The extracellular matrix in higher plants. 4. Developmentally regulated proteoglycans and glycoproteins of the plant cell surface.

The review focuses on recent evidence that two classes of cell-surface protein, one consisting largely of proteoglycans and the other of glycoproteins, may function during plant development. One class, the arabinogalactan proteins (AGPs), includes some of the extracellular proteoglycans in plant secretions, as well as related molecules that are found at the outer face of the plasma membrane where they present an array of complex carbohydrate structures to the extracellular matrix (cell wall). Recent evidence implicates cell-surface AGPs in the control of cell proliferation and morphogenesis. For example, immunodetection methods have shown that the developmentally regulated appearance of carbohydrate epitopes in these proteoglycans correlates with the formation of anatomical patterns. Likewise, the members of a second class, the hydroxyproline-rich glycoproteins (HRGPs, or extensins) of the cell wall, are developmentally regulated and their occurrence also correlates with changes in anatomy. Recent observations suggest that HRGP-like domains are present in plasma membrane proteins that bridge the wall and cytoskeleton. The roles of oxidative cross-linking and wall protein insolubilization during development and defense responses are also discussed, with particular reference to the roles of hydrogen peroxide, oxidases, and HRGPs in the processes. The survey emphasizes the value of monoclonal antibodies for revealing molecular and developmental changes in AGPs and HRGPs at the plant cell surface.