The Underlying Mechanisms Of Heat- Shock, Drought Priming And CO₂ Enrichment On Thermo-Tolerance In Wheat

Wheat is one of the most important staple crops in the world by virtue of its key contribution in food security. Heat stresses, especially those occurred during grain filling,cause severe grain yield losses. Importantly, heat stresses are predicted to become more and more frequent and severe due to the climate warming. Priming, a short exposure of plants to sub-lethal biotic or abiotic stress, is reported to induce the tolerance to a reoccurring severe stress. The effects and underlying mechanism of heat-shock and drought priming, and CO2 enrichment on thermo-tolerance of wheat plants were studied. Three experiments were conducted in the present study: (1) Seeds of winter wheat were firstly pretreated with heat-shock (HS, 40?, 4h), and then subjected to high temperature stress during germination or grain filling. The effects of heat shock on thermno-tolerance of later occurring heat stress were investigated. (2) Winter wheat plants were firstly subjected to a drought stress during grain filling, and their offspring were exposed to a post-anthesis high-temperature stress. (3) A climate chamber experiment was conducted to simulate CO2 enrichment (550 ?mol mol-1)and post-anthesis heat stress to explore their combined effects on growth and yield of spring wheat.1.Germinating heat-shock induced thermo-tolerance of wheat at both germination stage and grain filling periodHeat stress during seed germination decreased gennination rate, improved respiration rate and aggravated oxidative damage. Heat-shock pretreatment increased the gennination rate, weight and length of coleoptile, and promoted the degradation of storage starch and protein in seeds. In addition, antioxidant systems were activated by heat-shock pretreatment,and effectively reduced the content of malondialdehyde (MDA) and production rate of superoxide anion radical (O2-)in germinating seeds.Post-anthesis heat stress significantly depressed wheat grain yield while germinating heat-shock pretreatment alleviated this negative effect. The underlying mechanisms could be ascribed to the robust signal perception and transduction of heat stress, enhanced expression of stress related genes (including genes encoding heat shock proteins, osmotins and etc.),which further decreased the oxidative damage to membrane lipids, and improved the photosynthetic rate of flag leaf. These contributed to the less yield loss of heat-shock pretreated plants, and successfully improved tolerance to post-anthesis heat stress in wheat.2. Drought priming during grain filling improved thermo-tolerance to post-anthesis high temperature in the offspring of winter wheatDrought priming decreased the relative water content of flag leaves and thousand-kernel-weight of the harvested grains. However, drought priming improved the grain yield of their offspring under post-anthesis high-temperature stress. Comparing with the non-primed offspring, drought-primed offspring showed higher net photosynthetic rate (Pn),SPAD value, and maximal photochemical efficiency of PSII. In addition, the activities of SOD,POD in primed plants was higher than non-primed plants,resulting in a lower production rate of O2·- and lower concentration of MDA. This proved that an acquired drought-to-heat cross tolerance in offspring plants was conferred by drought priming in parent plants. This could be explained by the improved photosynthesis because of the regulated expressions of proteins involved in light reaction,Calvin cycle and photorespiration processes, and also by the enhanced anti-oxidation capacity. In addition, the more robust signal perception and transduction of heat stress, better maintenance of protein structures,up-regulation of sucrose synthesis and accumulation of heat shock proteins could also contribute to the enhanced thermo-tolerance in the offspring.3. CO2 enrichment increased grain yield, but decreased thermo-tolerance to post-anthesis high temperature of spring wheatCO2 enrichment increased canopy height,numbers of tillers and ears,resulting in a higher grain yield of spring wheat plants. Post-anthesis heat stress decreased Pn, stomata conductance (gs) and SPAD values, resulting in a significant lower grain yield with the shortened grain filling duration. In addition, thermo-tolerance was extenuated by the CO2 enrichment treatment, which could be concluded by the significant decrease of leaf area index of green leaves after high temperature stress, and Pn and SPAD values in the recovery period.The LC-MS proteomics analysis of flag leaves indicated that the processes of light reaction,TCA cycle, glycolysis, metabolism of ascorbic acid and glutathione were regulated under different treatments, which could partially explain the effect of CO2 enrichment on thermo-tolerance in wheat plants.In conclusion, heat shock priming during germination, and drought priming of the parental plants alleviated the adverse effects of heat stress on growth and yield in winter wheat while CO2 enrichment aggravated the negative effect. The physiological,transcriptional and proteomics analysis revealed that thermo-tolerance of wheat could be related to the capacities of photosynthesis and antioxidation, expression levels of stress related genes and proteins, signal transduction, synthesis and structure maintenance of proteins.