VIEW ARTICLE

The Physiological Basis of Carboxin Sensitivity and Tolerance in Ustilago hordei. Y. Ben- Yephet, Graduate Assistant, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel; A. Dinoor(2), and Y. Henis(3). (2)(3)Associate Professor, and Professor, respectively, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel. Phytopathology 65:936-942. Accepted for publication 24 March 1975. DOI: 10.1094/Phyto-65-936.

The effect of carboxin (5,6-dihydro-2-methyl-1, 4-oxathiin-3-carboxanilide) on six strains of Ustilago hordei was studied, using two carboxin-sensitive parent strains (1 and 2), two tolerant mutants derived from them (car-1 and car-2, respectively), and two progenies obtained from the cross between the tolerant mutants (car-3 and car-4). These six strains were tolerant to 0.2, 0.2, 25, 50, 50, and 150 µg/ml of carboxin, respectively. No leakage of cell metabolites from 1 and car-1 was detected in the presence of growth-inhibiting carboxin concentrations. No active uptake of carboxin-U-³H by any of the four strains tested (1, car-1, car-3 and car-4) as well as by Saccharomyces cerevisiae could be detected. In both sensitive and tolerant strains, minimal growth-inhibiting concentrations of carboxin totally inhibited succinate oxidation and ³²P uptake, whereas glucose oxidation was inhibited only by 42-75%. Activity of succinate-DCIP reductase in cell-free extracts and in mitochondrial fractions of these strains was less affected than that of ³²P uptake or succinate oxidation, whereas succinate-PMS-DCIP reductase activity was only slightly inhibited by the fungicide. Lineweaver-Burk plot of succinate-DCIP reductase activity of strain no. 1 in presence of carboxin revealed that the inhibition was both competitive and noncompetitive. Growth-inhibiting concentrations of carboxin decreased the ratio of C₆/C₁ of ¹⁴CO₂ produced from glucose-6-¹⁴C and gluclose-1-¹⁴C, respectively, in four out of the six strains. Development of tolerance to carboxin in U. hordei was accompanied by an increase in carboxin tolerance of succinate-DCIP reductase activity, a decrease in its level, and with an increased isocitrate lyase and malate synthetase activities which were not affected by carboxin. It is concluded that the sensitivity of U. hordei to carboxin is due to inhibition in succinic dehydrogenase activity, as well as to uptake through membranes. Carboxin tolerance in U. hordei mainly depends on the increased activity of the glyoxylate pathway, a tolerant system of phosphate uptake, and only partially on the development of tolerant succinic dehydrogenase.