Chapter 20. The extracellular matrix20.1. The relationship of the extracellular matrix of plant and animal cells.In the seventeenth century, the presence of thick walls in wood and cork made it possible for cells to be readily identified in plants (Hooke, 1665). The presence of a thick wall was the original sine qua non for defining a cell, and cells were defined and characterized by their walls and not their contents. Thefact that the extracellular matrix of most animal cells is thin or nonexistent was an impediment to the realization that animals were also composed of cells.Most cells, to a greater or lesser degree, secrete macromolecules into the surrounding medium. These macromolecules amalgamate into an organized structure, which has been called a cell wall, an extracellular matrix, an apoplast, a periplast, slime, a glycocalyx or a cell covering (Okuda, 2002; Niklas, 2004). Although plant cells generally have a thicker extracellular matrix than animal cells, there is a continuum between the two extremes. Some plant cells, like those of Dunaliella, have a thin or nonexistent extracellular matrix and some animal cells, like those of the tunicates, have anextracellular matrix composed of cellulose. Among plant biologists, the terms cell wall and extracellular matrix are used interchangeably; and the continuumof cell walls is usually referred to as the apoplast. Each term carries with it a slightly different shade of meaning. The term cell wall emphasizes the supporting role of the structure, and accentuates the apparent gulf between plant and animal cells. The term extracellular matrix emphasizes the primary importance of the plasma membrane and not the cell wall as an active barrier to diffusion in separating the protoplasm from the external environment. The term extracellular matrix also emphasizes the lively and dynamic aspects of the region external to the plasma membrane, and has never carried the connotation of being a dead part of cells, as does the term apoplast, which comes from the Greek for “without form”. The use of the term extracellular matrix has been productive in understanding the unity of nature while still allowing an appreciation of its diversity. This is not a new view. Thomas Huxley (1853), in his essay, entitled, “The cell-theory”, considered the 790extracellular matrix of plant and animal cells to be homologous, and gave them a common name, periplast.Young plant cells are surrounded by a thin extracellular matrix that prevents them from lysing when placed in dilute solutions typical of lakes, streams and soils. By contrast, animal cells with a thin extracellular matrix will lyse unless they are surrounded with a solution that is isotonic with the cell. In fact, the differences in the ionic basis of action potentials in plant and animal cells can be traced to the fact that plant cells, unlike animal cells, are not typically bathed in solutions containing high concentrations of NaCl (Wayne, 1994; Johnson et al., 2002). In order to live on land or in dilute aqueous environments, plant cells have evolved specializations in the extracellular matrix (including the hollow tracheary elements), while animal cells have evolved specializations in the circulatory systems. While the extracellular matrix allows plant cells to live in dilute environments, it contains numerous fixed charges that accumulate considerable amounts of essential nutrients in a nonosmotic form (Grignon and Sentenac, 1991; Gabriel and Kesselmeier, 1999). Thus the cell carries a suitcase around itself that contains the necessary nutrients found in its ancestral seas.In the meristems of plants, the extracellular matrix begins to form during cytokinesis (chapter 19). The primary cell wall is approximately 100 nm thick (Roberts, 1989,1990,1994), and is only slightly rigid so that the cell can still expand. The primary cell wall contains even thinner regions, known as primary pit fields, through which plasmodesmata pass (chapter 3; Figure 20-1). Correlated with the cessation of expansion, the secondary cell wall begins to be deposited either by thickening the primary cell wall and/or by depositingnew layers of wall material approximately 10 m thick between the plasma membrane and the primary cell wall (Figure 20-2). The primary and secondary cell walls are defined based on their order of formation (Esau, 1965). The secondary wall is specialized for mechanical support; and often, cells surrounded by a secondary wall are dead at maturity. The correlation between large cell walls and dead cells has historically caused plant biologiststo view the cell wall as dead wood instead of the dynamic organelle it is (Bolwell, 1993; Kieliszewski and Lamport, 1994; Carpita et al., 1996; He et al., 1996).The extracellular matrix of plants thickens during development. This allows each cell to generate an internal hydrostatic pressure or turgor pressure791in the order of 0.1-1 MPa, which provides the cell with a certain amount of rigidity and mechanical strength. Consequently, plants can grow tall and wide in order to maximize their ability to intercept the sun’s rays for photosynthesis. While a thick extracellular matrix is extremely useful for a phototrophic organism, it prevents plants from moving, and makes them susceptible to attack by predators.It is a general biological principle that crises lead to opportunities, and consequently species evolve by natural selection (Darwin, 1859). The very extracellular matrix that prevents plants from moving has evolved a number ofproperties that allow it to function in the defense of plant cells. In some cases, the extracellular matrix becomes extremely thick in order to prevent the entrance of pathogens (Israel et al., 1980; Kobayashi et al., 1997). In other cases, it acts as a lookout at the frontier of the cell, and signals the appearanceof a pathogen to the rest of the cell (Bergey et al., 1996; Brady and Fry, 1997;Gelli et al., 1997; Stratmann and Ryan, 1997; Xing et al., 1997). In this role, fragments of the extracellular matrix known as elicitors are released by the hydrolytic enzymes of pathogens and signal the rest of the cell to form antiseptic compounds known as phytoalexins (Darvill and Albersheim, 1984; Ryan and Farmer, 1991; Zhao and Last, 1996). Agrobacterium-mediated transformation also depends on a functional extracellular matrix (Zhu et al., 2003).The extracellular matrix of plant cells can be considered a keeper of positional information,