Plasma Membrane-Cell Wall Contacts

In the simplest sense, cell walls are a carbohydrate and protein structures that surround and separate cells. Perhaps the most familiar cell wall structures are cork and wood, where the cell wall remains after the cells within have degraded. Cotton, the fiber that keeps some of us clothed in our complex world, is also a type of cell wall. These are specialized examples, and it is clear that living cells have a dynamic interaction with their surrounding wall and with each other, perhaps through the wall. This extracellular matrix (ECM) has the potential to influence almost every aspect of cell function simply because of its position and physical properties. For this reason many have speculated on its role in a plant's development and response to the outside world. Cell growth can occur in many dimensions, such as the polarized expansion of a pollen tube tip, the creation of elongated cells characteristic of many vegetative tissues, or even the jig saw-like arrangement of cells at the leaf surface. As these cells expand in a regulated fashion they must necessarily modify and enlarge their ECM to permit the subsequent increase in volume, but it is thought that the cell wall laid down by the same or an adjacent cell itself might also influence this process. Currently our understanding of these processes is quite limited. Although there are as yet no direct answers to how the cell wall functions in development and plant responses, there have been some recent advances in understanding what molecules might be involved, and how they might interact with each other and the cells. For cell walls and cells to influence each other, there must necessarily be contact, and although there are numerous potential interactions at the plasma membrane-cell wall interface, this essay will focus on the direct physical connections that are known to occur between the plasma membrane and the ECM in angiosperms. Recent reviews describe other cell wall components and their role in expansion and plant development and the role of chemical modifications in response to environmental influences (Carpita and Gibeaut, 1993; Showalter, 1993; Reiter, 1994;Cosgrove, 1997). What emerges is the idea that the angiosperm cell wall is more than an exoskeleton; it is also a dynamic substrate for interacting cells. Diagramatic representation of primary cell wall assembly and structure between two adjacent cells. Cellulose is synthesized and secreted by a complex of cellulose synthases (CSAs) on the plasma membrane, and forms a bundle of crystalline microfibrils (black cables; cellulose). Cellulose synthase may be associated with cytoplasmic sucrose synthase (SUSY) that provides the sugars for polymer synthesis. Pectin (red bars) and hemicellulose (orange wiggles) are synthesized in and secreted through the endomembrane system and are complexed with cellulose in the ECM. How the synthesis of the wall is coordinated between two adjacent cells and between the cellulose synthase and the endomembrane system is not known. Cell walls have been classified as primary or secondary. The primary wall is laid down during cell division and expansion, and material deposited on the primary wall once growth has ceased is termed the secondary wall (Cosgrove, 1997). For simplicity, and since many dynamic interactions are found in expanding cells, the discussion of the plasma membrane-wall interface will be restricted to the primary cell wall. Many have equated the plant cell wall with the ECM of metazoans, and indeed the terms are often used interchangeably (Roberts, 1990; Carpita and Gibeaut, 1993). Although the cell wall has a radically different composition from the metazoan ECM, they do play similar roles; one could equate wood with cartilage at one extreme, and primary cell walls with basement membranes. Defining the cell wall is a battle with semantics, and raises some important points. Until recently the cell wall has been considered in the most part an exoskeleton of protein and carbohydrate that is secreted by its own caged or adjacent cell. In this way the cell defines its immediate environment and shape. The metazoan ECM is approached more from the view that the ECM is a carbohydrate substrate that influences the behavior of the surroundings through receptors and modifying proteins (Bissell and Nelson, 1999). Perhaps the most parsimonious view of the cell wall would encompass both views (Roberts, 1990). An easy working definition for the cell wall would see it as a carbohydrate matrix that provides a dynamic scaffold with which a variety of other carbohydrates and proteins associate. Whether these carbohydrates and proteins are “cell wall ” components is only a matter of definition, and it is often hard to distinguish whether they are structural or regulatory or both. The primary cell wall of angiosperms is in part laid down through the ordered secretion of 1-4-linked β-d-glucose polymers by plasma membrane-associated cellulose synthases (Amor et al., 1995; Pear et al., 1996). These polymers are woven together into linear bundles of cellulose fibers that have an average diameter of 7 nm and are thought to form a liquid crystalline array. Hemicellulose is a term used to describe a family of polymers rich in glucose, xylose, or arabinose that, unlike cellulose, have extensive side chains often including xylose, galactose, and fucose. The dicots and monocots differ substantially in their hemicellulose composition and comprehensive descriptions can be found in a number of reviews (Carpita and Gibeaut, 1993; Reiter, 1994; Cosgrove, 1997). The hemicellulose structure permits these complex sugars to lie along the surface of, and perhaps intercalate within the cellulose bundles, providing a linked matrix. The hemicelluloses are secreted through the endomembrane system (Fig. 1). How the