Potato Cell Plasma Membrane Receptors to Ring Rot Pathogen Extracellular Polysaccharides

The interaction of extracellular polysaccharides (EPS) of the potato ring rot bacterial pathogen Clavibacter michiganensis ssp. sepedonicus (Spieck. et Kott.) Skaptason et Burkh. (Cms) with protoplasts isolated both from leaf cells of plants grown in vitro and microsomal membrane fractions obtained from cell suspension cultures of two potato (Solanum tuberosum L.) cultivars contrasted by their resistance to this pathogen was studied. The EPS intensively bind to protoplast surfaces and microsomal membranes of the susceptible cultivar but not to those of the resistant cultivar. Treatment with protease, excess of unlabelled EPS, and with dextran, did not lead to the binding of fluorochrome‐labelled EPS to protoplasts and microsomal membranes (from both cultivars). It is proposed that (a) a great number of receptors to EPS Cms are present in the plasma membranes of potato cells of susceptible cultivars, (b) these receptors contain proteinaceous sites exposed on the external side of the plasma membrane which participate in EPS binding, and (c) the plasma membranes of cells of resistant cultivars contain a small but sufficient quantity of receptors to EPS able to induce defensive responses in plants.