Screening and antioxidant characteristics of Ustilago esculenta heat resistant mutants
|Keywords||Ustilago esculenta P. Henn. Irradiation induced mutation Thermo-tolerantmutant Oxidative stress Antioxidant response|
The culm-swelling of Zizania latifolia is inhibited by the high temperatures in summer season when fresh vegetables are few in variety and the swollen culm is welcomed with much higher prices than in other seasons. Since there is no known germplasm capable of culm-swelling in hot summer days, breeding for such cultivars is therefore of practical importance. Studies by others indicated that the thermo-sensitivity of Ustilago esculenta, which infects the plant and is responsible for the culm-swelling, can be a major factor prohibiting culm-swelling under high temperatures in summer. Introduction of thermotolerant U. esculenta into Z. latifolia provides a viable route for creating new cultivars capable of culm-swelling in hot summer days.In the present study, we obtained a series of Ustilago esculenta mutants, tolerant of40℃, from M-T strain irradiated by γ-rays from a60Co source. The nitrogen requirement and, the oxidative stress and antioxidant characteristics after heat stress of the mutants were studied. Two strains of the smut fungus, M-T strain from the non-sporulating culm and T strain from the sporulating culm of Zizania latifolia, were used as the reference in the study. Major findings are as follows,1. The U. esculenta was found to be extremely tolerant against ionization irradiation; it can survive under10kGy γ-irradiation from a60Co source.2. Twenty mutants, tolerant of40℃, were obtained from M-T strain irradiated by5kGy or10kGy of γ-rays from a60Co source. These thermotolerant mutants were named as5K1～10or10K1～10, respectively.3. Non-thermotolerant strains (reference strains) of U. esculenta were unable to propagate under32℃and above. Among the20thermotolerant strains, the growth rates of5K7and5K.8were, respectively, the highest and the lowest. It took～17.5hrs of5K7,～30.1hrs of5K.8,～62hrs of M-T and38.5hrs of T strain to reach the mid-log phase. All thermotolerant strains exhibited the phenotype of unable to utilize nitrate as nitrogen source, but were ready to use organic nitrogen source, such as asparagine.4. After heat treatment (40℃for1hr), the levels of reactive oxygen species (ROS), malondialdehyde (MDA) and protein carbonylation were increased substantially in the reference strains. In contrast, ROS level of the thermotolerant strain was only50～70%that of reference before heat stress and, there were only small changes (-10%) in levels of MDA and protein carbonylation in thermotolerant strain subjecting to heat treatment. It was evident that the oxidative stress of thermotolerant strain was significantly lower than the reference strains following heat treatment.5. Compared with reference strains, thermotolerant strains possessed smaller activities of glutathione peroxidase (GPx) and glutathione-S-transferase (GST) and similar levels of superoxide dismutase (SOD) activity, but its catalase (CAT) activity was about3-7-fold of the reference and its GSH content was also higher.6. After2mM MD treatment for1hr. activity of CAT and content of GSH drastically declined in all strains.These results indicated that high levels of CAT activity and GSH are likely to be the major components of antioxidant system that contribute to the thermotolerance of thermotolerant strains of U. esculenta.