secretion of hemicellulose and the synthesis of cellulose are coordinated is not known but this may be important in defining localized wall architecture and its interface with the cell. Pectins are a family of polygalacturonic acids that can vary in their side chains, usually arabinose, galactose, or a complex branched arrangement of monosaccharides (Cosgrove, 1997). The pectins are also secreted through the endomembrane system such that they may form a jelly like matrix that is intercalated with the cellulose/hemicellulose structure (Carpita and Gibeaut, 1993). The abundance of negative charges on pectins allows Ca2+-mediated cross-linking that may be regulated by the masking of pectic negative charges through the addition of methyl esters. Antibodies directed to either pectin or methyl-esterifed pectin detect epitopes that are distributed unevenly in a variety of tissues, including pollen tubes, providing evidence that this modification could have a regulatory function (Knox, 1997). When pollen contacts the stigma there is a rapid expansion of membrane at the pollen tip and the continued tip growth has been correlated with the de-esterification and Ca2+ cross-linking of pectins peripheral to the growing tip. The cross-linking leads to an increased ridgidity of the lateral pectin matrix of the pollen tube thereby permitting only tip expansion (Yang, 1999). Similar models are proposed for root hair growth (Wen et al., 1999). Nothing is known of how the synthesis of cellulose and the secretion of pectins are coordinated although their respective matrices can exist independently (Roberts, 1990). Traditionally “cell wall” proteins have been classified by their association with one or more of the complex carbohydrates secreted by plant cells. These include the abundant hydroxy-Pro-rich glycoproteins (HRGPs; Showalter, 1993), Pro-rich proteins (Showalter, 1993), Gly-rich proteins (GRPs, Keller 1993), arabinogalactan proteins (AGPs; Oxley and Bacic, 1999; Majewska-Sawka and Nothnagel, 2000), wall-associated kinases (WAKs; He et al., 1996, 1999), lectins (Herve et al., 1999), and expansins (Cosgrove, 1997). But the list is far more extensive and includes peroxidases, methyltransferases, galactosidases, glycanases, and proteases to name just a few (Showalter, 1993). Analysis of genome information and detailed gel analysis (Robertson et al., 1997) will likely provide an exhaustive list of additional cell wall proteins. It may not be a useful exercise to anoint a protein the honor of being a “cell wall” component, but rather deal with this large class of secreted proteins from a functional standpoint. Indeed perhaps the best example is provided by the protein ligand SCR for the receptor kinases that regulate self-incompatibility inBrassica sp. (Schopfer et al., 1999). SCR is secreted by the pollen grain and resides on its surface to be presented to its receptor on the plasma membrane of stigma cells. SCR is on the surface of the pollen and thus is in direct contact with and part of the pollen cell wall, but is it a “cell wall” protein? It is also important to remember that recent rapid freezing methods show the distances between the plasma membrane and the ECM are in fact smaller than previously observed (Roberts, 1990), such that it is possible for proteins to extend well into the carbohydrate matrix and perhaps even contact proteins or carbohydrates on another cell surface. One could also include in a discussion of cell walls the numerous receptor kinases on the plasma membrane (Kohorn, 1999). An example would be the CLAVATA 1 receptor (Trotochaud et al., 1999) on the lower meristem layer that influences cell identity and proliferation. The CLAVATA 3 protein is secreted by the uppermost meristem layer (Fletcher et al., 1999) and is postulated to bind CLAVATA 1 and serve as a ligand. To avoid the exclusion of many interesting proteins, it might be best to refer to the carbohydrates as the cell wall and to view the proteins as influential visitors. This indeed seems to be the view taken for the study of most other kingdoms (Bissell and Nelson, 1999). The question pertinent here then becomes which visitors have an influence that requires contact with both the plasma membrane and the extracellular carbohydrate. Physical connections between the cell wall and the plasma membrane have been observed in a number of ways. Most electron micrographs show that the plasma membrane is appressed against the extracellular material, and thus they are apparently in direct contact (Roberts, 1990). It is assumed that turgor pressure is responsible for this appression, because disruption of the turgor by osmotic shock induces plasmolysis and results in the separation of the membrane from the cell wall. In most cells this separation is quite complete, although appressed regions do remain and can be enhanced in frequency in salt-adapted cultured cells (Carpita and Gibeaut, 1993). Plasmolyzed cells have thin lingering strands of membrane that extend from the collapsed plasma membrane to the cell wall which have been termed Hechtian strands (Roberts, 1990). It remains to be determined if these are in fact sites of plasmodesmata that form cytoplasmic passages between cells (see below; Crawford and Zambryski, 1999), but the fact that they occur on the outer walls of the epidermis makes this less likely. The nature of the contact sites in either the Hechtian strands or the salt induced contacts is unknown, although they have been called “adhesion sites.” The term adhesion invokes homology with similar sites in metazoan cells, where integrins and similar receptors bind the ECM. These adhesion sites are clustered into islands that are associated with regulatory kinases, their ligands, and the cytoskeleton (Bissell and Nelson, 1999). Convincing evidence for such islands is still lacking in angiosperms, despite