Physiological Mechanism Of Exogenous Calcium Ions Alleviating Drought Induced Photosynthetic Damage In Tobacco

There are about 6.1 billion hectares of land in the world’s arid areas.Soil drought is one of the important environmental obstacles to agricultural production development.Improving the biological productivity of plants in arid areas can meet people’s increasing demand for crops and animal by-products.Soil drought will lead to the reduction of crops or even no harvest in the plant growth period.Therefore,increasing plant yield in the soil drought environment is an urgent technical measure for agricultural production.In recent years,people have paid attention to the physiological effects and molecular biology of Ca²⁺.It not only provides essential nutrients in the process of plant growth,also inhibit the increase of reactive oxygen species caused by drought which protect the structure of the plasma membrane to maintain the photosynthetic rate of plants,and participate in drought signaling to adjust the physiological response of plants.However,the physiological and molecular mechanisms related to the protection of plant photosynthetic apparatus by exogenous calcium are still unclear.Therefore,we spray a solution containing exogenous calcium on the model plant tobacco（Nicotiana tabacum L.）which is planted under the drought environment.This experiment revealed the physiological and molecular mechanism of exogenous calcium through changes in plant phenotype,photosynthetic mechanism,antioxidant enzymes,abscisic acid（ABA）and photosynthetic mechanism transcription levels,which provides a reference for the study of plant drought resistance mechanism.The main results are as follows:1.Drought stress led to water loss and wilting of tobacco leaf,photosynthetic pigment content decreased,photosynthetic pigment synthesis gene and antenna protein expression inhibited,in which the content of chlorophyll a（Chl a）changed more than that of chlorophyll b（Chl b）.After spraying exogenous calcium,leaf wilt symptoms were significantly reduced,the expression of photosynthetic pigment synthesis genes and photosystem-related genes were up-regulated,photosynthetic pigment content was increased,and the light-capturing ability of photosynthetic mechanism was maintained.2.Drought stress decreased the maximum photochemical efficiency（F_v/F_m）,photosynthetic performance index(PI_abs)and total photosynthetic performance index(PI_total)of PSII in tobacco leaves;and increased the fluorescence intensity of J,K and L points on the chlorophyll fluorescence kinetics curve（V_J,V_K,V_L）and the heat dissipation across the thylakoid membraneΔp H(Y_NPQ).Exogenous calcium increased F_v/F_m,PI_abs and PI_total of tobacco under drought stress,decreased V_J,V_K,V_L and Y_NPQ,and up-regulated the expression of OEC-related protein genes,alleviating the over-reduction state of electron transport chain.3.Under drought stress,cyclic electron transfer（CEF）was decreased,ABA content was increased,stomatal conductance（G_s）,net photosynthetic rate（P_n）,intercellular CO₂concentration（C_i）and transpiration rate（T_r）were decreased,and expressions of genes related to Calvin cycle were decreased.After the application of exogenous calcium,leaf CEF increased,ABA content decreased,leaf aquaporin（AQPs）gene expression was up-regulated,and G_s,P_n,C_i and T_r values were significantly increased.Therefore,the application of exogenous calcium increased the production of ATP and maintained the stomatal opening by regulating ABA signals to keep the normal operation of Calvin cycle.4.Drought stress increased the contents of hydrogen peroxide（H₂O₂）,superoxide anion（O₂·^-）and malondialdehyde（MDA）in tobacco leaves,and increased the activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）,while significantly decreased the activity of ascorbate peroxidase（APX）compared with CK.The ROS content decreased significantly after the application of exogenous calcium,while the activities of SOD,POD,CAT and APX were further enhanced.At the same time,gene expression of related antioxidant enzymes is up-regulated,suggesting that the exogenous calcium promotes the antioxidant system?s ability to clear ROS,which is conducive to maintaining membrane stability and reducing oxidative damage to cells.5.Transcriptome sequencing analysis showed that the expressions of photosynthetic PSI and PSII reaction center genes,antenna protein related genes,ATP synthesis and electron transport related genes in tobacco leaves were significantly down-regulated under drought stress.Ca²⁺alleviated the decrease of photosystem gene expression by up-regulating calcium signal related genes,which provided help for plant drought resistance.In summary,this study creatively shows that the damage sites of photosynthetic apparatus of tobacco leaves caused by drought stress and the mechanism of tobacco response to drought stress.Based on the transcriptome data analysis,it can be seen that exogenous calcium could up-regulate the expression of calcium signal,reduce plant water loss,increase plant leaf pigment content,repair PSII reaction center and ensure photosynthetic electron transfer;participate in ABA network signal regulation of stomatal movement to ensure Calvin cycle;improve the activity of antioxidant system to maintain the stability of photosynthetic membrane and reduce the damage of light system.The results supplement the drought response of plants and provide a new idea for plant drought resistance breeding.