Mitigative Effect Of Cold Priming And Salicylic Acid On Low Temperature Stress In Wheat And Its Underlying Physiology Mechanisms

Freezing stress in winter and spring is an important limiting factor for high yield and stable yield of winter wheat in the mid and lower reaches of the Yellow,Huai,and Yangtze Rivers.Priming,defined as a temporally limited experience to an environmental stimulus(biotic stress,abiotic stress or chemical agent),can enhance the plant’s level of resistance to a future environmental stress.The effects of salicylic acid and cold priming on freezing tolerance in wheat cultivar Yangmai 16 and its underlying physiology mechanisms were studied under hydroponics,pot,and field conditions in this study.Four experiments were conducted in this study:(1)Firstly,the setting conditions of cold priming that could effectively induce freezing tolerance in wheat plants were screened,and then the dynamic changes of SA metabolism in wheat leaves during cold priming were analyzed.Finally,the role of SA in cold priming-induced freezing tolerance was explored by using SA synthesis inhibitor.(2)Wheat plants were firstly primed with SA or cold temperature,and then grown without any treatment for 8 days,and subsequently subjected to a freezing stress.The effects of cold priming and S A on freezing tolerance in wheat plants and its underlying physiology mechanisms were studied.(3)A field air temperature control system(FATC)was used to simulate spring low temperature to study the effects of SA on low temperature tolerance in wheat plants and its underlying physiology mechanisms under field conditions.(4)Firstly,the changes of hydrogen peroxide(H2O2)and abscisic acid(ABA)levels in wheat leaves were analyzed after S A treatment,and the sources of endogenous H2O2 were investigate by using inhibitors.Finally,the roles of H2O2 and ABA in SA-induced freezing tolerance were explored by using inhibitor or scavenger.The main results are as follows.1.SA mediated cold priming-induced freezing tolerance in wheatCold priming could effectively induce wheat freezing tolerance.Our results suggested that cold priming-induced freezing tolerance was independent on the C-repeat binding factor(CBF)-Cold regulated(COR)pathway.Cold priming induced SA accumulation in wheat leaves through the phenylalanine ammonia-lyase(PAL)enzymatic pathway.L-α-aminooxyβ-phenylpropionic acid(AOPP,a specific inhibitor of PAL)pretreatment abolished cold priming-induced SA accumulation and attenuated the positive effects of cold priming on antioxidant capacity,cold-responsive gene expression as well as freezing tolerance,and application of SA could counteract the negative effects of AOPP.2.Cold priming and SA had similar physiology mechanisms in inducing wheat freezing tolerancePriming wheat plants with SA and cold temperature effectively enhanced tolerance to the later freezing stress as exemplified by higher water potential and photosystem activity,and lower MDA content,electrolyte leakage and cell death under freezing stress,and higher biomass accumulation during the recovery period as compared with non-primed plants.Further studies showed that both cold priming and SA could alleviate freezing-induced oxidative damage by increasing the antioxidant capacity of wheat leaves,and could alleviate freezing-induced dehydration injury by coordinating carbon and nitrogen metabolism to promote the accumulation of soluble sugar and free proline in wheat leaves.In addition,cold priming and SA could also up-regulate the expression of genes encoding cyanide-resistant respiratory terminal oxidase,antifreeze protein and heat shock protein in wheat leaves under freezing stress.3.Alleviation of field low temperature stress in wheat by exogenous application of SALow temperature stress in spring significantly inhibited plant growth,prolonged growth period and reduced grain yield.SA treated plants showed higher photochemical efficiency,osmo-protectants content and antioxidant capacity under low temperature stress,higher biomass at the recovery stage and higher grain yield at the maturity stage as compared with non-SA treated plants.In addition,pretreatment with SA effectively alleviated low temperature-induced prolonging of growth period of wheat plants.4.H2O2 and ABA mediated SA-induced freezing tolerance in wheat plantsExogenous SA induced the accumulation of endogenous H2O2 and ABA.The pharmacological study indicated that SA first induced H2O2 production in the apoplast by activating cell wall peroxidase(Prx).The increased H2O2 triggers ABA production,which in turn give rise to a further accumulation of H2O2 by activating NADPH oxidase.These results suggested that H2O2 and ABA may form a positive feedback loop to response SA signal.Pretreatment with H2O2 scavenger or ABA synthesis inhibitor significantly inhibited SAinduced increase of antioxidant enzyme activity and cold responsive gene expression level as well as freezing tolerance.In conclusion,endogenous SA was involved in cold priming-induced freezing tolerance in wheat plants.Exogenous SA and cold priming had similar physiological mechanisms in improving freezing tolerance.Both cold priming and SA could improve the antioxidant capacity,osmotic adjustment capacity and cold response gene expression of wheat plants under freezing stress.Exogenous SA treatment could alleviate the adverse effects of spring low temperature on the growth,development and yield of wheat plants.In addition,the interaction of SA,H2O2 and ABA signals might play an important role in acquired freezing tolerance in wheat plants.