Physiological Responses And Gene Express To Drought Stress Of Rhododendron Delavayi Franch

Rhododendron is a world famous ornamental plant with high ornamental and garden value.Drought stress is a major obstacle in Rhododendron application in landscaping.However,the research on Rhododendron adaptation to drought is still not enough;especially for the molecular mechanism of drought stress is still a blank.In this study,we used R.delavayi a wide distribution and drought tolerant species as material to investigate the changes of morphological structure,physiological and gas exchange during drought and rehydration,so as to understand the physiological mechanism of R.delavayi adapation to drought.Combined with the transcriptome analysis in the control and drought stress,we tried to understand the gene expression characteristics and clarify the main physiological reaction and the related genes response to drought stress.The study results of morphology,physiology and molecular biology perspectives can provide theoretical and practical guidance for the development of water-saving garden plants and breeding of drought-tolerant varieties of Rhododendron.1.Drought stress had a significant effect on the ultrastructure of mesophyll cells and chloroplasts of R.delavayi.With the increasing of drought stress,the degree of persecution of cells was aggravated.After 12 days of drought stress,plasmolysis and thylakoid vacuolization had intensified,while the chloroplast shape had changed from long and oval to circular or nearly spherical,and the lamellae were bent to produce an overall disorganized arrangement.The structure of chloroplasts was abnormal.2.The changes of physiological and biochemical substances in the leaves of R.delavayi showed strong adaptability to the drought and rehydration.With the increasing of drought stress,the content of malondialdehyde?MDA?,proline and soluble sugar increased significantly,and the activity of superoxide dismutase?SOD?also increased.After re-watering,the content of MDA,proline,soluble sugar,and the activity of SOD were decreased.3.The gas exchange characteristics of R.delavayi showed strong adaptability to drought.After 4 days of drought stress,the net CO₂ assimilation rate?A_N?declined slightly while stomatal conductance?g_s?and mesophyll conductance?g_m?were not affected.After 8 days of drought stress,A_N,g_s and g_m had decreased notably.A_N and g_s decreased rapidly after 9 days of continuous drought stress,the value of g_s was 0.02mol CO₂ m^-2s^-1 and reached the level of severe drought stress.After re-watering for 1day,A_N,g_s and g_m were quickly restored to more than 80%of the control level.4.With the increase of drought stress,chlorophyll fluorescence parameters such as the photochemical quenching?qP?,the actual photochemical efficiency of photosystem II??PSII?and the electron transport rate?Jflu?were decreased,but the maximum quantum efficiency of photosystem II?F_v/F_m?did not decline.After re-watering for 1 day,all of the chlorophyll fluorescence parameters were recovered to the control level and showed strong recovery capacity.5.The analysis on photosynthetic quantitative limitation showed that after 4 days of drought stress,stomatal limitation?S_L?was negligible,mesophyll conductance limitation?MC_L?and biochemical limitation?B_L?were the main limitation factors.After 8 days of drought stress,S_L and MC_L increased markedly and MC_L even more so,while B_L remained relatively constant.After re-watering,MC_L remained the main limitation factor irrespective of the degree of photosynthetic recovery.In conclusion,MC_L was the main limiting factor during the process of drought stress and rehydration,g_m may play an important role in the photosynthesis of R.delavayi.6.Based on high-throughput sequencing technology,a transcriptome database of R.delavayi under drought stress was established.After de novo assembly 86,855unigenes were generated with an average length of 1870 bp.A total of 22,728differentially expressed genes?DEGs?were indentified between the control and drought conditions.The genes related to photosynthesis,plant hormone signal transduction and flavonoid pathway were highly enriched,suggesting that these metabolic pathways may play an important role in response to drought stress of Rhododendron.7.The analysis of enriched metabolic pathways found that the transcript abundance of the DEGs encoding the components of light-harvesting chlorophyll-protein complex?LHC?,photosystems II and I,and electron-transport chains were significantly increased during drought stress,indicates that it played an important role in protecting and repairing photosynthetic system.In plant hormone signal transduction pathway,the DEGs encoded brassinosteroids?BRs?signaling pathways were significantly enriched to cope with drought stress,suggesting BR metabolic pathway may play a vital role in response to drought stress.In flavonoid biosynthesis pathway,flavonol and anthocyanin synthesis compete intermediates through the dynamic equilibrium of branch point,and the flavonoid biosynthesis transfered from flavonol to the anthocyanin.In conclusion,drought stress had a significant effect on the ultrastructure of mesophyll cells and chloroplasts of R.delavayi.The changes of physiological and biochemical substances in the leaves showed strong adaptability to the drought and rehydration.At molecular level,R.delavayi adapted to drought stress mainly through regulating expression changes of genes related to photosynthesis,plant hormone signal transduction and flavonoid biosynthesis.The results supplemented and polished the mechanism of Rhododendron adaptation to drought streess,which would serve as important theoretical and practical guidance for genetic improvement and the development of water-saving garden plants of Rhododendron.