CELLULAR-RESISTANCE IN RICE TO CERCOSPORIN, A TOXIN OF CERCOSPORA

Four rice cultivars differing in resistance to Cercospora oryzae were examined for resistance to purified cercosporin, the red toxin secreted by the fungal mycelium. The cultivars were red rice (most resistant to pathovars of C oryzae), Lemont and Leah (intermediate resistance), and Labelle (most susceptible). Resistance to the toxin was assessed in seedlings (inhibition of growth), leaves (chlorosis and necrosis), callus (inhibition of growth and hydrolysis of fluorescein diacetate), and cell suspensions (ion leakage). Toxicity of cercosporin depended on illumination. Labelle proved to be the most sensitive cultivar in each test, whereas red rice showed extreme resistance to the toxin and grew in the presence of cercosporin concentrations that were completely toxic to Labelle. Red rice cells contained about one-tenth as much cercosporin as cells of Labelle; this suggests that resistant cells have a mechanism for excluding, exporting, or destroying the toxin. Treating cell suspensions of red rice with norflurazon abolished the cercosporin resistance in this cultivar, and thus carotenoids are implicated in the resistance mechanism. These results show that plants can display cellular resistance to illuminated cercosporin and suggest a method of screening cells of other crop plants for variants resistant to the toxin.