Laying down the bricks: logistic aspects of cell wall biosynthesis

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Plant cell wall polysaccharides are synthesised at the plasma membrane and in the Golgi apparatus. Current research efforts mainly try to address how these molecules are synthesised or modified. However, it is clear that polysaccharide synthesis in the two compartments needs to be carried out in a coordinated fashion, and that carbohydrates and proteins that are delivered from the Golgi to the cell surface have to undergo a range of modifications. Consequently, there appears to be a need for a fine-tuned system that coalesces signals from the wall, synthesis of carbohydrate-based molecules and vesicle shuttling. Several recent papers have scratched the surface for an initial understanding of these linked processes. For example, the impairment of the proton pumping activity in the trans-Golgi network, which is part of the cell’s trafficking system, results in growth defects, changes in Golgi stack morphology and cellulose deficiency. An increased understanding of how cell wall synthesis is coordinated with the secretory machinery may facilitate avenues for modulating cell wall contents and therefore overall plant biomass.

Introduction

Plant cell walls determine the shape and architecture of the plant body and play major roles during cell differentiation, defence against pathogens and cellular growth [1]. Thus, cell walls are suggested to be highly dynamic and responsive structure that, are able to sense and integrate external stimuli. Both primary and secondary cell walls contain predominantly polysaccharides, which are embedded in an aqueous matrix. Cellulose, the main polysaccharide of plant cell walls, is made of β-1,4-linked glucan chains, which form crystalline microfibrils and act as a framework for the deposition of other wall components [1]. The matrix polymers, such as hemicelluloses and pectins, are structurally diverse polysaccharides and most of them harbour heavily substituted side chains [2, 3]. Whereas cellulose is synthesised directly at the plasma membrane, pectins and hemicelluloses are made in the Golgi apparatus and are secreted to the plasma membrane via exocytosis [4] (Figure 1). The ordered supramolecular architecture of the wall and the compartmentalised production of the polymers thus raise the question of how the deposition of different polysaccharides is coordinated, both during cell expansion and during cell wall maintenance. A reasonable explanation may be that structural deviations are sensed by a signalling system that triggers the synthesis of wall polymers, and stimulates the secretory machinery of the endomembrane system. Here we aim to review some of the latest developments in primary cell wall synthesis with an emphasis on the roles of vesicle transport and the cytoskeleton. The topics of cell wall biosynthesis and cell wall integrity signalling have been extensively covered in excellent recent reviews (e.g. [5, 6, 7]).

Section snippets

Cell wall polysaccharides are synthesised in different cellular compartments

Cellulose is synthesised at the plasma membrane by hexameric cellulose synthase (CESA) complexes, which are believed to hold 36 individual CESA proteins [1]. The Arabidopsis thaliana genome harbours 10 CESA genes [8], and mutant analyses show that a functional complex requires at least three different CESA isoforms [9, 10•, 11•]. A number of other mutants deficient in cellulose have also been described, which suggests that cellulose production requires a range of different components [6, 12].

Sensing the structural design of the cell wall

Cellular growth requires synthesis and deposition of new cell wall material as well as remodelling of the existing cell wall matrix [23]. Also, external stimuli, such as biotic and abiotic stresses, affect growth and are sensed and integrated into cellular responses. Several putative cell wall sensors are found at the plant plasma membrane, including wall-associated kinases (WAKs) [24], and other members of the receptor-like kinase (RLK) family [7]. Mutations in one RLK, THESEUS1 (THE1), was

Modification and secretion of cell wall components

The demand for cell wall biosynthetic events that may be transduced by cell surface signalling devices needs to be transformed into cellular responses. These may include post-translational modifications of enzymes, modulation of the rate of secretion and transcriptional activations or repressions. A recent report revealed that a functional KOR1 requires correct N-linked glycosylation and may constitute a direct substrate for STT3a, a catalytic subunit of the oligosaccharyltransferase complex in

The ‘yellow brick road’ to the wall

Deposition of cargo during cell wall synthesis involves exocytosis, that is transport of secretory vesicles from the Golgi stacks to the plasma membrane, and most probably membrane recycling at the cell surface [45, 46, 47]. The coordination of these vesicle trafficking events is far from understood, but at least some steps seem to be highly dependent on the cytoskeleton [48]. The cytoskeletal framework is formed by polymers of multiple actin and tubulin proteins, and is regulated by a vast

Future perspectives

The importance of the plant cell wall in plant morphology and industrial applications, such as bioethanol and food production, has prompted incentives and investments to elucidate how the different wall polymers are synthesised. However, to understand regulatory aspects of the wall assembly we need to consider that the wall is an integral part of the plant cell. The recent break-throughs concerning cell wall integrity signalling are in this sense exciting, and hold great promises for future

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgement

This work was supported by funds from the Max-Planck-Gesellschaft.

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