Effect Of Active Oxygen Species On Mycelial Growth And Antioxidative Systems Of Magnaporhe Grisea

Oxidative burst plays an important role in plant defense mechanisms. As active oxygen species (AOS) are toxic to microorganisms, the antioxidant enzymes of animal pathogens were viewed as one of the virulence factors. The objectives of this study were to explore the effect of active oxygen species on the mycelial growth and the antioxidant enzyme activities of Magnaphorhe grisea exposed to exogenous AOS, and to explore role of antioxidant enzymes of the fungus against the oxidative stress.In the liquid culture of Magnaporthe grisea exposed to different concentrations of H202, the mycelial growth was significantly inhibited at 6 mmol/L IrkOi, and completely inhibited at 10 mmol/L I-^Oa (Fig 1.1). As a generator of superoxde anion(CH), methyl viologen at 600 umol/L and 800 umol/L markedly retarded the mycelial growth, and completely inhibited the growth at 1000 umol/L (Figl.2). The hydroxyl radical (OH) was derived from system of Fe2+ and H2O2, based on the Fenton reaction. The mycelia were treated with different concentration compositions of H2O2 and Fe2+. The inhibition of mycelia exposed to the composition of H202 and Fe2+ was more significant than the treatment with H2O2 alone, and the inhibition was enhanced with increased concentration of Fe "*". It was suggested that OH strongly restraint the mycelial growth (Fig 1.3).After the mycelia of M grisea was exposed to exogenous H2O2 for 2 h, the activities of catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) of the fungus were higher than the contrasts, but the activities of superoxide dismutase (SOD) was not affected by exogenous H202- The effects of exogenous H202 on the activities of peroxidase (POD) and glutathione peroxidase (GPX) could not be explained based on the recorded data (Fig 2.1-Fig 2.6). The fungal cultures were exposed to H2O2 at 4 mmol/L for 10 h to examine the activity time-courses of theantioxidant enzymes. The date showed that activities of SOD and CAT in the fungus were significantly higher than the contrasts after 2 h treatment, and decreased later. The activities of APX and GR in the fungus were markedly higher than the contrasts after 6 h treatment, and then decreased. The GPX activities in the fungus were higher than the contrast after 8 h treatment, and then declined. The data, however, did not sustain effect of HiOi on activity dynamics of POD (Fig 2.7-Fig 2.12). These data showed that at lower concentration of exogenous HiO-. and shorter time, activities of antioxidant enzymes in the fungus increased with concentrations and time, suggesting that F^Oa was an inductive signal of antioxidant enzymes of M. grisea. Induction of antioxidant enzymes by HiO? may be involved in the protection of M grisea against AOS. The concentrations of F^Oj and the treated time could affect activities of the antioxidant enzymes in the fungus.The activities of antioxidant enzymes in mycelia of M. grisea were monitored after the fungal cultures were exposed to different concentrations of methyl viologen for 2 h. At low concentrations of methyl viologen (200 umol/L), activities of CAT, APX, POD and GPX were significantly higher than those in the contrasts. When treated at 600 umol/L methyl viologen, the fungus showed was markedly higheractivity of GR than the contrasts (Fig3.2-3.6). The results indicated that 02 also was inductive signal of antioxidant enzymes of M griease, and could activate the synthesis of the antioxidant enzymes in the fungus at the lower concentrations, but inhibit their activities at the higher concentrations. And the sensitivities of the antioxidant enzymesto O2 were different depending on the enzyme species.The fungal culture was treated with 600 umol/L methyl viologen for 10 h to monitor activities of the antioxidant enzymes in M. griease. The data showed that activity of GPX in the fungus reached its maximum after 2 h treatment, SOD, CAT, POD and GR reached their maximal activities after 4 h treatment; and activity of APX reached its peak after 8 h treatment (Fig 3.7-3.12). The resul...