Mechanism Of Oxalic Acid On Inducing Cold Resistance In Postharvest Hami Melon Fruit

Hami melon(Cucumis melo L.)is an economically important crop in Xinjiang province in northwestern China with a delicious flavor and crispy and juicy flesh.It ripens and ages rapidly postharvest during ambient temperature storage.Cold storage can extend the storage period of Hami melon fruit.However,it is sensitive to low temperatures and therefore susceptible to chilling injury(CI).Some methods for alleviating CI in cold-stored Hami melons have been developed.But oxalic acid(OA)as a non-biological inducer to enhance the cold tolerance of fruit during low temperature has not been studied clearly,and the mechanism of regulation is limited known.To exclude the influence of OA on fruit chilling injury.Postharvest Hami melon fruit were treated with exogenous OA.The fruit cold resistance process through the RNA-Seq research methods and the metabolisms of antioxidant,membrane lipid and proline were investigated to interpret the function of OA on fruit cold tolerance.The results were as follows:1.'Xi zhou mi 25 hao' Hami melon(Cucumis melo var.reticulatus Naud.)as raw material,selection of appropriate concentration of OA processing mode.The results showed that 10,15 and 20 mmol/L OA treatments all inhibited the decrease of fruit hardness and vitamin C content during the whole storage period compared with the control group,but the effect of OA treatment of 15 mmol/L was more significant.At day 42,the respiration rate,electrolyte eleakage and CI index of fruits treated with 15 mmol/L OA were low,which were 6.05 mg · Kg · h-1,13.72%and 0.55,respectively.Therefore,15 mmol/L treatment was selected as an ideal treatment model.2.Fruits were treated with exogenous OA and analyzed the pericarp using RNA-seq.The results showed that OA treatment upregulated the related genes of ROS scavenging pathway(3 POD genes,I MDHAR gene,2 SOD genes,1 APX gene and 1 GR gene),and decreased the ROS stress and maintained its balance.The transcriptional levels of 3 LOX genes and 1 PLD gene in the membrane lipid oxidation pathway were decreased,and the peroxidation of membrane lipid was inhibited.It also had an impact on the related genes of proline synthesis pathway.1 P5CS gene and 1 P5CR gene were increased,the transcription level of 1 ProDH gene was reduced,but the transcription level of GDH gene had less effect.OA treatment up-regulated the gene transcription level of 1 SPS,1 SuSy and 1 Glu gene in the soluble sugar accumulation pathway,and had no effect on other genes.3.OA treatment increased the activity of APX,MDHAR and DHAR in pericarp of Hami melon at the early cold storage,and increased the activity of POD,GR and SOD at the middle cold storage,and genes expression of CmGR,CmAPX and CmPOD were enhanced.The content of AsA and GSH and the ratio of AsA/DHA and GSH/GSSG in OA treated-fruits pericarp were higher than that of the control group,OA reduced MDA content,membrane permeability,H2O2 and O2-production,and regulated the balance of ROS metabolism,and alleviated fruit chilling injury.4.Compared with the control group,OA treatment decreased the activity of LOX,PLD and genes expression of CmLOX and CmPLD in pericarp of firuits.Decreased the relative content of palmitic acid(C16:0)and stearic acid(C18:0)(except 35 days),and improved the relative content of skin linoleic acid(C18:2),linolenic acid(C18:3)and arachidonic acid(C20:4),and inhibited the decrease of relative content of oleic acid(C18:1).Which reduced the degradation of unsaturated fatty acids of membrane lipids and maintained higher unsaturated degree of fatty acids,alleviating the physiological effects of chilling injury.5.OA treatment induced the relative expression of transcription factor Cm-CBF1 shortly which was related to the osmotic regulation substances,and improved the increase of P5CS activity and gene expression of CmP5CS during the cold storage.We speculated that the transcription factor Cm-CBF1 may participate in proline metabolism regulation by activating CmP5CS gene expression in OA-treated Hami melon,meanwhile OA treatment promoted the decrease of ProDH activity and gene expression of CmProDH.The content of proline was accumulated to a certain extent,which reduced the sensitivity of Hami melon to low temperature.