Physiochemical Property And Transcriptomic Analysis Of Bupleurum Chinense DC. Seedlings Under Drought Stress

During the growth and development of plants,plants are often subjected to various abiotic stresses,resulting in physiological imbalance and declining production in plants.Drought stress is the most common abiotic stress that affects plants.Global warming means that droughts will occur more frequently,which would severely affect the production of medicinal plants.Bupleuri radix is a common bulk medicinal material with immunomodulatory,anti-inflammatory,antioxidant and hepatoprotective effects.Bupleuri radix in the market is mainly from cultivated medicinal materials in the field,where de demand is far higher than the production,especially since COVID-19 pandemic.Drought stress is one of the important environmental factors that affect the quality and yield of medicinal materials,and also the main factor restricting the planting of B.chinense in the field.Seedling is a critical period for growth and development,and B.chinense is more sensitive to water requirements during the seedling period.The lack of water at the seedling stage will lead to the dwarf of seedlings,even the death of seedlings,and severely affect the crop yield.Although previous studies have provided experimental basis for the molecular and physiological basis of drought tolerance of B.chinense,the differences of transcriptome in roots and leaves under drought stress at seedling stage have not been explored.Objective:In this study,the effects of drought on photosynthesis and physiological and biochemical indexes were determined,and the changes of DEGs expression and metabolic pathway in response to drought stress were identified at the molecular level.The aim is to provide theoretical basis for analyzing the biological pathway and mechanism of plant drought resistance.Methods:B.chinense seedlings before and after drought were used as the research object,and vernier caliper and analytical balance were used to measure the length and weight.The content of chlorophyll in leaves was measured by UV spectrophotometer.The contents of soluble sugar,soluble protein and free proline were determined by anthrone colorimetry,Coomassie brilliant blue G-250 colorimetry and acid ninhydrin colorimetry respectively.SOD,POD and CAT were determined by nitroblue tetrazole method,guaiacol method and UV absorption method respectively.Use Trizol? Reagent extracted total RNA from leaves and roots of B.chinense seedling.Sequencing was performed based on Illumina hiseq 2500 sequencing platform.Result:The results showed that when the soil water content reached about 6.67%,the leaves of B.chinense seedling began to curl and wilt.After drought,the contents of chlorophyll a,b and total chlorophyll increased by 13.51%,15.09%and 13.93%respectively,and the value of chlorophyll a/b decreased.The results showed that chlorophyll b was more sensitive to drought stress than chlorophyll a.Under drought stress,the content of soluble sugar in roots increased by 13.24%,the content of soluble protein in leaves and roots decreased by 26.72%and 19.96%,and the content of free proline in leaves and roots increased by 19.13%and 67.58%,respectively.After drought,the activities of SOD,POD and CAT in leaves increased by 89.09%,32.99%and 156.59%respectively;The activities in roots decreased by 49.89%,7.20%and 22.47%respectively.A total of 59.82 GB of clean data was obtained by transcriptome sequencing,and unigenes was compared with six databases such as Nr,Swissprot,string,go,KEGG and Pfam.Under drought stress,3,737 and 6,816 differentially expressed genes(DEG)were identified in the leaves and roots of B.chinense seedling;The DEGs of leaves and roots involved 37 and 36 GO terms and 222 and 253 KEGG pathways,respectively.A total of 33,728 loci were identified by SSR analysis,of which dinucleotide accounted for the largest proportion.Drought stress mainly affected the photosynthesis and phenylpropanoid biosynthesis of B.chinense seedling leaves,the diterpenoid biosynthesis and biosynthesis of unsaturated fatty acids in roots.The expression level of DEGs encoding glutathione S-transferase(GST)was significantly down regulated,indicating that the expression of GST gene may play a key role in preventing oxidative damage to plants.B.chinense seedling respond to drought stress by up regulating the gene encoding copalyl diphosphate synthase(CPS)and increasing the biological content of terpenoids.Moderate drought stress can promote the synthesis of terpenoids in Bupleurum chinense seedlings in field planting.Conclusion:Under drought stress,B.chinense seedling respond to drought stress by adjusting the content of Osmoregulation Substances and the activity of antioxidant enzymes in leaves and roots.Transcriptome sequencing showed that the expression levels of reactive oxygen species and superoxide radical scavenging genes were overexpressed in B.chinense seedling,indicating the defense function of these related enzymes and hormones in alleviating drought stress.