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. WHAT IS THE CELL WALL?