ABSTRACT
Regardless of their relationship to substratum (pathogenic or saprophytic), these classes of organisms produce extracellular enzymes, which interact with the environment outside of the cell or hypha. Enzymes are complex proteins that are manufactured in the bacterial cell or fungal hypha, transported across the plasmalemma (cell membrane) and cell wall to the outside environment. These are typically hydrolytic or oxidative and systematically degrade or break down very complex plant (and animal) polymers into simple molecules. In turn, the
Concept Box
simple molecules such as sugars, amino acids, fatty acids, and so on resulting from enzymatic actions are absorbed through the cell wall, transported across the plasmalemma, and used for growth, energy, reproduction, and other life processes. Some enzymes that degrade plant cell walls may also be involved in plant pathogenesis. For example, the fungus Rhizopus stolonifera and the bacterium Erwinia amylovora both produce a battery of pectinolytic enzymes (pectinases) that degrade the middle lamella, which is composed chiey of pectin and lies between plant cells. These organisms cause soft rots via the actions of pectinases. Many obligate parasites, such as Peronospora tabacina (blue mold of tobacco), utilize cell wall degrading enzymes (e.g., cellulases) to establish contact between haustoria and plant cell membranes.
