Methyl Jasmonate Regulates Physiological Metabolism Of Catharanthus Roseus Seedling In Response To UV-B Stress

Catharanthus roseus is a medicinal plant containing a variety of alkaloids,and its rich alkaloids have been widely recognized by domestic and foreign counterparts.At the same time,phenolic compounds in C.roseus also have various pharmacological effects such as antioxidant,hypoglycemic,antitumor,and antibacterial.UV-B（280～320nm）stress can damage plant proteins,photosynthetic pigments and cell membranes,affecting plant growth,development and metabolic processes.Methyl jasmonate（MeJA）is considered to be an important signaling molecule in plant defense responses to biotic and abiotic stresses.In this experiment,C.roseus seedlings were used as experimental materials,and MeJA solutions of different concentrations(0μmol·L^-1,50μmol·L^-1,100μmol·L^-1,150μmol·L^-1)were added exogenously.The effects of different concentrations of MeJA on the growth and development,photosynthetic characteristics,osmotic regulation system,antioxidant enzyme system,accumulation of primary secondary metabolites and gene expression of enzymes related to secondary metabolic pathways in C.roseus seedlings under supplemental 10μW·cm² UV-B irradiation.In order to preliminarily explore the mechanism of MeJA promoting C.roseus to antagonize UV-B stress.The main research results are as follows:（1）After UV-B treatment,the growth,development and photosynthesis of C.roseus were inhibited,the contents of MDA,proline and H₂O₂ increased significantly,the activity of antioxidant enzymes decreased,the content of total phenols decreased,and the content of total flavonoids accumulated;The plant height and root length of C.roseus seedlings were recovered to a certain level after exogenous addition of MeJA.Photochemical quenching coefficient（q P）,electron transfer rate（ETR）,maximum photosynthetic rate（Fv/Fm）,stomatal conductance（Gs）,transpiration rate（Tr）,photosynthetic pigment content,proline,antioxidant enzyme activity and total phenolic content all increased,while the contents of MDA,H₂O₂,initial fluorescence（F0）and non-photochemical quenching coefficient（NPQ）decreased.（2）A total of 39 primary metabolites were identified by GC-MS technology.18compounds with significant differences were screened by the OPLS-DA model,among which the content of acid compounds in C.roseus under UV-B stress decreased and the sugar compounds accumulated.The contents of Rhamnose,Galactoside,Sucrose and Fructose were significantly lower than those under UV-B treatment after exogenous addition of low concentration of MeJA,which was close to the level of CK.Metabolic pathway enrichment analysis was performed on the differential metabolites,and finally the five most severely affected metabolic pathways were identified:Sulfur Metabolism,Ascorbic Acid and Aldose Metabolism,Citric Acid Cycle,Amino Sugar and Nucleotide Sugar Metabolism,and Galactose Metabolism.（3）Based on UPLC-MS technology,29 phenolic metabolites and 9 alkaloids in leaves of C.roseus seedlings under different treatments were determined.The precursor L-phenylalanine accumulated significantly in the leaves of C.roseus seedlings under UV-B stress,and decreased after adding MeJA,and its content increased with the increase of MeJA concentration.Phenolic acids and flavonoids were significantly accumulated under UV-B stress,and the content of phenolic acids was significantly decreased after exogenous addition of MeJA.Alkaloids and phenylpropanoid compounds were significantly inhibited under UV-B stress,and exogenous addition of MeJA could regulate the secondary metabolic pathway of C.roseus,promote the synthesis of alkaloids,and cause significant accumulation of alkaloids.Pearson correlation cluster analysis was performed between the secondary metabolites measured by UPLC-MS,the primary metabolites measured by GC-MS and tolerance indicators,and it was found that the primary metabolites and alkaloids mainly responded to the antioxidant activity of C.roseus enzymatic system,while phenolic compounds mainly affect the osmoregulation system of plants.（4）Real-time quantitative PCR was performed to determine the expression levels of four key enzyme genes（Cr CHI,Cr CHS,Cr PAL and Cr F3H）in the phenolic metabolic pathway and four key enzyme genes in the alkaloid metabolic pathway（Cr D4H,Cr SLS,Cr STR and Cr TDC）.It was found that UV-B stress promoted the expression of phenolic key enzymes genes,and the expression level of exogenous addition of MeJA decreased to varying degrees,but was still higher than that of the control group.UV-B treatment significantly inhibited the expression of Cr D4H and Cr STR genes,and exogenous application of MeJA promoted the expression of alkaloid key enzymes gene.Pearson correlation cluster analysis was performed between tolerance indicators,secondary metabolites and related enzyme genes.Alkaloid metabolic pathways and phenylpropanoid compounds affect plant antioxidant enzyme systems,while flavonoids and phenolic acid compounds affect C.roseus osmotic regulation ability.In conclusion,exogenous addition of appropriate concentration of MeJA is beneficial to alleviate the inhibition of UV-B stress on the photosynthetic system,antioxidant enzyme system,osmotic regulation system and alkaloid content of C.roseus.In practical production applications,when encountering strong UV-B radiation,it can be considered to improve the stress resistance of C.roseus and obtain more medicinal components by exogenously spraying a suitable concentration of MeJA.