Any microorganism that attempts to colonize a plant must contend with the cell wall. One of the most conspicuous effects of microorganisms on plant cell walls is enzymic degradation. Our interpretation of the significance of this depends on our concept of the plant cell wall, which, however, is paradoxical. On the one hand, the wall is an inert mechanical support and barrier; on the other hand, the wall is a dynamic, metabolically active organelle (Robinson, 1991, and refs. therein). The wall is a nutritional source for microorganisms and animals, yet it contains noxious peroxidases, phenolics, and activated oxygen (Cooper, 1984). The wall is mainly carbohydrate, yet its proteins are disproportionately studied (Showalter, 1993). Degradation poses another paradox. On the one hand, plant cell walls are the world's most abundant source of Glc and other sugars; on the other hand, parts of it are resistant to degradation. Since all the polymers of a plant cell wall are eventually degraded by microorganisms, for every type of chemical bond in the wall there must be an enzyme that can cleave it. Some polysaccharidases such as a-amylase are widespread in nature; others, such as ligninase, are very restricted in their distribution.