Study On The Relationship Between Pericarp Browning And The Peroxidase Gene Expression Of Postharvest Litchi

.Litchi is an important fruit tree in south China. The rapid postharvest pericarp browning reduced the shelf-life and commercial value of litchi, and is considered as the main limitation to the development of litchi industry. It has been proved that the enzymatic browning is the main reason for pericarp browning due to the brown substances formed by the oxidation reaction of oxidase on polyphenols in the pericarp. Peroxidase is one of the major oxidases involving in the oxidation of phenols. Cloning the POD gene and exploring the relation between the pericarp browning, POD activity and the gene expression could provide the basis for elucidating the mechanism of pericarp browning and developing the technology for fruit preservation.. However, the relation between pericarp browning and POD gene expression need to be further studied.In this study, fruits of three litchi cultivars, Feizixiao (FZX), Wuhelizhi (WH) and Ziningxi (ZNX) were used to compare their postharvest pericarp browning process and the physiological changes under different treatment. Two POD genes were cloned, and their expression levels were studied. Then the relation between pericarp browning and POD gene expression was discussed.There were differences on the browning process among the three cultivars. Fruit of WH turned brown most rapidly and ZNX turned browning most slowly. Water loss in browned fruit was the highest in WH and the lowest in ZNX. POD activity in the pericarp of all the three cultivars increased firstly, and then decreased. POD activity in WH pericarp had the greatest variation, the highest peak value and the earliest peak occurrence. That in ZXN showed the opposite changes. Controlling loss of water by fruit bagging delayed the pericarp browning of all the three cultivars, and delayed the occurrence of the peak of POD activity for WH and FZX. Stored fruit of the three cultivars at4℃significantly prolonged the storage period, reduced the water loss, weakened the variation of POD activity and lagged the occurrence of its peak.There were131POD genes obtained from the genome of litchi. They were clustered into five categories according to their sequences similarity. We have got2POD segments by sequencing the litchi pericarp cDNA library previously. On these bases, we cloned the full length of the two POD genes. The POD1was1309bp in length with a248bp3’-UTR and a48bp5’-UTR. It had a1062bp ORF, encoding an polypeptide with353amino acid residues. The POD2was1293bp in length with a309bp3’-UTR and a1077bp ORF, which encoded a polypeptide with358amino acid residues. The deduced amino acid sequence of POD1had higher similarity with Hevea brasiliensis, Populus alba and so on, while that of POD2showed higher similarity with Theobroma cacao, Fragaria vesca et al. The expression level of P0D1and POD2were all the lowest in root, but was the highest in young stem for POD1and in leaves for POD2. During the fruit development of all the cultivars, the two POD genes had low expression level and slight change in the pericarp. However, they increased sharply when the fruits were harvested. During the storage, the expression of the two genes increased, firstly, and then decreased, the same as the POD activity. The expression peak of POD genes occurred earliest in WH. The peak of POD gene expression and POD activity occurred on the same day for WH and FZX, while the peak of POD gene expression appeared earlier than POD activity for ZNX.There was no difference on the expression of the two POD genes within three days between the bagging treatment and control when the FZX fruits were stored at room temperature. However, the expression of POD2was reduced on day4and5in the pericarp of bagged fruit. When the fruits were stored at4℃, expression of both of the two POD genes were decreased at the early stage of postharvest storage. The expression peak of POD2was also delayed by low temperature. Thus, POD2should have more close relationship with pericarp browning.In a summary, the expression of POD gene had close relationship with pericarp browning. At the early stage of postharvest storage, the up-regulated expression of POD genes may enhance the POD activity, and then resulted in the rapid increase of browning index.