Effects Of Melatonin On Senescence Process Of Gardenia Leaves Under Darkness

Gardenia(Gardenia jasminoides Ellis)is an evergreen small shrub that belongs to the family Rubiaceae.In addition to being widely used in a landscaping function,gardenia is often used as a leaf-cutting material,cut gardenia foliage is greatly favoured due to its shiny,dark green,leathery leaves.However,whether to fill a vase alone or as the choice material in a flower bouquet,insufficient indoor light greatly accelerates its senescence,which tremendously shortens the viewing time and seriously affects its further application.Little is known about the relevant solutions,therefore,taking measures to delay leaf senescence under low light conditions requires investigation.Several independent studies have all demonstrated that melatonin can delay leaf senescence under extremely low light condition(darkness);these studies mainly concentrated on the physiological and biochemical responses.In this study,excised gardenia leaves were treated with melatonin under darkness to explore the mitigation effect of melatonin-treated gardenia leaves on senescence caused by dark stress,and further to carry out corresponding basic research on physiological indexes,anatomical characteristics and molecular level of leaves.The main results were shown as follows:(1)Four different melatonin concentrations(0.05 mM,0.1 mM,1.0 mM and 2.0 mM)all delayed dark-induced leaf senescence compared with the control,and 1.0 mM melatonin concentration(MT1.0)treated leaves maintained their green color,whereas a higher concentration(2.0 mM,MT2.0)attenuated the effect.Moreover,the related color and physiological indices,including the hue angle(H0),soil plant analysis development(SPAD)and the ratio of variable fluorescence to maximum fluorescence(Fv/Fm),all first increased and then decreased,peaking at MT1.0.Conversely,the malondialdehyde(MDA)content exhibited a significant decrease in the MT-treated leaves,and was minimized at MT1.0.(2)In this study,1.0 mM melatonin concentration(MT1.0)treated can delay gardenia leaf senescence when exposed to extremely low light condition(darkness).At 1.0 mM melatonin concentration,chlorophyll contents and chlorophyll fluorescence parameters(Fv/Fm,Fv/Fo and Y(II))increased,while the carotenoid and flavonoid contents decreased.Meanwhile,stress physiological indices including hydrogen peroxide(H2O2),superoxide anion free radical(O2-),the relative electric conductivity(REC)and malonaldehyde(MDA)content decreased in response to exogenous melatonin application,whereas an increase in glutamine synthetase(GS)activity,water and soluble protein contents was observed.Moreover,exogenous melatonin application also increased the endogenous melatonin level,expression levels of tryptophan decarboxylase gene(TDC),superoxide dismutase(SOD)and catalase(CAT)activities and the ascorbate-glutathione(AsA-GSH)cycle.(3)In this study,the microstructure of the leaf surface was observed.Under darkness,the cuticle of the leaf epidermis in MTl.O-treated leaves was still obvious and corrugated,whereas its texture in the control was flat.Regarding the density,length,width and opening degree of stomata,there were no obvious differences.Under darkness,MT1.0-treated leaves had more intact mesophyll cell ultrastructures and more chloroplasts than the control.Furthermore,the chloroplasts had a more rounded shape and more lipid spheres were enlarged in the control leaves.The membranes of some chloroplasts in the control appeared to be diffuse,not intact,leading to leakage of the grana lamellae.(4)Total RNAs extracted from the leaves of the control and 24-day MT1.0 treatment under darkness for transcriptome sequencing.After removal of the low-quality reads,the other reads were assembled into a total of 94812 high-quality unigenes with an average length of 1289 nt.Transcriptome sequencing(RNA-Seq)revealed that various biological processes responded to exogenous melatonin application,including carbohydrate metabolism,amino acid metabolism,lipid metabolism,plant hormone signal transduction and pigment biosynthesis.In our study,exogenous melatonin resulted in the up-regulation of genes in carbohydrate metabolism and amino acid metabolism,whereas the down-regulation of genes in lipid metabolism and pigment biosynthesis in gardenia leaves.Genes in plant hormone signal transduction are partly up-regulated and down-regulated.