Toughing It Out--Disease-Resistant Potato Mutants Have Enhanced Tuber Skin Defenses.

Common scab, a globally important potato disease, is caused by infection of tubers with pathogenic Streptomyces spp. Previously, disease-resistant potato somaclones were obtained through cell selections against the pathogen's toxin, known to be essential for disease. Further testing revealed that these clones had broad-spectrum resistance to diverse tuber-invading pathogens, and that resistance was restricted to tuber tissues. The mechanism of enhanced disease resistance was not known. Tuber periderm tissues from disease-resistant clones and their susceptible parent were examined histologically following challenge with the pathogen and its purified toxin. Relative expression of genes associated with tuber suberin biosynthesis and innate defense pathways within these tissues were also examined. The disease-resistant somaclones reacted to both pathogen and toxin by producing more phellem cell layers in the tuber periderm, and accumulating greater suberin polyphenols in these tissues. Furthermore, they had greater expression of genes associated with suberin biosynthesis. In contrast, signaling genes associated with innate defense responses were not differentially expressed between resistant and susceptible clones. The resistance phenotype is due to induction of increased periderm cell layers and suberization of the tuber periderm preventing infection. The somaclones provide a valuable resource for further examination of suberization responses and its genetic control.