When considering the first steps in plant glycobiology, summarised in this review, it may be wise not to forget the suggestion of a French philosopher about the period that precedes attempts at addressing a new issue:
L'important est plus de poser les vraies questions que d'apporter les vraies réponses.Levi-Strauss
Laboratories interested in plant glycobiology are currently asking some very basic questions such as:
What is a plant glycoprotein glycan made of?
How is a peptide glycosylated?
How is an N-linked glycan processed in a plant cell?
Why are some plant proteins N-glycosylated?
The present review is an attempt to summarise the way these questions are being addressed and the preliminary answers that have already been obtained.
Structures of N-linked glycans
Plant glycoproteins have oligosaccharide sidechains attached to their protein backbone via an N-linkage (amide nitrogen of asparagine) or O-linkage (hydroxyl group of serine, threonine or hydroxyproline). The N-linked oligosaccharides, or glycans, found on plant glycoproteins fall into two general categories already described for other eukaryotes: highmannose and complex glycans. The high-mannose type oligosaccharides have the general structure Man5–9(GlcNAC)2. High-mannose type oligosaccharides are found associated with immature and mature glycoproteins of higher plants (Faye et al., 1986). High-mannose glycans, first described for soybean lectin (Lis & Sharon, 1978), were observed in many other mature, vacuolar or extracellular plant lectins and enzymes (Paul & Stigbrand, 1970; Ericson & Chrispeels, 1973; Basha & Beevers, 1976; Sturm et al., 1987a).