Effect Of MeJA On Brassica Rapa L. Seedling Growth And Physiology Under Low Temperature Stress

Low temperature affects the normal growth cycle of most plants,leading to a reduction in crop yields and can cause plant death in severe cases.Brassica rapa L.is an important oilseed crop in China.It is widely planted in the northwestern region,and the temperature in the northwestern region is relatively low in winter.Therefore,low temperature severely reduces the winter rapeseed output in the northwestern region.Many studies on other crops have shown that exogenous application or endogenous induction of MeJA can enhance the cold tolerance of crops,but there are few reports on the specific regulatory mechanism and related pathways of MeJA to alleviate low temperature stress in plants.In this study,Brassica rapa L.variety"Longyou 8"was used as the experimental material,and the analysis of related genes during the low temperature damage of rapeseed by exogenous MeJA combined with transcriptome sequencing technology.From Brassica rapa L.seedlings ROS activity,osmoregulation substance accumulation and membrane damage in terms of antioxidant enzyme activity,photosynthesis-related indexes,and cell wall components,the molecular and physiological mechanisms of MeJA regulating low-temperature resistance in Brassica rapa L.seedlings were studied.It is hoped to provide a new theoretical basis for the regulation mechanism of MeJA in alleviating low temperature stress in plants.Below are key research findings:1.MeJA sprayed Brassica rapa L.seedlings with different concentrations of MeJA?0,0.001,0.01,0.1,0.5,1 mM?were pretreated,and then treated at 4?after 24 hours?compared with low temperature stress alone,The relative electrical conductivity and malondialdehyde content of Brassica rapa L.seedlings were reduced to varying degrees,and the mitigating effect of 0.1 mM was the most significant.This indicates that exogenous MeJA pretreatment can effectively alleviate the low temperature effects on Brassica rapa L.damage,enhanced low temperature resistance of Brassica rapa L.seedlings,and this alleviating effect was related to the concentration of MeJA applied.2.It was used RNA-Seq technology that Brassica rapa L.transcriptome was sequenced and differentially expressed genes were analyzed.High-throughput transcriptome sequencing was performed on Brassica rapa L.leaves that had been pretreated with MeJA.After sequencing quality control,9 samples obtained 69.15 Gb Clean Data.The Clean Data of each sample reached 6.06Gb,the Q30 base percentage was 93.27%,and the GC content of each treatment was about 46%.The clean reads of each sample were sequenced with the Brassica rapa L.rape genome,and the alignment efficiency ranged from 82.72%to 88.05%.Based on the comparison results,functional annotation,enrichment analysis,and gene expression analysis were performed.According to the gene expression in different treatment samples,13318 differential genes were identified,of which 43 genes were differentially expressed in all three treatments.Combining gene function annotation,GO,and KEGG enrichment analysis,19 genes were selected as candidate genes that were associated with the MAPK cascade pathway,Ca²⁺-mediated signal transduction pathway,photosynthesis-related pathway,and cell wall synthesis,and verified by qRT-PCR,and the quantitative results were consistent with transcriptome results.3.Compared with low temperature stress,the relative conductivity,malondialdehyde content and O₂.^-accumulation of Brassica rapa L.seedlings treated with MeJA+4?decreased,and the relative water content in leaves increased,which promoted the expression of antioxidant enzyme-related genes.Significantly increased the activity of antioxidant enzymes,and the braICE1,braCOR,braSOD,braPOD,braCAT,braMAPK3,braMAPK5,braCPK4 and braCPK5 genes were up-regulated expression.Combined with the results in the transcriptome,it was shown that MeJA pretreatment could alleviate the damage of Brassica rapa L.to low temperature stress and enhance the low temperature resistance of Brassica rapa L.4.Compared with MeJA+4?,U0126+MeJA+4?,EGTA+MeJA+4?treated Brassica rapa L.relative conductivity,malondialdehyde content,O₂.^-accumulation significantly increased,while the relative water content of leaves decreased the expression of antioxidant enzyme-related genes decreased,and the activity of antioxidant enzymes decreased.braICE1,braCOR,braSOD,braPOD,braCAT,braMAPK3,braMAPK5,braCPK4 and braCPK5 genes are down-regulated,and the MAPK cascade pathway and Ca²⁺-mediated signal transduction pathway are involved in the process of MeJA alleviating low temperature damage to Brassica rapa L.,and these two pathways affect each other.5.Compared with low temperature stress,the chlorophy?fluorescence parameters and photosynthetic pigment content of Brassica rapa L.seedlings treated with MeJA+4?increased,and the content of pectin and cellulose in the cell wall increased significantly,and the cell wall was more complete.Compared with MeJA+4?,U0126+MeJA+4?and EGTA+MeJA+4?reduced the chlorophyll fluorescence parameters and photosynthetic pigment content of Brassica rapa L.seedling leaves,and fructose,pectin and cellulose contents were significantly reduced,which led cell wall synthesis is blocked.It shows that low temperature stress can affect photosynthesis and cell wall integrity of plants,and exogenous MeJA treatment can effectively alleviate this damage.This mitigation process involves the MAPK cascade pathway and Ca²⁺-mediated signal transduction pathway.The above results show that 0.1 mM of exogenous MeJA can effectively alleviate cell damage to Brassica rapa L.seedlings at low temperature,improve its photosynthetic capacity and effectively protect cell wall integrity.In addition,exogenous MeJA can induce the expression of braMAPKs and braCPKs in Brassica rapa L.seedlings under low temperature stress.The MAPK cascade pathway and Ca²⁺-mediated signal transduction pathway are involved in the process of exogenous MeJA to relieve Brassica rapa L.seedlings from low temperature damage,which can further affect and stimulate various Brassica rapa L.seedling physiological and biochemical response,enhance the cold resistance of Brassica rapa L.seedlings.