The Study Of The Molecular Mechanism Of Salt Acclimation In Arabidopsis Thaliana

Glycophyte plant species can acquire tolerance to lethal high salinity stress by previous short-term exposure to low level,non-lethal salinity stress,a process known as salt acclimation.Salt acclimation plays an important role in improving plant salt tolerance and promoting agricultural practice.However,the molecular mechanisms underlying salt acclimation remain largely unknown.In this study,firstly an experimental system has been established to study the mechanisms of salt acclimation in Arabidopsis.In most previously reported experimental systems,seedlings were treated with low salinity followed by an immediate high salinity treatment.Using such approach,it is hard to distinguish the genes related to salt acclimation from those genes that are just simply responsive to low salinity stimulus.To solve this problem,a recovery period(under normal growth condition)is included after mild salinity pretreatment,as a part of the priming process,before exposing primed seedlings to high salinity conditions.Using this experimental system,we found that the effect of salt acclimation could be maintained even after the recovery period.This result shows that plant could imprint or memorize a history of salt exposure.Further investigations indicate that salt acclimation is induced by specific ion,and could not improve the resistance to osmotic stress or freezing.In this study,one day of priming followed by one day of recovery is applied,and is demonstrated suitable for the study on memorable salt acclimation.Next,digital gene expression profiling(DGE)is performed to analyze geme expression changes during salt acclimation,and to reveal genes related to imprinted salt acclimation.By analyzing gene expression levels at different stages(right after low salinity treatment,recovery,or after high salinity treatment,respectively),genes are divided into six different transcriptional responses groups.Among these groups,genes in Group C are likely memorable salt acclimation-related genes,which expression levels are increased by priming and are able to retain the response after recovery period.We propose that these genes may be involved in the sustainable salt acclimation.Gene ontology(GO)enrichment analysis is adopted for Group C genes,and shows that genes in the oxidation-reduction processes are the most enriched,including genes participated in ROS production and scavenging.Therefore,investigation on the members of the respiratory burst oxidase homolog(Rboh)gene family was conducted.The effect of salt acclimation was weakened in mutants of atrbohC,atrbohD,and atrbohF.Additionly,exogenous H2O2 can partially simulate a salt acclimation-like effect.These data suggested that ROS induced by low salinity is required for salt acclimation.However,further observation of ROS accumulation in Arabidopsis roots using the fluorescence probe shows that low-salinity-induced ROS is scavenged after removal of the stimulus.This suggests that ROS triggered by the 50 mM NaCl priming may act as a signaling molecule to establish salt acclimation,but ROS per se is not a memorable factor in salt acclimation.In addition,many proteins encoded by genes in Group C are predicted to be located in the endoplasmic reticulum(ER)based on GO molecular function classification.It has been shown that ER stress related unfolded protein response(UPR)and ER-associated degradation(ERAD)are involved in salt tolerance in plants.Basic-leucine zipper 17(bZIP17)and Hmg-CoA reductase degradation 3A(HRD3A)are two important genes in UPR and ERAD.Their expression levels are increased by 50 mM NaCl priming and are able to retain after recovery.Salt acclimation is less efficient in primed bzipl7 and hrd3a mutants than in wild-type.In summary,these results imply that salinity conditions increase the accumulation of unfolded and/or misfolded proteins.The expression of bZIP17 and HRD3A are increased by the treatment of low concentration of salt,and remains at stably high levels after the removal of stimulus,which improves plant tolerance to future high salinity challenges.Taken together,we established a novel experimental system to study memorable salt acclimation,and found ROS induced by mild salt acts as a signaling molecule to establish salt acclimation.Furthermore,expression of important genes in UPR and ERAD can imprint the history of salt exposure in plant,and enable more potent responses to future severe salt stress.This study provids new insights into the mechanisms underlying salt acclimation in plants.