Adaptive Strategy Of Allohexaploid Wheat(Triticum Aestivum L.)to Long-Term Salinity Stress

Most types of stresses influence crop plants at intervals.Unlike other stresses,salinity stress influences crop plants for their entire life cycle.The ultimate aim of research into salinity tolerance is the improvement of salinity tolerance in crop plants and the subsequent utilization of extensively salinized lands.Unfortunately,most investigations of crop salinity tolerance are conducted in a greenhouse under short-term stress condition within a single growth stage.Hence,an understanding of the mechanisms of crop response to long-term salinity stress?LSS?will be more valuable than that of short-term stress for achieving the improvement of crop salinity tolerance.To our knowledge,the mechanisms by which plants respond to salinity stress have been largely investigated using the short-term salinity stress system,whereas LSS conditions are rarely used to investigate salinity tolerance.In the current study,we exposed allohexaploid wheat seeds to LSS conditions across the germination and seedling stages for 30 days.To elucidate the gene expression and physiological response strategies of allohexaploid wheat to LSS,we analyzed chloroplast ultrastructure,leaf anatomy,transcriptomic profiling,concentrations of plant hormones,organic compatible solutes,and inorganic ions,comparing stressed and control plants.We also discussed that the expression partitioning of salt tolerance homeologous genes among A,B and D subgenomes.Major results included following five aspects:1.Chloroplast feature.The thylakoids in chloroplasts of stressed plants had a higher packing density than those of control plants,with more and larger starch grains in control plants than in stressed plants.Higher-density thylakoids of wheat plants subjected to LSS may produce more ATP and NADPH to fuel these salinity stress responses.2.Physiological response.Under LSS,the concentrations of amino acids were particularly enhanced in the leaves,but not in the roots.In contrast,under LSS,increased carbohydrate concentration was more apparent in the roots than in the leaves.Alanine,proline,maltose and sucrose were the dominant compatible solutes in allohexaploid wheat leaves,whereas alanine,fructose,glucose and sucrose were the dominant compatible solutes in allohexaploid wheat roots.Taken together,amino acids and carbohydrates made similar contributions to increasing the osmotic potential of the leaf cytoplasm,whereas carbohydrates played more important roles in osmotic adjustment of the root cytoplasm than did amino acids.3.Transcriptomic response.RNAseq data showed that 69 late embryogenesis abundant?LEA?genes and 39 dehydrin genes were significantly up-regulated in either roots or leaves.LEA and dehydrin proteins play important roles in dehydration tolerance and the prevention of protein aggregation under salinity stress or drought stress.Another important function of LEA and dehydrin proteins is their interaction with carbohydrates to form intracellular glasses that slow molecular mobility and reduce metabolic rate.The intracellular glasses may also limit the movement of Na⁺and Cl^-in the cytoplasm.Co-expression of a very high number of LEA and dehydrin genes should result in an increased accumulation of LEA and dehydrin proteins,which should,in turn,cause crucial effects in terms of dehydration tolerance and the prevention of protein aggregation in wheat plants under LSS.A combination of increasing carbohydrate concentrations and increased expression of LEA and dehydrin genes under LSS conditions may also result in a marked accumulation of intracellular glasses in wheat cytoplasm,which will slow the general metabolic processes and alleviate ion toxicity.4.Plant hormone response.LSS did not significantly affect the concentrations of trans-zeatin and dihydrozeatin in either leaves or roots.LSS increased ABA concentration in both leaves and the roots.Accordingly,we observed that nine ABF?ABA response element binding factor?genes,representing the final function of the ABA signaling system,were greatly up-regulated in wheat leaves under LSS.Up-regulation of ABF genes can not only mediate stomatal closure and limit growth but also up-regulate the expression of the salinity-responsive genes that contain ABA-responsive elements?ABRE?in their promoter regions.LSS also induced a reduction in JA and GA₃ concentrations and the up-regulation of three DELLA genes?suppressing gene of GA pathway?and 10 JAZ genes?suppressing gene of JA pathway?in wheat leaves.5.The expression partitioning of salt tolerance homeologous genes among A,B and D subgenomes.We further distinguished the expression status of the A,B,and D homeologs of any gene triad of allohexaploid wheat,and also surveyed the effects of LSS on homeolog expression bias for salinity-tolerant triads.We found that most of salinity-tolerant triads showed balanced expression?with the three homeologs having similar expression levels?under LSS,which may have contributed to the development of the high stress tolerance of allohexaploid wheat because many adaptive mechanisms of plants to stress conditions rely on the dosage effects.Taken together,energy partitioning between general metabolism maintenance and stress response may be crucial adaptive strategy of allohexaploid wheat to LSS.According to these plant hormone expression data,changes in carbohydrate concentrations,and above LEA and dehydrin gene expression results,we propose that,under LSS,wheat might shift energy partitioning from general maintenance to stress response through the following processes:?1?inducing the ABA pathways-stomatal closure and growth limitation;?2?suppressing the pathways controlled by GAs and JA in the leaf;?3?the large-scale accumulation of intracellular glasses produced by a mixture of carbohydrates and the LEA and dehydrin proteins.From an evolutionary perspective,many of the duplicate salinity-tolerant genes generated by hexaploidization may have been retained in extant allohexaploid wheat and were co-expressed during response to LSS to show dosage effects.