Study On The Mechanism Of Exogenous L-histidine Regulating Salt Tolerance In Maize

Soil salinization in China disrupts the soil biological environment and chemical structure,resulting in reduced soil quality and reduced crop production,causing harm to the eco-environment and human health,and severely hindering national economic development.Amino acid can alleviate the damage caused by salt stress on maize and improve the salt tolerance of maize.In this study,we screened 20 amino acids and found that histidine could improve the growth of maize under salt stress,and then explored the mechanisms of histidine regulation of maize salt stress tolerance by related physicochemical indexes and transcriptomic techniques.1.Effect of histidine on maize root growth under salt sress.In plant hydroponic experiments,we found that maize roots under salt stress were more sensitive to amino acids than plant height when 20 amino acids were applied to the roots.By comparing the total root length,we found that the root system of maize was more sensitive to histidine under salt stress.By comparing the total root length,root surface area,root projected area,root volume,root tip number and root bifurcation number among the six concentrations of histidine（0.001μM,0.01μM,0.1μM,1μM,10μM,100μM）,the best result was achieved at the concentration of 0.1μM.2.Effect of L-histidine on physicochemical properties of maize under salt stress.0.1μM histidine could significantly increase the activities of antioxidant enzymes（The activities of SOD,POD and CAT were improved by 21.92%,12.84%,and59%,respectively）in maize roots under salt stress,significantly increase the content of osmoregulation substances（The contents of proline,soluble sugar and soluble protein were increased by 26.77%,24.69%and 19.12%）,significantly reduce the accumulation of ROS（The contents of O₂.^-and H₂O₂were decreased by 36.32%and42.7%,respectively）,and alleviate the damage caused by salt stress on roots.Histidine treatment of maize roots also significantly increased the activity of nitrogen metabolizing enzymes（The activities of NR,GS and GOGAT were increased by53.2%,9.26%,and 176.52%,respectively）and the use efficiency of nitrate and ammonium nitrogen（The contents of NO₃^-and NH₄⁺were decreased by 69.54%and34.48%,respectively）.3.Transcriptomic analysis revealed that the number of differentially up-regulated genes was significantly increased in the roots treated with histidine for 12 h and 24 h under salt stress compared with the control roots,and the differentially expressed genes were mainly involved in plant hormone signal transduction pathways,glycolysis pathways,and nitrogen metabolism pathways.In addition,the differentially expressed transcription factors were mainly ERF and WRKY families when treated with histidine for 24 hours under salt stress.GRMZM2G102583 and GRMZM2G120320 involved in negative regulation of gibberellin signaling in WRKY family may jointly regulate the expression of downstream cytokinin receptor（LOC732762 and LOC732835）and transporter（Zm SFP2,Zm SFP6 and LOC100281461）.4.Effect of histidine on maize photosynthesis.Pot experiment of plants showed that when the concentration of histidine sprayed on the leaves was 0.5 m M,the photosynthetic promotion effect of maize under salt stress was the best.0.5 m M histidine significantly improved the photosynthetic performance（P_N,g_s,E and Chl a/Chl b were increased by 74.09%,92.14%,77.74%and 9.5%,respectively）,improved the fluorescence parameters（F_v/F_m,ΔF/F’_mand q _Pwere increased by13.8%,39.1%and 23.2%,respectively.NPQ was decreased by 11.4%）,significantly increased the activity of photosynthetic enzymes（The activities of PEPC,NADP-ME,PPDK and Rubisco were increased by 65%,41.7%,81.8%,and 70%,respectively）and the relative expression of photosynthetic genes（Zm PEPC,Zm NADP-ME,Zm PPDK and Zm RCA were increased by 135%,77%,291%,and170%,respectively）,increased the length of vascular bundles on the cross section of leaves,and played a protective role in vascular bundles of maize under salt stress.In addition,histidine also changed the antioxidant system of maize leaves（The contents of O₂·^-,H₂O₂and MDA were decreased by 41.67%,42.19%and 27.15%,respectively.The activities of POD and CAT were improved 14.84%and 67.19%,respectively）and significantly increased the content of osmoregulation substances in leaves（The proline,soluble sugar and soluble protein were increased by 28.16%,23.89%and 16.61%,respectively）under salt stress.The study showed that root application of 0.1μM and spraying of 0.5 m M L-histidine had a certain effect on the growth of maize under salt stress,which could regulate the salt tolerance of maize through photosynthesis,enzyme activity and the expression of related genes.