Progress in the biological synthesis of the plant cell wall: new ideas for improving biomass for bioenergy

https://doi.org/10.1016/j.copbio.2011.12.003Get rights and content

Lignocellulosic biomass feedstocks for biofuels are primarily the thickened secondary cells of vascular plants. Recent advances have been made in our basic understanding of how cellulose and the non-cellulosic polysaccharides of the plant cell wall are synthesized, assembled, and integrated with the synthesis of lignin. New complexities have been elucidated in the ways cellulose microfibrils are deposited at the plasma membrane surface and integrated with non-cellulosic polysaccharides are assembled and lignified into functional form. Current strategies focus on the transcriptional events that specify vascularization and fiber formation and how the composition of lignin is modified in expression variants in the natural population. This knowledge base will yield new ideas for how to enhance lignocellulosic composition and cell wall architecture in biomass tailored for its end use.

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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