Exogenous Melatonin Treatment Positively Affects Ripening And Storage Behaviour Of Apple Fruit

Apple is a widely grown tree fruit with delicious taste and high nutritive value.Cultivation and daily uptake of this fruit was increasing day-by-day all over the world.Successful and profitable business can be assured only by the proper storage of apple after harvesting.Cold-storage,Controlled Atmosphere?CA?storage and chemical treatment e.g.1-MCP are popularly used to prolong the storage life and quality of apple fruits.Most important physiological change upon ripening is climacteric ethylene burst in apple fruit.Controlling the ethylene production can reduce firmness loss,rate of metabolic activities and ultimate postharvest senescence and decay of fruit.However,other plant hormones related to the ripening circle is still indistinct.Modification of ripening shift is primarily depending on understanding of ripening behavior to extend the period from commercial to physiological ripening stage.Transcriptome analysis was carried out in apple fruit to clarify the ripening regulation of apple by ripening hormones rather than ethylene.Results revealed 9187 differentially expressed genes?DEGs?in the post-ripening?HY₂0^oC₅ vs HY₄th?samples and comparatively lower DEGs?922 DEGs?in the pre-ripening?HY₄th vs HY₃rd?fruit.Crucial involvement of ABA and auxin signaling genes were identified from the DEG analysis.Furthermore,several transcription factors were also up/down regulated,which may positively or negatively regulate apple fruit ripening.'Fuji'apple fruit is very popular for taste and color,but unfortunately this fruit is severely susceptible to cold-storage disorder and CO₂ injury even at CA storage.Postharvest development of peel injury symptom causes devastative loss of market quality of apple fruit.Use of several treatments have been generously studied previously to eliminate the storage disorder and retardation of climacteric burst by reducing the ethylene production.Most widely practiced 1-MCP treatment in the apple storage also developed scald symptoms upon cold storage and reduced eating quality in some apple cultivars.As a prominent antioxidant inducer in postharvest produce,melatonin is a mostly studied molecule.Melatonin is an indoleamine produced by the pineal gland which has minimal toxicity for human health in very wide range.Combined effects of melatonin in stress tolerance,defense response development and hormonal regulations have been observed in the plant system.Depending on several prospective characteristics of melatonin application in plant system and convenient application process,it has been selected as a treatment in this study.Taken into consideration of this defending behavior of Fuji apple,this fruit at commercial maturity stage was used as sample materials.Fruit were treated with different concentration of melatonin solution,including MT0.1(0.1mmol L^-1 melatonin),MT1.0(1.0 mmol L^-1melatonin),MT10(10 mmol L^-1 melatonin)and controlled?ddH₂O?fruit.The fruit were stored at 1°C and 90-95%relative humidity for 56 days.After 28 days of storage,control fruit developed sunken black necrotic sports on the peel whereas melatonin treated fruit maintained better peel appearance.Among the different melatonin treatment,the fruit treated with 1.0 mmol L^-1 melatonin solution showed significantly reduced weight loss and ethylene production compared to the control and other treatment groups.Moreover,the fruit peel quality was comparatively better than other treatment groups up to 56days of storage.To evaluate the metabolic shift among the control and MT1.0 fruit,the samples of 0,28 and 56days were subjected to UPLC-Q-TOF-MS based metabolites quantification analysis.Metabolome analysis revealed that exogenous melatonin led to elevated endogenous MT level in melatonin treated fruit up to 26-fold,indicated probable influence of melatonin in stored fruit quality retention.Several other metabolites group such as sugars,organic acids,amino acids,sugar alcohols and phenols were also regulated by melatonin treatment,which indicated possible involvement of these chemicals in cold-storage of'Fuji'apple.Furthermore,RNA-Seq based transcriptome analysis was also carried out to elucidated the genomic changes in melatonin treated fruit?MT1.0 mM?compared to the control.The reduced ethylene production through regulation of the expression of MdACO1,MdACS1,AP2.4 and ERF109 were exposed by RNA-Seq analysis and validated by quantitative RT-PCR.In addition,the antioxidant systems in'Fuji'apple were enhanced by melatonin treatment by increasing the activity of peroxidase?POD?,superoxide dismutase?SOD?,catalase?CAT?and phenylalanine ammonia lyase?PAL?enzymes.Taken together,this study highlights the inhibitory effect of melatonin in ethylene biosynthesis and occurrence of peel injury symptoms,and indicated influencing factor in postharvest fruit ripening and quality of'Fuji'apple.However,optimization of treatment method,concentration,maturity stages and storage conditions are necessary for successful implementation in commercial industry.