Research On The Adaptation Mechanism Of Sugar Beet In Response To Salt Stress And The Regulation Of Exogenous Allantoin

Salt stress seriously affects crop growth and is one of the main abiotic stresses that limit crop yields.Studies on the adaptive mechanisms of crops in response to salt stress and improving salt tolerance have been widely concerned.Sugar beet is an important sugar crop in the world and has a strong adaptability to salt stress.In order to explore the adaptive mechanism of sugar beet to salt stress,this study,through transcriptome and metabolomics analysis,locked key genes and metabolites of the important metabolic pathway of sugar beet under salt stress,and explored the key metabolites to improve the salt tolerance of sugar beet.This study provides new insights for elucidating the adaptive mechanism of sugar beet to salt stress,and provides a theoretical basis for the application of exogenous allantoin to improve the salt tolerance of sugar beet,and is of great significance for the development and utilization of saline land and promoting the su stainable development of sugar beet industry.In this study,hydroponic and soil culture?2017-2019?salt stress experiments were conducted.The study first used hydroponic methods to analyze the transcriptome and metabolomics of sugar beet under salt stre ss?300 m M Na⁺,Na Cl and Na₂SO₄ in a 2:1 molar ratio?on day 1 and day 7 to explore the mechanism in response to salt stress,and it is possible to lock in the key regulator?allantoin?.This article then studied the regulation of different concentrations of exogenous allantoin?0,0.01,0.1,and 1 m M?on sugar beet seedling growth,photosynthesis,antioxidant protection system,ion balance,osmotic adjustment system,and nitrogen metabolism under salt stress,and this study clarified the role of allantoin in improving the salt tolerance of sugar beet.?1?Transcriptomics results showed that compared with non-salt controls,1,279 and 903differentially expressed genes?DEGs?were identified on day 1 and day 7 of salt stress,respectively,and up-regulated genes were more than down-regulated genes.Day 7 contained more down-regulated DEGs than that on day 1.DEGs were mainly enriched in 10 pathways in the KEGG database.Most genes were related to carbon metabolism,phenylalanine,and amino acid biosynthesis.They were also mainly enriched in 22 GO annotations,including carboxylic acid metabolism and redox process.?2?A total of 127 metabolites were identified by metabolomics analysis,including 24 kinds of amino acids?AAs?,21 kinds of organic acids?OAs?,9 kinds o f amines,9 kinds of carbohydrates,11 kinds of lipids,9 kinds of alkaloids,7 kinds of nucleic acids,4 kinds of vitamins,2 kinds of cofactors,2 kinds of sugar alcohols,2 kinds of hormones and 28 kinds of other compounds.The sucrose metabolism and tricarboxylic acid cycle of sugar beet increased after salt stress,and 73 and 85 metabolites were significantly affected on day 1 and 7,respectively.On day 1after salt stress,the sucrose content decreased and OAs such as L-malic acid and?-ketoglutarate content increased.On day 7 after salt stress,nitrogen metabolites such as AAs,betaine,allantoin and melatonin increased significantly.?3?Transcriptomics and metabolomics analysis showed that sugar beet genes were time-specific in response to salt stress,and allantoinase was significantly down-regulated on day 7after salt stress,leading to a large accumulation of allantoin.Correlation analysis showed that most genes were significantly related to allantoin and xanthine?r>0.8,P<0.05?.Under salt stress,the carbon and nitrogen metabolism of sugar beet roots were readjusted.Nitrogen metabolism played a major role in the later stages of salt stress.The xanthine dehydrogenase?XDH?,allantoinase?ALN?or allantoin in purine metabolism pathway was the key gene or metabolite of sugar beet in response to salt stress.?4?Exogenous allantoin significantly improved the salt tolerance of seedlings,biomass,leaf area and photosynthetic capacity,which including the photosynthetic rate?Pn?,transpiration rate?Tr?,actual photochemical quantum efficiency of photosystem II?Y?II??,and non-photochemical quenching coefficient?Y?NPQ??.Exogenous allantoin also increased the activity of ribulose1,5-diphosphate carboxylase?Ru BPcase?.In addition,exogenous allantoin increased the activities of glutamine synthetase?GS?,NADH-glutamic acid synthase?NADH-GOGAT?and glutamate dehydrogenase?GDH?,and promoted the nitrogen metabolism of sugar beet seedlings under salt stress.?5?Exogenous allantoin improved the antioxidant system capacity of sugar beet leaves and roots under salt stress,and enhanced the activities of superoxide dismutase?SOD?,peroxidase?POD?,catalase?CAT?,ascorbate peroxidase?APX?,the content of the antioxidant substances,and reduced ascorbic acid?ASA?,flavonoids,and the content of reactive oxygen species?ROS?,including malondialdehyde?MDA?content and superoxide anion?O₂·^-?generation rate.The enhancement of enzyme and non-enzyme antioxidant systems by exogenous allantoin is an effective mechanism to improve the salt tolerance of sugar beet seedlings.?6?Exogenous allantoin contributed to the ion homeostasis and osmotic adjustment of sugar beet seedlings.The Na⁺content increased and the K⁺,Ca²⁺and Mg²⁺content reduced of sugar beet under salt stress,while exogenous allantoin increased K⁺/Na⁺ratio,Ca²⁺/Na⁺ratio,and Mg²⁺/Na⁺ratio in leaves and roots under salt stress.At the same time,exogenous allantoin increased the osmotic adjustment ability of sugar beet leaves and roots under salt stress,and significantly increased the accumulation of betaine and soluble sugars?sucro se and reducing sugar?.Under salt stress,exogenous allantoin enhanced the carbon metabolism of sugar beet seedlings and increased the content of oxalic acid,malic acid and succinic acid in sugar beet leaves under salt stress and reduced the content of lactic acid;it also increased the content of oxalic acid and lactic acid in beet roots,and reduced malic acid and succinic acid content.?7?Exogenous allantoin increased the content of endogenous allantoin and allantoic acid in sugar beet.Salt stress reduced putrescine?Put?content in leaves,and increased spermine?Spm?content in both leaves and roots,but reduced S-adenosylmethionine decarboxylase?SAMDC?and diamine oxidase?DAO?activities.Exogenous allantoin pretreatment increased the arginine decarboxylase?ADC?,DAO,SAMDC and polyamine oxidase?PAO?activities in leaves and roots of sugar beet seedlings.Exogenous allantoin also increased Put and Spm contents in leaves and roots under salt stress.In addition,exogenous allantoin reduced the abscisic acid?ABA?content in beet leaves,and had no significant effect on the ABA content in beet roots.?8?Principal component analysis?PCA?showed that different exogenous allantoin concentrations had different effects on the growth of sugar beet seedli ngs under salt stress.In the first principal component?PC1?,allantoin showed a higher positive load,and O₂·^-showed a higher negative load.In the second principal component?PC2?,allantoin showed a negative load.Pearson correlation analysis showed that the dry weight of whole sugar beet plants was positively correlated with polyamine metabolism,antioxidant system,osmotic adjustment and nitrogen metabolism.?9?Salt stress reduced the weight and sugar content of sugar beet roots and increased the sugar content.However,0.1 m M exogenous allantoin pretreatment significantly increased sugar beet weight and sugar content under salt stress.Therefore,purine metabolism is an important way for sugar beet to respond to salt stress;exogenous allantoin improves the salt tolerance of sugar beet by enhancing polyamine metabolism and antioxidant protection.