walls accumulate approximately seventy percent of the land pant photosynthesized products on the globe, cell wall biomass is the most abundant renewable resource for production of bioenergy and biomaterials. However, only a small portion (~2%) of the plant cell wall resource has been used by human society so far. More efficient utilization of lignocellulosic cell wall biomass is highly desired. Great effort has been invested in understanding of how cell walls are formed in plant in recent years. Plants cell walls contain cellulose, hemicelluloses, pectic polysaccharides and lignin. These polysaccharides and phenolic polymers are constructed in concrete cell walls for supporting plant growth and development. Cellulose is composed of β-1,4-linked glucose units and a bundle of multiple -1,4-glucan chains together form microfibrils, providing a skeleton of cell wall structures. Cellulose is synthesized in plants by cellulose synthase (CesA) complexes which are integrated and localized in plasma membrane. There are a family of CesA genes in plants which direct cellulose synthesis in different layer of cell walls or regulate cellulose microfibril structures. Although cellulose has been used in many aspects of human life, such as fiber cloth, paper, and modified cellulosic products, Cellulose, the most abundant biopolymer on the Earth, promises more utilization in supporting for sustainability of human society development. Hemicellulose is a group of diverse polysaccharides, such as xyloglucan, heteromannans, heteroxylans and mixed- linkage glucan. Their diverse structure and sugar composition varies in plant species, tissues, even cell types. Hemicellulose polysaccharides, amorphous and soluble in aqueous solutions, are composed of heterogeneous monosaccharide units or linkages in their backbone and branched side chains. Hemicelluloses are usually covalently linked to cellulose and lignin in cell wall matrix. Hemicellulose has been used in many applications such as food products and nutritional supplements. How is hemicellulose synthesized? How do the diverse hemicelluloses function in plant development? How can the abundant hemicellulose sugars be used? Many questions remain to be investigated. Lignin is polymerized from phenolic monolignols and mainly deposited in the secondary cell walls of vascular plants. Evolved in higher plants, lignin is essential for long distance water transport, mechanical support of up-growth, pathogen defense and for other stress responses. In the cell walls, lignin is usually cross-linked with cellulose and hemicellulose. Although biosynthesis of monolignols including p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) monolignols has been elucidated genetically and biochemically in various plant species, regulation of monolignol formation in different species, tissues and cell types is still unclear. As the second most abundant plant biopolymer after cellulose on the globe, utilization of lignin is highly attended. Lignin is considered as a main recalcitrance during cell wall biomass conversion, therefore modification of lignin biosynthesis is of great interest in engineering plant cell walls for efficient biomass utilization. Laboratory research has showed potential benefits of lignin engineering for lignocellulosic biomass conversion, while application of the technology in the field requires more evaluation. This review summarizes recent studies of the biosynthesis of plant cell wall components such as cellulose, hemicelluloses and lignin as well as the modification of cell walls for the improvement of lignocellulosic biomass utilization.