Study On The Role Of Melatonin In Controlling Chilling Injury And Related Mechanism In Mango Fruit

Mango（Mangifera indica L.）belongs to the fruit of mango tree of the sumac family,and is widely planted in tropical and subtropical areas due to its important economic status.Mangoes are typical respiratory climacteric fruit that undergo fast ripening and senescence after harvest,severely restriciting the circulation of products.Low temperature storage is one of the common methods to prolong the shelf life of fruits.However,mango fruit are prone to develop chilling injury（CI）during refrigeration due to sensitivity to low temperature.The symptoms of CI become serious after removal of the fruit from cold conditions,resulting in quality deterioration and ripening failure,which lead to huge loss of products and cause unfavorable impact on industrial development.Cell membrane is the main site for plant cells to respond to the external environment.Low temperature can cause damage to cell membrane.Therefore,maintaining the stability and fluidity of cell membrane can improve the capactity of plants to resist low temperature stress.In addition,ripening ability may be lost in some fruits（e.g.mango,banana and tomato,etc.）when suffering CI,so promoting riepening-related physiological metabolism would be helpful to reduce the fruit CI symptoms.Melatonin（MT）has been recognized as a vital biomolecule to regulate a variety of physiological processes in plants.In recent years,the application of MT on preservation in fruits and vegetables has been widely reported.The effect of MT on suppression of CI in various fruits and vegetables（including mango）has attracted much attention.However,there is no report on the role of MT in regulating membrane lipid components during refrigeration in mango fruit,and is also a lack of information about effect of MT on ripening during shelf life after removal of refrigeration.Therefore,in this study,we employed physio-biochemical and lipidomic analyses to investigate the effects of exogenous MT on lipid metabolic enzymes and lipid molecule profiles in‘Guifei’mango fruit during refrigeration.Then,the effects of MT on ripening and softening and related mechanism in chilled‘Guifei’mango fruit during shelf life after low temperature storage were studied.The main results are shown as follow:1.The effect of exogenous MT treatment on CI in mango fruit（1）Compared to the control fruit,0.5 mmol L^-1 MT treatment effectively inhibited the increases in CI index and chromaticity a*value,while maintaining a higher chromaticity L*value and Fv/Fm in mango fruit during refrigeration.（2）Exogenous MT treatment inhibited the increases in relative electrical conductivity（RC）and malondialdehyde（MDA）content,decreased the activities of phospholipase D（PLD）,phospholipase C（PLC）and lipoxygenase（LOX）,while increasing the activity of phospholipase A₂（PLA₂）during cold storage,thereby preventing phospholipid degradation,reducing membrane lipid peroxidation and maintaining the degree of membrane lipid unsaturation.（3）Exogenous MT treatment alleviated the symptoms of CI in mango fruit by affecting membrane lipid metabolism.To elucidate the relationship between cell membrane lipid composition and CI,the changes of lipid molecular abundance values of different types of lipids（phospholipids,lysophospholipids,sphingolipids and triglycerides）under the action of MT were identified using lipomic analysis technology.These results indicate that MT could trigger remodeling of lipid molecules by regulating membrane lipid metabolism,which would be conducive to maintaining membrane structure and activating downstream signaling pathways,contributing to improved cold tolerance in mango fruit.2.The effect of exogenous MT in ripening and softening in mango fruit after refrigeration（1）Exogenous MT(0.5 mmol L^-1)treatment markedly decreased CI index,while enhancing soluble solids content（SSC）,respiration rate and ethylene biosynthesis,while accelerating the decline in hue angle and firmness in chilled mango fruit during shelf life after refrigeration.（2）Compared to the control fruit,exogenous MT treatment promoted the increases in contents of water-soluble pectin（WSP）and CDTA-soluble pectin（CSP）during shelf life in mango fruit after refrigeration.Moreover,MT treatment promoted the increase in activities of polygalacturonase（PG）,β-galactosidase（β-Gal）,endo-1,4-β-D-glucanase（EGase）and the decrease in pectin methylesterase（PME）activity,thereby maintaining cell wall metabolism in chilled mango fruit after refrigeration.（3）Exogenous MT treatment enhanced the ethylene production,which was attributed to MT-induced increases in activities of 1-aminocyclopropane-1-carboxylic acid（ACC）oxidase（ACS）and ACC oxidase（ACO）and the expression of corresponding encoding genes（Mi ACS and Mi ACO）.MT up-regulated the expression of Mi ETR1,Mi ERS1,Mi ERF1and Mi EIN2 while down-regulating the expression of Mi CTR1,thus promoting ethylene signal transduction and accelerating the ethtlene-dependent ripening process in mango fruit during shelf life after refrigeration.In addition,MT treatment promoted the expression of Mi CBF1 and Mi ICE1,which indicated exogenous MT treatment could improve the chilling tolerance of mango fruit via regulating CBF signalling pathway.