The Cell Wall

The cell wall surrounding plant cells is an extracellular matrix which can be viewed as a multifunctional sub-cellular compartment with many important functions in plant growth, development and response to environment cues [1,2]. The cell wall provides living plant cells with the mechanical framework that supports mechanical cell strength and controls the rate and direction of cell growth from which stable plant architecture and form are produced [1]. The cell wall also protects plant cells against pathogens, dehydration, and other stressful environmental factors, and regulates the distribution of water, minerals, and other small nutrient molecules in and out of the cells [1]. The cell wall is a storage site of many biologically active signalling molecules that; a) signal cell-cell recognition and interaction, b) regulate cell differentiation and growth, c) sense and react to the presence of pathogenic microbes [2]. The presence of a cell wall is one of the most important distinguishing features of plant cells.

Morphologically, there are four major regions of the cell wall, 1) middle lamella, the outermost layer composed primarily of pectic polysaccharides that glues adjacent cells, 2) primary wall deposited during active cell division and growth in order to provide cells mechanical strength but also allow them to divide and grow, 3) secondary wall deposited inside the primary wall in some cells after cell size and shape are permanently established, in order to provide cells additional mechanical support, and 4) apoplastic continuum that facilitates the transport of water and nutrients [2]. Finally there are also special conduits termed plasmodesmata [3], that penetrate the middle lamella, the primary and secondary cell walls to allow symplastic, cell-to-cell communication occur.

The actual structure and content of a cell wall may vary with plant species, different developmental stages, tissues and sub-domains of cells. In contrast to their static appearance, plant cell walls undergo substantial spatial and temporal changes in architecture in response to both developmental and environmental cues. Dicot primary cell walls surrounding growing and dividing plant cells are composed of cellulose microfibrils that are crosslinked by hemicelluloses and embedded in a gel-like matrix of pectins [4]. This polysaccharide network is supplemented by structural proteins (e.g. extensin, proline-rich proteins) and interacts with proteins ranging from cell surface transmembrane receptors (e.g. arabinogalactan proteins) to secreted signal proteins (e.g. clavata3, hydroxyproline-rich glycopeptide signals) to cell wall modification enzymes (e.g. invertase, polysaccharide hydrolases) [2]. The secondary walls are composed predominantly of cellulose, lignin, and hemicellulose. The polysaccharides in cell walls form one of the largest biomasses in the world, which are utilized for the production of food, wood, paper, fiber and textiles. They are now also target of biotechnology for overcoming technical challenges to make cellulose and other cell wall polysaccharides more readily available for bio-refineries, including fuel-grade ethanol fermentation [5].

While knowledge of specific biochemical activities associated with cell wall polysaccharide biosynthesis is growing [3], the distribution pathways of cell wall polysaccharides and modifying agents for the generation of proper cell walls are still largely unknown. The biosynthesis of a cell wall starts during cell division when the cell plate is formed between daughter cells. It is generally believed that the formation of the cell plate requires the fusion of Golgi-derived vesicles that carry non-cellulosic polysaccharides and cell wall proteins. Recently, it has been demonstrated that endocytosis of existing cell wall and plasma membrane materials might also contribute significantly to the formation of the cell plate during plant cytokinesis [6]. As the cell plate is made, further matrix polysaccharides, hemicelluloses and pectins, and cell wall proteins are synthesized and/or modified in the Golgi apparatus, and delivered by post-Golgi membrane trafficking pathways to specific regions of growing cell walls at the right times in order for normal plant cell growth and development to occur [7,8]. Cellulose is synthesized by the cellulose synthase complex that is organized in a rosette complex at the plasma membrane, however, post-Golgi membrane trafficking is required for proper delivery of the cellulose synthase complex [9].

Fluorescent protein-based labeling of the cell wall has not been easy. Table 1 is a list of cell wall-associated fusion protein probes that have been created. When GFP is fused with an ER targeting peptide, the fusion protein termed secGFP is transported to the apoplast when transiently expressed in tobacco epidermis [10, 11], or stably expressed in wild type Arabidopsis Col-0 [12]. However, the fluorescence is low due to rapid degradation of the fusion protein and sub-optimal conditions for GFP fluorescence in the apoplast [12]. When secRFP, a variant of RFP fused with the same ER targeting signal peptide is used, the fluorescence is very stable in the apoplast in both transient expression in tobacco epidermis [11] and stable Arabidopsis line [13]. Thus it is necessary to point out that when cell wall labeling is required, RFP, not GFP should be a choice. Finally, plant cell walls can also be perfectly labeled by propidium iodide [14].

References

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