Research On The Response Mechanism Of Rhododendron Rex,An Endemic Plant In Southwest China,to Drought Stress

Drought is the most common and main abiotic stress factor that affects plant growth and development.It usually causes serial physiological changes such as dehydration of the plant body,cell dehydration and plasmolysis,damage to cell membranes and enzyme systems,loss of cell function,and metabolic disorders,which integrally caused abnormal development of plants.Especially in the regions with obvious dry and wet seasons in south-western China,drought is the main environmental obstacle faced by ornamental plants in garden applications.The plentiful Rhododendron plants in this area are famous woody flower and have important application value in landscaping.However,there are few reports about the response of plants from Rhododendron to drought stress in garden application and introduction and cultivation.In this thesis,the plant Rhododendron rex Levl.,which is endemic in Southwest China and has great potential for garden application development,was taken as the research objective plant.Through indoor controlled treatments of drought stress,the effects of leaf stoma,physiological and biochemical and photosynthetic characteristics under different drought levels were analyzed to explore the physiological mechanism of Rhododendron rex in response to drought;Through transcriptomics,weighted gene co-expression network analysis(WGCNA)and other methods,the differential expression of genes(DEGs)in response to drought stress and the corresponding functions and metabolic pathways of related modular genes were revealed.Finally,the mechanism of Rhododendron rex in response to drought stress was discussed from multiple levels including morphology,physiology,biochemistry and gene expression.The results will help us to reasonable scientific understanding of its biological characteristics response to drought stress,providing scientific basis for the introduction and cultivation of Rhododendron rex and even its sustainable development and protection management.The main findings are as follows:(1)Different drought stress gradients were set as control group(CK: 75-80%),mild drought(D1: 55-60%),moderate drought(D2: 40-45%),severe drought(D3: 30-35%).The results of seedling leaf stomata and physiological and biochemical data measurement of Rhododendron rex showed that drought stress significantly(P<0.05)affected its phenotype and physiological and biochemical indexes.With the increase of drought stress,the stomata density of Rhododendron rex showed slightly decreased,while its stomata area increased;the content of chlorophyll a,chlorophyll b and total chlorophyll decreased significantly under severe drought stress(D3);the content of malondialdehyde increased,and the content of soluble sugar and proline decreased;?PS?,ETR,and q P decreased significantly under moderate(D2)and severe(D3)stress;membership function analysis showed that the order of membership values among the treatments was CK>D1>D2>D3,and The average value of each treatment of Rhododendron rex seedlings was 0.4815,which showed that the plant belonged to the medium resistance type.(2)Based on the Illumina Hi Seq sequencing platform,the transcriptome(RNA-Seq)sequencing analysis of different drought stress treatments of Rhododendron rex showed that 52312048,50617448,4557733,48856750 high-quality Clean reads were obtained under different drought stresses,with a total of 72118(48.68%).A unigene successfully annotated to the public database.A total of 1,536 differentially expressed genes were obtained based on the DESeq2 analysis of the significantly differentially expressed genes(DEGs)of the four treatment groups of Rhododendron rex,176 were obtained from the pairwise difference analysis of different drought stress treatments(D1,D2 and D3)and the control group.1,228 and 1132 DEGs.The functional enrichment analysis of significantly different genes showed that these differential genes are mainly involved in the process of carbohydrate metabolic process,plant-pathogen interaction,response to stimuli,Phenylpropanoid biosynthesis,amino sugar and nucleotide sugar metabolism,and plant hormone signal transduction,porphyrin and chlorophyll metabolism and other processes;compared with the control group,drought stress promoted the citric acid cycle,metabolism(biosynthesis of other secondary metabolites,carbohydrate metabolism,cofactors and vitamin metabolism),signal Transduction and other related genes are up-regulated,while environmental adaptation and lipid metabolism-related genes are down-regulated.(3)The Weighted Gene Co-expression Network Analysis(WGCNA)analysis of Rhododendron rex under different degrees of drought stress showed that 19,541 genes were selected from 16 Rhododendron rex samples to construct a total of 9co-expression modules,of which 6 modules were associated with each phenotype and physiological and biochemical indicators are significantly related.The GO and KEGG enrichment analysis of the genes enriched in these 6 different modules showed that the Brown module was significantly enriched in the pathways of photosynthesis,Porphyrin and chlorophyll metabolism and chlorophyll biosynthetic process,and the Blue and Black modules were both significantly enriched in the pathway of Plant-pathogen interaction,both Green and Red modules are significantly enriched in plant hormone signal transduction pathways.Based on the above research results,it could be summarized that under drought stress,the affected plant of Rhododendron rex showed changes in stomata density,stoma area,malondialdehyde,proline,chlorophyll and other physiological and biochemical indicators as response.It was assessed as a medium drought resistance type according to average value of each drought treatment,which means that the severe drought should be avoided in its introduction and cultivation.And the RNA-Seq analysis showed that it mainly responds to drought stress by the gene expression related to carbohydrate metabolic process,photosynthesis,ROS system,and osmotic regulation.