Cellulose synthesis: a complex complex
Introduction
Plant cell walls regulate cell growth and provide structural integrity and mechanical support for the plant [1]. Cellulose is a major component of the wall and is an important source of industrial raw material [2]. In a more long-term perspective cellulose is also expected to become a large source for biofuel production [3].
The cellulose polymer is arranged in 3-nm thick microfibrils, each estimated to be comprised of roughly 36 crystalline, parallel β-1-4-glucan chains [2]. These polymers are believed to consist of 8000 (primary cell wall) to 15 000 (secondary cell wall) glucose molecules, making the cellulose microfibril one of the longest bio-macromolecules known in nature [2]. The microfibrils wrap tightly around the cell and provide the major mechanical resistance to external stresses and internal osmotic pressures [4]. The cellulose microfibrils also serve as scaffolds for other cell wall polymers such as hemicelluloses and pectins [1].
In higher plants, cellulose is synthesized at the plasma membrane by rosette complexes (Figure 1) [1]. These transmembrane complexes are organized as hexamers, presumably consisting of 36 individual cellulose synthase (CESA) proteins. The CESA complexes are assumed to be assembled in the Golgi and then exported to the plasma membrane via exocytosis [2]. The deposition of primary cell wall cellulose is highly ordered and thought to be guided by cortical microtubules (MTs; Figure 1) [5, 6, 7], at least in rapidly expanding cells [8••]. Hence, cellulose microfibrils tend to be deposited parallel to the MTs in these cells. Several recent papers have offered new insights into the interplay between MTs and cellulose deposition, and into the constitution and regulation of the primary wall CESA complex. This review will focus on the primary cell wall cellulose production in context of these recent advances.
Section snippets
The CESA complex
As microfibrils are believed to contain 36 glucan chains, it has been proposed that one rosette complex holds 36 individual CESA proteins [9]. On the basis of immunological evidence the CESA proteins also form six subunits within the complex [10], probably resulting in synthesis of six glucan chains per subunit. Genomes from higher plants harbour multiple CESA genes; for example, there are ten in the Arabidopsis genome, 18 in poplar, and at least nine in rice [2, 11].
Importantly, at least three
Cellulose biosynthesis
The utilization of a functional YFP-tagged CESA6 has greatly advanced our understanding for how cellulose is synthesized [8••]. Time average images of such fluorescently labelled CESA complexes in vivo show that they move with an average velocity of approximately 300 nm min−1 [8••]. This corresponds to the addition of 300–1000 glucose molecules min−1, assuming that the microfibril is immobilized in the cell wall. The cellulose microfibrils are generally perpendicular to the axis of cellular
Future perspectives
The ability to directly visualize the rosettes localized in plasma membrane has rapidly increased our understanding of how cellulose is deposited. Several questions, however, remain unanswered in regard to the constituents of the rosette complex and also concerning identification and characterization of components that affect cellulose deposition. It appears, for example, that the primary complex contains CESA1 and 3 and a CESA6-like CESA. It is, however, not clear when and where the different
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
We would like to thank Daniela Geisler for useful comments on the manuscript and the Max-Planck Society for financial support (M.M. and S.P.). This manuscript is published with permission of the Director of the Kentucky Agricultural Experiment Station as article 08-11-007 (SD).
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