Sensitivity of yeast cells to reactive oxygen species generated in the extracellular space

Resistance of postharvest biocontrol yeasts to oxidative stress: a possible new mechanism of action

ABSTRACT We detected the generation of the reactive oxygen species (ROS) superoxide anion ( O.(-) (2)) and hydrogen peroxide (H(2)O(2)) in apple wounds 2 immediately after wounding, and assessed the relationships between (i) timely colonization of apple wounds by biocontrol yeasts, (ii) resistance of these microorganisms to oxidative stress caused by ROS, and (iii) their antagonism against postharvest wound pathogens. We analyzed a model system consisting of two yeasts with higher (Cryptococcus laurentii LS-28) or lower (Rhodotorula glutinis LS-11) antagonistic activity against the postharvest pathogens Botrytis cinerea and Penicillium expansum. LS-28 exhibited faster and greater colonization of wounds than LS-11. In contrast to LS-28, the number of LS-11 cells dropped 1 and 2 h after application, and then increased only later. In vitro, LS-28 was more resistant to ROS-generated oxidative stress. The combined application of biocontrol yeasts and ROS-deactivating enzymes in apple wounds prevented the decrease in number of LS-11 cells mentioned above, and enhanced colonization and antagonistic activity of both biocontrol yeasts against B. cinerea and P. expansum. Polar lipids of LS-11 contained the more unsaturated and oxidizable alpha-linolenic acid, which was absent in LS-28. Resistance to oxidative stress could be a key mechanism of biocontrol yeasts antagonism against postharvest wound pathogens.

Selection and analysis of superoxide dismutase mutants of Neurospora

A survey of 12 genetically distinct, heat-sensitive mutants of Neurospora revealed three (un-1, un-3, and un-17) that are specifically deficient in the superoxide dismutase (SOD) isozymes SOD-2 (mitochondrial), SOD-3 (mitochondrial), SOD-4 (exocellular), respectively. Genetic analysis of the three mutants indicates that the enzyme deficiencies are probably the cause of the heat-sensitive phenotype. The phenotypes of the mutants are (1) no growth at the normally optimal temperature 35 degrees C and comparatively inferior growth at 15-30 degrees C; (2) inferior resistance to the oxidants paraquat or oxygen; (3) female sterility; and (4) inferior conidial viability and longevity. Paraquat or O2 inhibition is alleviated respectively by desferrioxamine-Mn (a SOD mimic) and tocopherol. Diverse antioxidants, including tocopherol, are therapeutic for the heat-sensitive and female-sterile phenotypes, and for inferior growth of wild type at stressfully high temperatures. The results support previous theories that heat stress is a form of oxyradical/oxidant stress and that antioxidant enzymes such as SOD are essential for normal growth, development, and longevity. Since the three genes may encode the three enzymes and are not closely linked to either one another or the family of antioxidant-enzyme regulatory genes Age-1, the latter apparently trans-regulate their expression.

Pharmacogenetics of cyclic guanylate, antioxidants, and antioxidant enzymes in Saccharomyces

Supplements of antioxidants, superoxide dismutase (SOD), catalase, cyclic guanylate (cGMP), and theophylline, or omission of iron and copper from the medium are therapeutic for the inferior growth and viability of yeast mutants doubly deficient in mitochondrial and exocellular SOD isozymes under oxidative stresses. Cyclic adenylate tends to be ineffective or counterproductive. Oxy-stress resistant revertants are cross-resistant to other oxy-stresses and acquire one, the other, or both isozymes. The principal conclusions are: i) a genetic defect in cGMP metabolism probably compromises regulation of the enzymes' synthesis; ii) the enzymes are only essential for growth and viability under oxidative stresses; iii) oxidative toxicity is mediated by both exo- and endocellular oxy-radicals, particularly hydroxyl radicals; and iv) the pharmacogenetic features and the mutants' phenotypes are quite similar to those of negative antioxidant enzyme regulatory mutants of the related ascomycete Neurospora.