numerous attempts to identify such sites (Carpita and Gibeaut, 1993; Canut et al., 1998; Laval et al., 1999). Ironic and perhaps most pertinent is that their abundance in metazoans is greatly exaggerated in cultured cells and quite diffuse if not rare in real tissues (Bissell and Nelson, 1999). The plant cytoskeleton may have a role in defining contact sites between the plasma membrane and cell wall as it is clear that both actin and tubulin play essential roles in plant morphogenesis (Kost et al., 1999). It remains to be established, however, if angiosperm cells have true adhesion sites in the sense that there are locations on the membrane whose major role is to anchor the cell to the cell wall. If adhesion sites are not required to maintain cell shape, they may have a function in keeping a cell from rotating within a cell wall frame. This appears to be unnecessary in most cells due to the presence of plasmodesmata. Plasmodesmata are membrane filled channels that connect adjacent cells in defined locations, are laid down during cell division, and may indeed provide sufficient structural force to fix cells in position and Zambryski, 1999). If do not have adhesion they might have sites of contact that are of not to be in abundance during these might be in and cell wall synthesis. This appears to be the in a of the known proteins that are both in the plasma membrane and the cell wall, as representation of proteins known to contact both the plasma membrane and the extracellular carbohydrate. Cellulose synthase forms a complex in the membrane and that direct cell expansion and growth influence the cytoplasmic in and the cellulose microfibrils in similar such that they have similar protein are of carbohydrate linked to a smaller protein of (red can be found to the membrane a which can be on the cell surface to a secreted are secreted in the of a membrane anchor the form and the secreted have been found to with a variety of cell wall components and have been to provide or perhaps as in that modify or the carbohydrate are also membrane and may to modify the matrix once it is laid family of cell wall-associated kinases has cytoplasmic and the membrane to have an extracellular in the ECM. is associated with and of is not to the ECM, but most is linked to is also associated with a secreted that is associated with both pectin and is a functional These proteins, in with secreted proteins such as like proteins, and likely a of yet to be proteins, and interact with the ECM to cell growth and and environmental One that the most between the plasma membrane and the cell wall is the that Cellulose synthase can form a of protein in the plasma membrane and is thought to with sucrose synthase on the cytoplasmic of the plasma membrane (Amor et al., 1995; Pear et al., 1996). The association of the with could the of from sucrose to a growing cellulose the complex is to it allows the of cytoplasmic to the of cell wall Cellulose synthase is by a large family in angiosperms and it is possible that the different have in in one synthase and leads to a in crystalline cellulose and the of cell surface structures et al., cellulose is still it is likely that the is in the of cellulose and other synthases can still 1-4-linked polymers in the of a plasma membrane It is quite possible that the itself is of whose representation within the membrane can be as to the cellulose composition in the cell wall. such as et al., 1999) describe cellulose in in primary cell walls secondary wall providing more evidence that the cellulose synthase family proteins of whose study may how localized synthesis at the plasma membrane can the architecture of the wall. only can the composition of the cellulose synthase complex have influences on the synthesis and of the cellulose but it is also clear that the is associated with the It has been observed that the the plasma membrane and the cellulose are both to the of cell This has to the idea that the the on the cytoplasmic of the plasma membrane as the in the membrane during cellulose synthesis (Kost et al., 1999). In of a number of do the of the cellulose some that it is the that cell expansion that the of both and cellulose but in the same analysis that from cellulose the although there are et al., 1999; and is potential in either for be it or to influence cell wall and the may plasma membrane contacts with the cell wall. many vegetative cell both root (Kost et al., 1999) and pollen (Yang, by tip In these specialized cells it is clear that the cytoskeleton and the of are coordinated with the of extracellular and it will be of to see if the from and pollen can be to other cell at and are by a large family in angiosperms. Many are secreted from the cell to modify the carbohydrate matrix. One class of is to the plasma membrane and a in one of by the the assembly of the matrix and cell expansion in growing cells et al., The of this in the membrane may of its with the assembly of the cellulose synthase and perhaps provide a direct to It is likely that as are and proteins a number of and will and our view of how the surface of the cell as an surface for the cell wall will are by a large family in a variety of angiosperms. are in the endomembrane and some for the addition of a anchor such that secretion remain on the plasma membrane to the cell wall and Nothnagel, Oxley and to of the of an can be carbohydrate that is in the endomembrane The structure has potential to bind to components of the cell wall, and numerous that with cell wall (Showalter, 1993; Cosgrove, 1997). family can be in many to that play roles in plant growth and The anchor can be at the cell surface and Bacic, 1999), as in cells where cell wall composition is by the of glycoproteins et al., 1999). It is easy to that can the carbohydrate of the cell wall to the cell. The which the carbohydrate of has