Analysis Of The Chloroplast Proteome Of Cucumber Leaves Under Elevated CO₂ Concentration And Drought Stresses

‘Jinyou No.35' cucumber was chosen as the test cultivar.We measured morphological and physiology traits of cucumber seedlings under elevated [CO₂] and drought stress,and chloroplast proteins isolated from cucumber leaves were examined through isobaric tags for relative and absolute quantitation?iTRAQ?based proteomic analysis.The experiment were arranged in a split-plot design.The main plot was [CO₂],the subplot was drought treatments.The CO₂ treatments included an ambient [CO₂] of ⁴00 ?mol·mol-1,designated A,and an elevated [CO₂] of 800±20 ?mol·mol-1,designated E.The water treatments included control?nutrient solution,designated 0?,moderate drought stress?nutrient solution + 5% PEG 6000,designated 1?,and severe drought stress?nutrient solution + 10% PEG 6000,designated 2?.The chief results are showed as follows:1.Elevated [CO₂] significantly increased the plant height,stem d iameter,leaf area and RWC of cucumber seedlings under three water treatments.Elevated [CO 2] significantly increased the photosynthesis?Pn?under moderate and severe drought stress.Compared to the ambient [CO₂],elevated [CO₂] reduced the degree of increase and decrease in the NPQ and Fm values under drought stress,respectively.Elevated [CO₂] remarkably decreased J and K peak of OJIP under moderate and severe drought stress.2.We identified 2046 proteins through iTRAQ and took a statistical analysis o f differentially expressed proteins with important function and highest credibility.The differentially expressed proteins were mainly associated with the photosynthesis,pigment biosynthesis,stress and defense responses,redox homeostasis and metabolic pathway.?1?Proteins related to the photosynthetic light reaction?photosystem II reaction center protein L,chlorophyll a-b binding protein,photosystem I assembly protein Ycf4 and photosystem I reaction center subunit VI-2?were down-regulated following exposure to moderate and severe drought stress.Elevated [CO₂] increased the abundance of psbJ and the chlorophyll a/b-binding protein of PSII under control,and psbJ and the PSI reaction center subunit VI-2 under moderate drought stress.?2?Proteins related to carotenoid biosynthesis?carotene epsilon-monooxygenase,protein LUTEIN DEFICIENT 5,phytoene synthase and zeaxanthin epoxidase?were down-regulated under moderate and severe drought stress.Proteins related to chlorophyll biosynthesis?protochlorophyllide reductase,tetrapyrrole-binding protein and magnesium-chelatase subunit ChlH?were down-regulated under moderate drought stress,and magnesium-chelatase subunit ChlI-2 were down-regulated under severe drought stress.Elevated [CO₂] increased the abundance of Mg-chelatase subunit ChlI-2 under control,but decreased the abundance of uroporphyrinogen decarboxylase and tetrapyrrole-binding protein under drought stress conditions.?3?Stress response proteins?probable plastid-lipid-associated protein,PLAT domain-containing protein,late embryogenesis abundant protein-like,glutathione S-transferase and heat shock protein?were down-regulated under moderate and severe drought stress.Elevated [CO₂] decreased contents of aldehydes?such as MDA?and keto nes caused by drought stress.?4?In the condition of ambient [CO₂],moderate and severe drought stress increased the abundance of ferritin,and the abundance of PHGPx was only up-regulated under moderate drought stress.Contents of GSH and ASA and the antioxidase?SOD,POD,CAT,APX?activities were increased under moderated and severe drought stress.Elevated [CO 2] increased antioxidase activities under moderate and severe drought stress and up-regulated the abundance of thioredoxin family protein,which indicates that elevated [CO₂] can improve the antioxidant ability of cucumber under drought stress.?5?Elevated [CO₂] up-regulated threonine dehydratase biosynthetic,the biosynthesis of isoleucine and proline contents under severe drought stress.The res ults imply that elevated [CO₂] can improve the ability of cucumber drought resistance by increasing the expression and content of osmotic adjustment substances.In conclusion,cucumber seedlings make response to drought stress by regulating expression of related proteins to improve its' adaption ability under drought environment.Elevated [CO₂] can improve Pn,antioxidant ability and osmotic adjustment ability,and relieve photo damage and accumulation of toxic substance under drought stress by regulating expression of related proteins,which al eviate cucumber drought stress to a certain degree.