Arabinogalactan-Proteins in Pollen Tube Growth

Abstract

Arabinogalactan-proteins (AGPs) were associated with reproductive tissues in the early days of their discovery (Nothnagel 1997). In particular, the stigmas and styles of flowering plants were abundant sources of AGPs, as revealed by Yariv phenylglycoside staining [(β-D-glucosyl) Yariv phenylglycoside [(β-D-G1C)₃]], a red-colored reagent thought to bind AGPs (Yariv et al 1962). Several species were examined in detail using the (β-D-Glc)₃ to precipitate the stigma and stylar AGPs (Clarke et al 1979, Gleeson and Clarke 1979 1980, Fincher et al 1983, Miki-Hirosige et al 1987). Only recently has DNA sequencing provided us with knowledge of the protein component of some of these large proteoglycans, and the first AGPs sequenced were from stylar transmitting tract tissues (Du et al 1994, Mau et al 1995). The carbohydrate branches of AGPs consist mainly of arabinose and galactose, and the protein core is typically rich in Hyp/Pro, Ala and Ser, but the entire molecular structure of any AGP is unknown (Nothnagel 1997). The exciting recent discovery that some plasma membrane AGPs have glycosylphosphatidylinositol anchors has suggested the possibility of their involvement in signaling cascades at the cell surface (Youl et al 1998, Schultz et al 1998, Svetek et al 1999). Polyclonal and monoclonal antibodies to AGPs have been produced with polysaccharide epitopes of mostly unknown composition (Pennell et al 1991, Knox 1992). There is good circumstantial evidence that they bind AGPs. These studies have confirmed that AGPs are abundant in stigmas, styles, and pollen tubes of many species. The antibodies have revealed the diversity of expression patterns for AGPs in development and have ignited interest in their roles in plant biology.