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 recogn ise extracellular matrix glycoproteins implicated in plant-microbe int eractions has been used to study glycoprotein antigens in petioles of turnip (Brassica campestris L.). While MAC204 recognised two glycoprot eins (gp120 and gp45) with apparent M-r 120 000 and 45 000 in petiole extracts made with 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris) bu ffer containing sodium dodecyl sulfate, MAC236 recognised gp120 but no t gp45, and MAC265 gave no or only weak reactivity. Tissue dissection studies established that gp120 was predominantly associated with the v ascular bundle whereas gp45 was largely associated with the pith. This was consistent with results fi-om tissue prints probed with MAC204 an d MAC236 which also suggested a vascular localisation for gp120. Immun oelectronmicroscopy showed that MAC204 and MAC236 both labelled three- way junctions between cells of the phloem and sclerid fibres. Both gp1 20 and gp45 were shown to carry epitopes in common with known hydroxyp roline-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 b y size-exclusion and ion-exchange chromatography. Amino acid analysis revealed gp120 to be a novel glycoprotein, particularly rich in prolin e, 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 aga inst microbes is discussed.