Regulatory Function Of Exogenous Melatonin On Fruit Development,Postharvest Fruit Quality And Ring Rot Disease Resistance In Pears

Pear originated in China,and in recent years,the area and yield of cultivation both are the first in the world.As a plant growth regulator,melatonin has multiple effects in plants.However,there are few reports about the effect of melatonin on the development and postharvest storage of pears.In this study,pear cultivars‘Starkrimson'?Pyrus communis L.?,‘Zaosu'?P.bretschneideri Rehd.?,‘AbbéFetel'?P.communis L.?,‘Red Anjou'?P.communis L.?and‘Korla'?P.sinkiangensis T.T.Yu?were used to study the regulation functions of melatonin on pear fructification development,postharvest storage and postharvest ring rot.The mechanism of action was also discussed.It provides theoretical basis and technical support for improving fruit setting and maintaining postharvest quality of melatonin.The main research results are as follows:1.Exogenous melatonin induced pear parthenocarpy.Pear cultivar‘Starkrimson'was taken as the material to study the effect of exogenous melatonin on pear parthenocarpy during flowering phase.It was found melatonin could induce pear parthenocarpy with formation of seedless fruit.In the early stage of fruit development,melatonin increased the division and expansion of mesocarp cells,and promoted fruit development.Melatonin induced the accumulation of GA₃ in ovary,which caused the fruit type index to increase and the fruit type to become more slender at fruit ripening stage.But the content of soluble solids and organic acids did not change.Using the gibberellin inhibitor paclobutrazol?PAC?,the relationship between melatonin and gibberellin was further investigated in the process of melatonin inducing pear parthenocarpy.It was found that after PAC pre-treatment,spraying melatonin could not cause seedless fruit,which indicated that melatonin could induce pear parthenocarpy through gibberellin pathway.Through transcriptome analysis,we found melatonin regulated PbGA20ox and PbGA2ox,the important genes in the process of gibberellin synthesis.Meanwhile,melatonin also regulated cyclin and expansion genes,and thereby promotes the division and expansion of mesocarp cells to eventually form seedless fruits.2.Exogenous melatonin regulated the fruit development.After irrigating‘Zaosu'pear with 0.1 mM melatonin,the content of soluble solids increased significantly to 13.82%during fruit development.The most significant change was the content of sucrose and sorbitol,which reached 6.24 mg g^-1 and 44.67 mg g^-1,respectively.At the same time,melatonin significantly increased the fruit weight,and the fruit weight reached 202.39 g.Melatonin can further regulate invertase,sucrose synthase and sucrose phosphate synthase activity by regulating the expression of sucrose-related synthase coding genes,so as to increase the sucrose and sorbitol content in fruits.3.Exogenous melatonin delayed postharvest softening and senescence of fruits.Using European pear cultivars‘Starkrimson',‘AbbéFetel'and‘Red Anjou'as materials,pear fruits were soaked with 0.1 mM melatonin after harvest to study the mechanism of melatonin delaying fruit senescence.During postharvest storage,melatonin could regulate the activities of antioxidant enzymes SOD,POD and DHAR,improve the antioxidant capacity of pear fruits,balance the released amount of ethylene,and inhibit the respiration rate,which finally delayed the senescence of fruits.After NO donor and inhibitor treatment,it was found that melatonin delayed the senescence of pear fruit by NO synthesis pathway.After inhibiting the synthesis of NO and then treating with melatonin,the regulation effect of melatonin on ethylene release and respiration rate was weakened,which indicated that NO was not the only downstream regulator of melatonin during maturation and softening.MT also increased the activity of nitric oxide synthase by up-regulating the expression of PbNOS,thus leading to the increase of NO content.During the softening process,NO may also participate in the removal of reactive oxygen species.4.Exogenous melatonin affected aromatic substance of pear fruit after harvest.Taking pear cultivars‘Korla'and‘AbbéFetel'as materials,the effect of melatonin on aroma components of postharvest fruits was studied.The results indicated that the main aroma components of‘Korla'and‘AbbéFetel'pears were C₆ aldehydes,alcohols and esters.In‘Korla'pears,melatonin down-regulated the expression of PbHPL and inhibited the activity of hydroperoxide lyase.While in‘AbbéFetel'pears,melatonin down-regulated the expression of PbLOX1 and PbLOX2,inhibited lipoxygenase activity,and promoted the activity of ethanol dehydrogenase and ethanol acyltransferase.Both in‘Korla'and‘AbbéFetel'pears,the contents of hexanal and hexenal in the fruit were reduced with melatonin treatment.5.Exogenous melatonin improved the resistance of pear fruits to ring rot.Pear cultivar‘Zaosu'was used to study the effect of melatonin on ring rot of pear fruits.Melatonin could increase the activity of antioxidant enzymes,such as CAT,SOD and POD,to remove the burst of reactive oxygen species caused by fungal infection,and increase the content of disease-resistant signaling substances salicylic acid?SA?and jasmonic acid?JA?.It was found JA acted on the downstream of melatonin by the synthesis inhibitor DIECA.After DIECA treatment,the content of JA decreased,so melatonin could not improve the disease resistance of fruits.Homologous analysis was used to find the PbOPR3 gene in pear that was homologous to Arabidopsis thaliana and could promote jasmonic acid synthesis.After melatonin treatment,the expression of PbOPR3 was significantly up-regulated.The promoter activation experiment verified that melatonin could activate the PbOPR3transcription.Therefore,melatonin promoted the synthesis of jasmonic acid by activating the PbOPR3 promoter.Finally the content of jasmonic acid was increased,and thus the disease ring rot resistance of fruits was improved.