Polygalacturonases: many genes in search of a function.

Pectins are a major component of the plant cell wall and comprise one of the two major coextensive networks in which cellulose microfibrils are embedded (Carpita and Gibeaut, 1993). Pectic polysaccharides exist in the cell wall as either “smooth” regions of a linear copolymer of a-(1– 4)-linked GalUA or “hairy” regions that have attached a-(1–2)-linked rhamnosyl residues that may be substituted with Araand Gal-rich side chains. The pectin structure is further elaborated by divalent cation cross-linkages and possible esterification to other cell wall polymers. Because of the contribution of both ionic and covalent linkages, the structure of pectin may be modified by the ionic strength of the apoplast, by enzymes that modify the charge of the GalUA residues, or by enzymes that cleave either the a-(1– 4)-linked GalUA backbone or side chains of the hairy pectin regions. Because plant cells undergo dramatic changes in shape and developmentally regulated episodes of cell separation, in which the pectin network is systematically disassembled, pectin metabolism is critical to many developmental processes. A wide range of enzymes are known to catalyze aspects of pectin modification and disassembly. The best characterized are exoand endo-PGs, pectate lyase, pectin methylesterase, and b-galactosidase, which has been proposed to have the capacity to reduce the apparent molecular size of pectic polymers by cleaving neutral side chain residues (De Veau et al., 1993). In addition, it is likely that there are as-yet-undiscovered enzymes that may play critically important roles in cleaving covalent cross-linkages that tether pectins to other structural networks within the cell wall. Because of the extensive study of PG-mediated pectin disassembly, this review summarizes what is known about the complexity and structure of genes encoding plant PGs and their role in developmental processes. PGs were first identified over 35 years ago and have been suggested to be involved in the disassembly of pectin that accompanies many stages of plant development, particularly those that require cell separation. For example, PG activity has been shown to be associated with organ abscission (Taylor et al., 1990; Bonghi et al., 1992), pod and anther dehiscence (Meakin and Roberts, 1991), and pollen grain maturation and pollen tube growth (Pressey and Reger, 1989; Pressey, 1991). PG activity has also been detected in rapidly growing tissues, indicating that it may be involved in cell expansion (Pressey and Avants, 1977; Sitrit et al., 1996). Although it is clear that PG participates in many plant developmental processes, the majority of research has focused on PG in ripening fruit, abscission zones, or pollen. Molecular cloning and the modification of PG gene expression in transgenic plants have provided new insights into the physiological function of this enzyme. In addition, it is now clear that PGs are encoded by relatively large gene families in plants and that they are expressed in a wider range of developmental contexts than previously appreciated.