Effects Of Exogenous Spd On Chlorophyll Metabolism In Cucumber Seedlings Under High Temperature Stress

As one of the major abiotic(environmental)stresses affecting plant growth,high temperature stress acts as an important limiting factor for plant yield and productivity(Boyer,1982a).In China,the crop yield reduction due to high temperature is 20%per annum(Zhao et al.,2014).Cucumber(Cucumis sativus L.)is a major vegetable crop planted worldwide.In China,cucumber is one of the vegetable crops with a large area and high efficiency in facility cultivation.As a thermophilic plant,cucumber requires an optimum temperature in the range of 25-30℃ for assimilation.High temperature above 38℃ can cause thermal injury in cucumber plants,while necrosis appears in cucumber leaves and stems within a short period at approximately 50℃(Oda,1993;Talanova et al.,2006).Substrate culture experiments were performed in an artificial climate chamber at 42℃/32℃ using the high temperature-sensitive variety ’Jinchun No.2’,with foliar application of 1.0 mmol·L-1 Spd.The aim of this study was to elaborate the effect of exogenous Spd onchlorophyll synthesis and degradation incucumber seedlings under high temperature stress.This study provides a reference for the practical use of PAs to improve heat resistance in cucumber plants.The main results were as follows:1.High temperature stress significantly reduced chlorophyll content of cucumber seedling,and chloroplast ultrastructure changes obviously,grana thylakoid lamella is extruded,chloroplast serious deformation,even appeared the phenomenon of plasmolysis,significantly inhibited the growth of the plant.Exogenous Spd can alleviate the harm of high temperature stress on cucumber seedlings,stable chloroplast structure,and increase higher chlorophyll content,promoting the growth of cucumber.2.Under high temperature stress,the conversion of porphobilinogen(PBG)into uroporphyrinogen III(UroIII)in the Chl biosynthetic pathway and the catabolic process of Chl were accelerated.Following the application of exogenous Spd,the conversion of PBG into UroIII was suppressed,and the accumulation of certain intermediates(e.g.),protoporphyrin IX(ProtoIX)and Mg-protoporphyrin IX(Mg-ProtoIX)was decreased in the Chl biosynthetic pathway.Exogenous Spd delayed the conversion of PBG into UroⅢ in the Chl biosynthetic pathway,effectively preventing oxidative bleaching of Chl in cucumber leaves.3.A BN-SDS-PAGE coupled with MALDI-TOF/TOF MS was performed to investigate the effects of exogenous spermidine(Spd)on proteomic changes in cucumber seedling under high temperature stress.A total of 22 differentially accumulated protein spots in response to stress and identified.The results show that the high temperature lead to seedling thylakoid membrane protein such as ATPaseCFI,CP47,D1,D2 fell sharply;analysis in transcriptional level showed the Lhcb2,Lhcb4,Lhcb5,Lhcb6,PsbA,PsbD,Lhcp,atpA gene exhibited a consistent transcription accumulation.These results suggested that Spd regulated the proteins expression to varying degree in transcription and translation levels.Spraying endogenous polyamine inhibitors MGBG under high temperature at the same time found that exogenous Spd in direct access to a complete chloroplast under high temperature stress,the protection of cucumber seedling photosynthetic organs,improve the resistance of cucumber seedlings under high temperature stress.4.Exogenous Spd reduced chlorophyllase(Chlase)and Mg-dechelatase(MDCase)activity and transcript levels and markedly downregulated pheophorbide A oxygenase(PaO),red Chl catabolite reductase(RCCR),Chl b reductase 1(CBR1)and stay-green reductase 1(SGR1)transcript levels.These results indicate that although high temperature stress accelerated Chl biosynthesis,it concurrently facilitated Chl catabolism in cucumber leaves.Exogenous Spd effectively prevented oxidative bleaching of Chl in cucumber leaves.Meanwhile,Spd slowed down the catabolic process of Chl.In conclusion,while high temperature stress accelerated the conversion of PBG into UroⅢ for Chl biosynthesis,it simultaneously facilitated Chl catabolism in cucumber leaves.Exogenous Spd markedly delayed the conversion of PBG into UroIII;this mechanism was conducive to reducing the risk of forming high-energy oxidising substances by certain Chl precursors with oxidation potential,thus preventing their oxidative damage to Chl.Meanwhile,Spd clearly decreased PaO pathway-related enzyme activity and transcript levels,thereby slowing down Chl catabolism and increasing Chl concentrations.Moreover,Spd increased Chi concentrations under high temperature stress,which might be related to SGR1,a key regulator of Chl catabolism.