Progress in the biological synthesis of the plant cell wall: new ideas for improving biomass for bioenergy
Highlights
► Genomic variation and expression defines cell wall type. ► Microtubules guide cellulose synthase complexes into plasma membranes. ► Genes discovered in xylan biosynthesis and acetylation. ► Transcriptional networks for vascularization and lignification defined.
Section snippets
Structure and synthesis of the secondary cell wall
The major types of biomass crop plants, grasses, angiosperms (hardwoods), and gymnosperms (softwoods), have three distinct types of cell wall compositions and architectures [1•], underscoring the need appropriate and convenient genetic model systems for each. The backbone genome sequences of Arabidopsis, rice and maize allowed comparison of the family structures of hundreds of genes involved in wall synthesis, showing divergencies responsible for the differences in cell wall structure among
Old and new partners for cellulose synthases
As recent reviews iterate [9•, 30, 31], both genetic and cell biological experiments have shown that several isoforms of CesA proteins are needed to form the synthase complex specific to primary and secondary cell wall synthesis. Most models suggest that one CesA polypeptide synthesizes a single glucan chain and that the Zn-finger domains function to couple CesAs into the larger complex. An alternative model has been proposed, where dimers of two isoforms of CesA form the catalytic unit,
Lignin biosynthesis
The genes involved in the synthesis from phenylalanine to hydroxycinnamates and monolignol substrates of lignin biosynthesis are well established [51•, 52]. Several genetic approaches to modify lignin content and composition have been employed in attempts to reduce inputs in processing for the pulp and paper industry or recalcitrance to enzymatic digestion for biofuels production [53, 54]. One of the more promising advances has come from enhancing expression of a ferulate-5-hydroxylase (F5H)
Gene expression networks: new ideas to alter lignin composition and architecture
Efforts to understand how vascular and fiber cell identity is defined is beginning to yield a wealth of information about how lignin formation can be modified [67, 68, 69]. Few systems are as refined as the Arabidopsis root tip, where the timing and balance of transcriptional regulation and microRNA expression during the early events of vascularization can be observed at the cellular level [70•]. For vascularization of the stem, the breakthrough came a few years ago with the discovery of a
Conclusions
Steady progress is being made in characterizing genes that encode the proteins of polysaccharide synthesis, but we have only rudimentary understanding the biochemical mechanisms of catalysis. Advances are needed to define protein structural features and interactions within synthase complexes to make possible manipulation of their synthase scaffolds and complex stoichiometries for fine-tuning wall architecture. Likewise, all the genes of monolignol synthesis and the consequences of
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The author thanks Maureen McCann and Clint Chapple, Purdue University, for their many suggestions during review of this manuscript. This review was completed through support of the Center for Direct Catalytic Conversion of Biomass to Biofuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DE-SC0000997).
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