Research On The Resistance And Response To Oxidative Stress Of Tall Fescue-endophyte Symbiont

The endophyte-infected and endophyte-free grasses of tall fescue were used for materials in this study. The endophyte-infected grasses were new symbionts by transferring those endophytes which were from other grass to tall fescue. After stability testing, drought stress and heat stress were carried out to study the differences between stress resistance and antioxidant defense responses and to obtaine the high stress resistance symbiotic plants eventually. These works provided a theoretical basis and materials for breeding new cultivars of tall fescue with a variety of resistance characteristics.Although the growth conditions and the genotype of the host plants have changed, these two kinds of endophytic fungi were already stably colonized in the tall fescue plants respectively, formed new tall fescue-endophyte symbiotic plants. The tall fescue and endophyte were established a good symbiotic relationship with each other.Compared with nonsymbiotic plants, tall fescue-endophyte symbiotic plants leaf water deficit was less, had a stronger regulation of membrane permeability and drought recovery ability. The endophyte infection not only improved the drought tolerance of tall fescue plants but also the heat stress resistance. Heat tolerance was evaluated based on the predicted LT50using the mathematical model in this pater, associated with plant growth and physiological responses to heat stress with high credibility. Heat tolerance of materials were showed as follows:the symbiotic M-t(LT50=51.82℃)> symbiotic M-s(LT50=51.29℃)> nonsymbiotic M(LT50=50.93℃).In the presence of endophyte, symbiotic plants significantly decreased the rate of superoxide anion and hydrogen peroxide generation and the content of MDA under drought and heat stress. Hence, symbiotic plants can maintain membrance stability and permeability. Endophyte infection also enhanced the root vitality of symbiotic plants. Therefore, the symbiotic plants had strong resistance to drought and heat stress. A direct relationship can be assumed between efficient antioxidant machinery and a delay in the onset of MDA accumulation. In symbiotic plants, endophyte caused a substantial reduction in the stress-increased content of O2-, and in this way lowered the level of oxidative stress in plant cells. This effect, even partial, is probably a result of the influence of endophyte on SOD activity. Endophyte caused less of a decrease in this enzyme activity, and therefore the dismutation of O2-into H2O2was more effective. However, stress-induced H2O2accumulation was lower in symbiotic plants. This effect is due to endophyte is able to affect the other H2O2scavenging enzyme activities-APX and CAT. CAT and APX activities increased in leaves of symbiotic plants with respect to nonsymbiotic plants. This response indicates a good H2O2scavenging ability in symbiotic plants, and the observed correlation between the two enzyme activities strongly suggests coordinated action of APX and CAT.Endophyte can enhance the drought resistance of the symbiotic tall fescue plants, only at the growth recovery period after the severe water stress was released. The advantage was not obvious under normal moisture conditions and mild of water stress conditions. Tall fescue plants themselves have a certain short time of heat tolerance, whereas only under the relatively longer period of heat stress, the advantage of tall fescue-endophyte symbiotic plants can be shown.