Mechanisms Of Salt Tolerance Improved By Epichloe Endophyte In Wild Barley

Under the scenarios of climatic variation, soil salinization is becoming to be one of the major reasons which constraint the crop yeild and agricultural productivity. Therefore, studies on the salt tolerance of plant are important for selecting and breeding for salt-torlerance species. On the other hand, endophytes confer stress tolerance abilities to host species, and play a critical role in the tolerance to high-stress environments, such as those where drought, salt, heavy metal and so on, may be present. By using wild barley (Hordeum brevisubulatum) as a sample plant, we built up two communities:one with asexual Epichloe endophyte infected (E+) and the other without (E-), attempting to identify the effects of endophyte on host-plant’s salt resistence. By compared the seed germination, plant growth, physiological parameters, nutrient absorption and antioxidase activities between plant with and without endophyte under different NaCl concentration treatments, we explored the growth and physiological mechanism of endophyte on salt-tolerance of wild barley. In addition, the effects of water or exogenous phytohormone on salt tolerance of H. brevisubulatum-endophyte symbiont were also observed. The main subjective of this study was to figure out the role of endophyte on the salt tolerance mechanism of the host grass, and the adaptive strategy formed by grass-endophyte symbiont under long time salt stress environment. Results showed as follows:1. Using TP method, Seeds of E+ and E- H. brevisubulatum were subjected to different concentrations of NaCl (0,50,100,200 and 300 mM), and PEG treatments which resulted in same osmotic potential as corresponding NaCl treatments. Our results indicated that, despited the endophyte infection, the germination rate, germination index, shoot length and root length of H. brevisubulatum decreased with the increasing of osmotic potential. Endophyte increased the germination rate, germination index, shoot length and root length of H. brevisubulatum under salt and PEG stress. However, under the highest NaCl concentration (300 mM), no significant differences were found between E+and E-plant on germination rate and shoot length. In addition, when compared to the corresponding PEG treatment, the germination rate and germination index of H. brevisubulatum in the NaCl treatment were significantly higher (P< 0.05). However, the shoot length and root length under high salt concentrations (200 and 300 mM) were lower than the corresponding PEG treatments. So the salt ion can affect seed germination positively, but was poison for seedling growth.2. In the pot experiment, the plant height, tiller number, and biomass of H. brevisubulatum declined with the increase of NaCl concentration (0、150、300、450 and 600 mM). With the time of salt stress expansion, E+ plant had higher growth rate and tiller number than E- plant (P< 0.05). Under the higher salt concentration (such as 600 mM), E+ H. brevisubulatum could support regular growth, while E-plants turned to yellow and even partly were killed. Endophyte significantly increased the photosynthetic rate, transpiration rate, intercellular CO2 concentration and stomatal conductance of H. brevisubulatum under salt stress. Additionally, the relative water content, chlorophyll content, carotenoid content and proline content of H. brevisubulatum under NaCl stress were significantly higher when infected by endophyte. Regarding to high concentration of NaCl (450 and 600 mM), proline content of E+ plants increased first and then maintained higher values, while for E-plants, the proline content increased first and went down later. So the endophyte could help host grass for growth and osmotic adjustment, and decreasing the negtive effects caused by salt stress.3. After the same NaCl treatment, the activities of SOD, POD and CAT in wild barley increased gradually with the increment of NaCl concentration, as well as the contents of H2O2 and MDA. The activities of SOD, POD and CAT of E+ plants were significantly higher than that of E-plants, and the contents of H2O2 and MDA were significantly lower than that of E- plants under salt stress(P< 0.05). Under higher salt stress (300,450 and 600 mM), the activities of SOD and POD of E+ plants went up quickly then maintain high levels, while the CAT activity, H2O2 and MDA contents showed parabolic patterns, In contrast, for the E-plants, the activities of SOD, POD and CAT increasing slowly, and H2O2 and MDA contents rised quickly then maintain high levels. In addition, under 150 mM salt stress, not like E+ plants, H2O2 contents and POD activity of E- plants increased gradually, and the difference of H2O2 contents and POD activity between E+ and E- plants became significant as the stress continue (in day 12 and 16). So the endophyte alleviated the unbalance of ROS in wild barley after salt stress.4.18 days after treatment, salt stress induced alternation on nutrient absorption and transportation of H. brevisubulatum, for example, as the NaCl concentration increased, C, P and K+contents increased, while contents of N and Na+ declined. Under higher salt stress (300,450 and 600 mM), the N, P and K+ contents were higher while the Na+ content was lower in E+ plants than in E- plants. The existence of endophyte had a trend to lower the ratios of C:N, C:P, N:P and Na+:K+. So endophyte increase the growth of wild barley by adjusting the nutrient stoichiometry under salt stress. Our findings indicated that endophyte could improve the salt resistence of H. brevisubulatum by promoting nutrient absorption and adjusting the ionic balance.5. After compared the performance of H. brevisubulatum under low NaCl concentration (100 mM) in dry and waterlogging conditions, we found that endophyte significantly increased the plant height, tiller numbers, blade width, root activity, chlorophyll and proline contents of H. brevisubulatum, and decreased leaf wilt, electrolyte leakage and MDA under both dry and waterlogging conditions. The low NaCl concentration improved the growth of H. brevisubulatum under dry and waterlogging treatments. Under dry condition, the heights of E+ plants induced by NaCl were higher, however, for E-plants, no significant differences were found. For the other parameters, NaCl did not cause significant difference.6. NaCl treatment was conducted after the addition of exogenous phytohormone. The results showed that under salt stress, addition of exogenous phytohormone changed the growth and physiological and biochemical properties of H. brevisubulatum:ABA significantly increased the shoot/root biomass, POD and SOD activities, and lower H2O2 content; GA3 only increased root biomass and decreased H2O2 content; IAA significantly declined shoot/root biomass as well as POD activity. In the CK, E+ plants had higher H2O2 content and POD activity than E-plants after added ABA, GA3 and IAA. However, under salt stress, the increasing ratios of root biomass, SOD and POD contents and the decreasing ratios of H2O2 induced by ABA were higher than E-plants, which indicated that ABA improved the adjustion of endophyte on salt resistence. Nonetheless, for GA3 and IAA, there was no significant effect on E+and E-plants.