| Alfalfa(Medicago)is leguminous forage with high yield,good quality,and strong adaptability.With the rapid development of animal husbandry production in China,the planting area of alfalfa increase year by year.Thrips damge leads to a decrease in the yield and quality of alfalfa,seriously affecting the development of the alfalfa industry.Alfalfa varieties ‘Caoyuan No.4’ and ‘Caoyuan No.2’ with and without thrips treated were used in this study.Based on the evaluation of thrips resistance of the varieties,primary and secondary metabolite content,extensive targeted met,and gene expression was nanlyzed to explore the metabolic regulation of thrips resistance in alfalfa.The results are as folllows:(1)The insect pest index(0.41)of ‘Caoyuan No.4’ alfalfa was significantly lower than that of ‘Caoyuan No.2’ alfalfa(0.78)(p<0.05),confirming that ‘Caoyuan No.4’ has stronger resistance to thrips.(2)Without thrips treatment,the ratio of reducing sugar to nitrogen,reducing sugar,total sugar,and total amino acid content in ‘Caoyuan No.4’ were significantly higher than those in ‘Caoyuan No.2’(p<0.05),while the total sugar to nitrogen ratio of ‘Caoyuan No.2’ was significantly higher than that of ‘Caoyuan No.4’(p<0.05).After thrips feeding,the content of total organic acids,total amino acids,and amino acid derivatives(aspartic acid,glutamic acid,arginine,proline,tyrosine,valine,cystine,isoleucine,phenylalanine and lysine)in two types of alfalfa significantly increased(p<0.05),while the content of total nitrogen and amino acid derivatives(methionine)significantly decreased(p<0.05).However,the content of reducing sugar,total sugar,and soluble protein significantly decreased only in ‘Caoyuan No.4’(p<0.05).It is speculated that the ratio of reducing sugar to nitrogen and total sugar to nitrogen are related to the constitutive defense of alfalfa,while the content of total nitrogen,total organic acids,soluble proteins,and some amino acid derivatives are related to inductive defense in alfalfa.The content of reducing sugars,total sugars,and total amino acids are involved in both the constitutive and inductive defense.(3)Without thrips treatment,the content of total flavonoids and total phenols in‘Caoyuan No.2’ were significantly lower than that in ‘Caoyuan No.4’(p<0.05).After thrips feeding,the tannin content of two types of alfalfa significantly decreased(p<0.05),the total alkaloid content of ‘Caoyuan No.2’ significantly decreased(p<0.05),and the total alkaloid content of ‘Caoyuan No.4’ significantly increased(p<0.05).It is speculated that the total flavonoid content may be related to the constitutive defense of alfalfa,while the content of tannins and total alkaloids may be related to the induced thrips defense in alfalfa.(4)774 metabolic products were detected by widely targeted metabolomic analysis.A total of 116,140,178 and 184 differential metabolites were screened in the group of R-CK vs R-T,S-CKvs＿R-CK,S-CK vs S-T and S-T vs R-T,respectively.Functional annotation and enrichment analysis of differential metabolites KEGG revealed that the isoflavone biosynthesis pathway and flavonoid biosynthesis pathway may be mainly involved in the induced thrips defense in alfalfa.There are 74 differentially expressed metabolites were detected between the S-T vs R-T and S-CK vs R-CK groups(flavonoids accounting for 29.73%),which are speculated to be related to alfalfa constitutive defense.In addition,there are 60 differentially expressed metabolites between the S-CK vs S-T and R-CK vs R＿T group(with flavonoids accounting for 51.67%),suggesting that they may mainly participate in alfalfa induced defense responses.(5)the expression level of flavonoid related genes in ‘Caoyuan No.2’ was significantly higher than that in ‘Caoyuan No.4’(p<0.05).After thrips feeding,the expression levels of flavonoid related genes in ‘Caoyuan No.2’ decreased significantly(p<0.05),while ‘Caoyuan No.4’ changed little(p<0.05).It is speculated that genes related flavonoid biosynthesis may be involved in induced thrips resistance in alfalfa. |
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