A novel proline-rich glycoprotein associated with the extracellular matrix of vascular bundles of Brassica petioles

A panel of monoclonal antibodies (MAC204, MAC236, MAC265) which recognise extracellular matrix glycoproteins implicated in plant-microbe interactions has been used to study glycoprotein antigens in petioles of turnip (Brassica campestris L.). While MAC204 recognised two glycoproteins (gp 120 and gp45) with apparent Mr 120000 and 45000 in petiole extracts made with 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris) buffer containing sodium dodecyl sulfate, MAC236 recognised gp120 but not gp45, and MAC265 gave no or only weak reactivity. Tissue dissection studies established that gp120 was predominantly associated with the vascular bundle whereas gp45 was largely associated with the pith. This was consistent with results from tissue prints probed with MAC204 and MAC236 which also suggested a vascular localisation for gp120. Immunoelectronmicroscopy showed that MAC204 and MAC236 both labelled three-way junctions between cells of the phloem and sclerid fibres. Both gp120 and gp45 were shown to carry epitopes in common with known hy'droxyproline-rich glycoproteins. Unlike gp45, gp120 could be extracted from petioles with Tris buffer alone and then isolated from this extract by trichloroacetic acid treatment (which left gp120 soluble), followed by size-exclusion and ion-exchange chromatography. Amino acid analysis revealed gp120 to be a novel glycoprotein, particularly rich in proline, lysine, valine and threonine but relatively poor in hydroxyproline. The most abundant sugars were arabinose and galactose. The potential role of this very basic cell surface glycoprotein in plant defence against microbes is discussed.