Peroxidase as a Component of the Signaling Pathway in Potato Cells during Ring Rot Infection

Changes in the activity of peroxidase, a component of the NADPH oxidase signaling pathway, in potato cells were studied. This activity increased sharply during ring rot pathogenesis. Two mechanisms of peroxidase activation were distinguished. One of them was the enzyme de novo synthesis; it was characteristic of the potato cultivar susceptible to the pathogen. Another mechanism characteristic of the resistant cultivar included not only the enzyme synthesis but also the activation of preexisting enzyme molecules. Bacterial infection and exopolysaccharides secreted by the pathogen induced changes in the pattern of intra- and extracellular peroxidases of the susceptible cultivar. No changes were noted in the peroxidase patterns of the resistant cultivar. A sharp activation of the extracellular peroxidase of R _f 15 occurred in the infected or exopolysaccharide-treated cells of the resistant cultivar.     

Plasma Membrane Receptors for Exopolysaccharides of the Ring Rot Causal Agent in Potato Cells

By means of differential centrifugation, microsomal fractions enriched in the plasma membrane were isolated from suspension cell cultures of two cultivars of potato (Solanum tuberosum L.) contrasting in their resistance to the causal agent of ring rot (Clavibacter michiganensis subsp. sepedonicus) (Cms). Electrophoresis of the fractions showed that they comprised a wide range of proteins from 15 to 75 kD. The protein bands were more brightly expressed in the microsomal membranes of the cells of susceptible cultivar. The proteins of 70 and 42 kD were present only in the cellular membranes of the resistant cultivar. In order to visualize the binding of exopolysaccharides (EPS) produced by Cms to the receptors of membrane fractions, a conjugate of EPS with a fluorescent marker was used. The membrane fraction isolated from the cells of the susceptible cultivar was found to be richer in receptors for EPS Cms than the membrane fraction from the resistant cultivar. It is supposed that numerous receptors for EPS present on the plasma membrane may partially account for potato susceptibility to Cms. These receptors may facilitate the binding of bacteria to the plant cells, the formation of colonies, and the development of the disease.     

Changes in Peroxidase Activity during Potato Ring Rot Infection

The effects of the ring rot causal agent Clavibacter michiganensis subsp. sepedonicus (a virulent strain 5369) on the peroxidase activity of various tissues of potato plants grown under axenic conditions were studied. Root infection enhanced peroxidase activity in all plant tissues (roots, leaves, and stems). In the resistant cv. Lugovskoi, peroxidase activity was much higher than in the susceptible cv. Luk'yanovskii. Co-culturing of the suspension cells of these potato cultivars with the bacterial pathogen also activated peroxidase in the cells of the resistant cultivar; in the cells of the susceptible cultivar, peroxidase activation was less pronounced. Treating suspension cell with exopolysaccharides secreted by the pathogen enhanced the activity of extra- and intracellular peroxidases, and the degree of this enhancement differed in the two potato cultivars.     

Necrotic Lesions as Unusual Symptoms of Ring Rot in the Potato Leaves

Test-tube plants and suspension cell cultures of two cultivars of the potato (Solanum tuberosum L.) differing in their resistance to ring rot caused by Clavibacter michiganensis subsp. sepedonicus and six strains of this bacterium were used to test the relationship between the virulence, the leaf ability to adsorb bacteria, and the symptoms of the disease. In addition to chlorosis and drying, heavy inoculation with virulent strains caused unusual symptoms, such as leaf necrotic lesions. In the resistant cultivar, the necrotic lesions were predominantly local, whereas in the susceptible cultivar, they expanded. Unlike the susceptible cultivar, suspension cells of the resistant cultivar weakly adhered bacteria of the tested strains. Bacteria entered the plants through the leaf stomata. The sorption and penetration were much more pronounced in the susceptible cultivar. It was concluded that strain virulence varies depending on the conditions of inoculation, and uncharacteristic symptoms (necrotic lesions) arise. The local necrotic lesions are considered a hypersensitive response, and exopolysaccharides of the pathogen as the factors of virulence.