Effect Of Endogenous Salicylic Acid And Ethylene On Arabidopsis Response To Polyethylene Glycol Stress

Drought has influence on crop production on the world, which causes a globalenvironmental and ecological problem. Plant hormones such as salicylic acid （SA） and ethylene（ET） which are performed just using exogenous application of chemicals have beendemonstrated to play an important role in plant response to abiotic stress. However few arecarried out by reverse genetics.In order to assess the role of SA or ET in plant response todrought stress, we designed this experiment using Arabidopsis （Columbia） wild type （wt） and itsSA-altering mutants, including snc1, nahG, npr1-1, sid2, and ET-altering mutants ein2-1andein2-1/npr1-1. In a mimic condition of drought stress, plants were treated with a series ofconcentrations of polyethylene glycol （PEG）, in order to reveal the role of SA or ET in plantsresponse to drought stress.The main contents and results are as followed:Based on the morphological phenotype and the content of dry weight, tolerance with asequence from a high to a lower extent being: snc1＞wt＞npr1-1＞sid2＞nahG＞ein2-1＞ein2-1/npr1-1. Under PEG stress, the other tested genotypes had a gradual decrease in relativewater content （RWC） as the increase of PEG concentrations except snc1in which the RWCmaintained unchanged upon PEG stress, with a dose-effectiveness relationship. Theinvestigation of antioxidative enzymes and their isozymes showed that all the tested genotypesincreased in SOD, POD and CAT activity under PEG stress except the dose of9%, with themost extent being observed in snc1、ein2-1and ein2-1/npr1-1, whereas the extent being lower innahG, sid2and npr1-1than in wt. When PEG reached a concentration of9%, POD activity oftested plants continued increased，when SOD and CAT activity in most of the tested plantsdeclined to their control levels. This suggested that the applied dose of9%may be a limitingvalue for SOD activity in Arabidopsis which were growed in water. The ein2-1/npr1-1washigher than ein2-1and npr1-1in SOD activity. With the increase of PEG concentrations,reduced form of GsH、H₂O₂and MDA content increased gradually. Compared to wild type,under normal conditions, GsH and H₂O₂contents in snc1was the least, other mutants werehigher than wt whereas the higher in ein2-1, ein2-1/npr1-1, npr1-1, sid2, and nahG. Undernormal conditions, level in snc1mutant significantly increased relative to the other lines,indicating that high accumulation of SA caused oxidative stress. When stress concentration wasthe highest, MDA contents of all the tested plants were elevated greatly, with the greatest extent being occurred in the ET-related mutant, whereas the least in snc1plants. Under the stress ofPEG, the chlorophyll contents decreased gradually, with the greatest extent being observed inthe ET-related mutant, whereas the least in snc1plants, higher in nahG, sid2than wt. Thissuggested that chlorophyll was accelerated to degradate by SA. The values of chlorophyll a/bunder the stress were the highest in snc1, and the least in nahG and ein2-1. As the increase ofstress concentrations, proline and soluble sugar contents increased gradually, with a dose-effectrelationship. Under stress concentrations, proline and soluble sugar contents in snc1were lowerthan that in wild type plants, and npr1-1, nahG and sid2were higher than wild type. Prolinecontent in ein2-1, ein2-1/npr1-1was the highest, but the soluble sugar contents in ein2-1,ein2-1/npr1-1were slightly lower than the wild type.In summary, this study suggested that high accumulation of endogenous SA protectedArabidopsis plants against PEG-mediated damages, whereas deficiency of SA intensifiedArabidopsis plant sensitivity to PEG stress. Ethylene insensitive mutation also increased plantsensitivity to PEG stress. This may be associated with the antioxidative capacity.