been used to has major on plant development and cell expansion in (Yang, pollen tube tip growth et al., and cell growth in and Showalter, 1999). is also evidence that an can direct pollen tube growth et al., It is what the is between the and and which is most by different have different and thus perhaps different wall and since they are in a variety of cells, it is to that to cell (Roberts, 1990; Showalter, 1993; Oxley and Bacic, 1999). This remains to be although it provides a where the cell wall be as a substrate for defined cell are cell in and in other angiosperm each have a cytoplasmic protein the plasma membrane and extend a into the cell wall et al., 1996, 1999). like the plasma membrane to the carbohydrate matrix but are in that they have the potential to through their The extracellular is between the and the family is in and are the most and of the and their are in vegetative of and of cell are also induced by and et al., 1999). in that they are essential for plant development and required during the response et al., et al., 1999) The but not cell wall to a of the cell wall in and can be from leaf or and this is and large of is also linked to pectin and most of that is to pectin is also is a of that is not to pectin or cell wall carbohydrate and this can be with The a where becomes to as a and then to pectin (Fig. How are in from the pectin matrix in with will be to our understanding of the cell role in cell expansion and to identify ECM plasma membrane receptors have not provided evidence that these molecules exist in angiosperms. bind the protein of which is to the metazoan ECM (Bissell and Nelson, 1999). number of have in the angiosperm plasma membrane, but have provided evidence for proteins with to integrins et al., 1998; Laval et al., 1999). to a of metazoan proteins that are known to the ECM to the cell have material in angiosperms see Canut et al., that identify these proteins or describe their that the plant epitopes are not in proteins in extracellular et al., 1996). This may not be the in carbohydrates between the two and a more likely may be in the cytoplasmic of ECM receptors where processes have a of being The of our of the interface between the plasma membrane and cell wall is at a rapid more and more of proteins that to be in the plasma membrane and either or contact the extracellular the interface becomes more This has on only components that have been to in the plasma membrane and cell wall, but in has some important that may to be For one would that membrane-cell wall contacts would be important in the of the and in the of the cell during cell of the proteins here may be in this and analysis of cell division (Kost et al., 1999) will identify additional components that could lie at the interface of the wall and plasma The study of pollen tube growth may also provide into cell to the in proteins that have to plant proteins et al., The name of these is since plant are not in in the same way as The adhesion in requires a large carbohydrate also found in the ECM for and it will be important to whether this complex to the plasma membrane or wall components in the pollen tube to the of an adhesion are by a family of that are in a variety of tissues, and have the potential to for other of cells. are a variety of other cell wall proteins that also have potential to membrane but as yet there is no clear evidence that These include the family of (Showalter, 1993), and that membrane-associated proteins (Herve et al., 1999). Many these proteins to be important in of In this the cell wall has been into two large structural carbohydrate and regulatory proteins. this separation has in this may be as to a clear between structural and regulatory Indeed some smaller carbohydrates have been in (Cosgrove, and secreted proteins can form structural extracellular matrices 1993). It is clear that the matrix not only provides structural but also as a substrate for cells to their and identity through protein This essay has and cellulose as these are proteins known to contact the and the carbohydrate is a that and perhaps and cell wall with cellulose synthase their regulated and association with the carbohydrate matrix may also a that in with turgor the cell in of only a few cell as but there are likely more to be that have been such as and it remains to be determined how these an interaction that cell shape, and Although some interactions between the wall and cell may be there is a of extracellular protein and carbohydrate that is into an matrix (Cosgrove, 1997). It is likely that this matrix a structural and its can be regulated both and and can extensive cross-linking between carbohydrate and protein in this matrix. that in the extracellular are likely responsible for some of the cross-linking that is and an understanding of how these proteins are regulated may be to how cells their But one must also the between this extensive material and the membrane proteins, such as and cellulose synthase that at have a dynamic interactions with the cell wall. Although this is not it is possible that components at the membrane-wall interface during cell expansion may well their function once the primary wall has been synthesized and be to the extracellular to serve as structural of a system that is a plant In this sense the proteins at the interface might serve two in and in a structural and perhaps less it will be important to include in our of the cell wall the force that it number of that force by the wall have a role in cell development and and 1997). Whether these are to the cytoskeleton or cell surface receptors remains to be In the few it is likely that the that the plasma membrane from the carbohydrate matrix will be filled with additional molecules whose interactions will some of the in this would like to and for the discussion that to this and and in the of the