Study On The Effect Of Melatonin On The Preservation Of Fresh-cut Lotus Roots

Lotus root(Nelumbo nucifera Gaertn.)is rich in nutrients and has high ornamental and edible value.Freshly cut lotus root is a convenient food that is hygienic and up to 100%edible.Browning of fresh-cut lotus roots during processing and preservation is a constraint to the healthy development of lotus root industry.Therefore,it is important to develop fresh-cut lotus root preservation technologies that can inhibit browning,effectively maintain the quality of fresh-cut lotus roots and are easy to operate.Melatonin is a key bioactive molecule that is involved in regulating various physiological processes in plants.New studies have shown that melatonin can be used as a novel preservation agent for fruits and vegetables to inhibit fruit decay.In this study,0.15 mmol/L melatonin was used to treat freshly cut lotus roots for 10 min,with distilled water as the control.The treated lotus root slices were drained and placed in plastic sealing bags in a refrigerator at 4℃ for 14 days.The aim was to investigate the effect of exogenous melatonin on the browning of freshly cut lotus roots during 14 days of storage and to reveal its regulatory mechanism for inhibiting browning.The effect of melatonin on the preservation of fresh-cut lotus roots was studied from the perspectives of quality and physiological and biochemical effects.The melatonin treatment reduced MDA,H2O2 and enhanced the catalytic activities of ROS scavenging enzymes such as SOD,APX and DHAR,and reduced the activities of PPO and POD compared with the control.In addition,melatonin treatment also had an effect on the flavonoid content of fresh-cut lotus roots.Compared with the control group,melatonin treatment significantly reduced the flavonoid content of fresh-cut lotus roots and inhibited the catalytic activity of4 CL,CHI and PAL,which are key enzymes in the flavonoid biosynthesis pathway.We screened key exogenous melatonin-regulated genes by RNA-seq to analyze their molecular mechanisms of inhibiting browning in fresh-cut lotus roots.We selected a total of15 samples from control and melatonin-treated groups at 0 day,8 day and 14 day,and produced 89.15 Gb of data to screen for differentially expressed genes(DEGs).The differentially expressed genes identified included 857 DEGs(384 up-regulated and 473down-regulated)for MT8 versus CK8 and 862 DEGs for MT14 versus CK14(433up-regulated,429 down-regulated).To determine the function of DEGs,we performed an enrichment analysis of the Kyoto Encyclopedia of Genes and Genomes(KEGG).Phenylpropanoid biosynthesis,flavonoid biosynthesis,and glutathione metabolic pathways were significantly enriched in the MT8 versus CK8 comparison group.RNA-seq showed that DEGs involved in the phenylpropanoid pathway,which were downregulated in MT8 versus CK8 or MT14 versus CK14,included PAL,POD,4CL,COMT,and HCT.To determine the effect of melatonin treatment on flavonoid biosynthesis We performed a heat map analysis of differential gene expression and found that melatonin treatment significantly reduced the expression of CHS in fresh-cut lotus roots.In addition,CHI expression was significantly decreased at MT8 compared to CK8.DFR expression was also significantly suppressed in the melatonin-treated group at 8 day,while there was no change at 14 day.A similar expression profile was detected in PPO,with a decrease in expression in the melatonin-treated group on 8 day and no change in both groups on 14 day.The decreased expression of PAL,CHI,4CL and PPO genes in melatonin-treated lotus roots is consistent with their down-regulation of enzymatic activity after storage.However,genes encoding SOD,APX and DHAR did not differ significantly in transcript levels between the melatonin-treated and untreated groups.the relevant differences in enzymatic activities of SOD,APX and DHAR may be attributed to post-transcriptional regulatory effects.Melatonin treatment was able to promote the expression of transcription factors MYB5,MYB6 and MYB308.Among them,gene co-expression analysis(GCN)showed that upregulated MYBs transcription factors were negatively correlated with the expression of PPO and related genes involved in flavonoid biosynthesis and phenylalanine biosynthesis,and positively correlated with the expression of genes related to glutathione metabolism.Our study shows that melatonin delays browning of fresh-cut lotus roots by activating MYB5,MYB6 and MYB308 expression and suppressing the expression of key genes for flavonoid and phenylalanine biosynthesis.