| Litch is one of the most important tropical fruit trees. However, pericarp browning reduces the commercial value and shelf-life of postharvest litchi fruit, which is one of the main factors limiting the development of litchi industry. Research on postharvest physiology proved that enzymatic browning was the major reason for the pericarp browning and polyphenol oxidase played a main role in the process. But there is still no report on the molecular biology of litchi polyphenol oxidase. Cloning litchi PPO gene and exploring the relationship among pericarp browning, PPO enzymatic activity and gene expression could provide the basis for further revealing pericarp browning mechanism and developing postharvest preserving technology.Storabilities of several litchi varieties were different.The results showed that for storage abilitity, Ziniangxi was the best and Wuhe was the weakest.Ziniangxi had lower PPO,POD activity, browning index, increase rate, pH value, total polyphenol content in pericarp than Wuhe.Bioinformatics analysis on the PPO gene Revealed that the cDNA was2091bp in length containing an 1800 bp open reading frame (ORF), encoding 599 aminoacid residues. The sequence of aminoacid contains three conservative structure domain,the protein is hydrophilic and contains more Ala and Thr.They were highly homologous with Ricinus communis, Populus trichocarpa, Populus balsamifera, Ziziphus jujube and so on. By Comparing of sequences of the gDNA, cDNA and deduced amino acid sequences of PPO genes from the three cultivars, we found that PPO genes contain introns, PPO gene of three cultivars had high identities, but some single nucleotide polymorphisms (SNPS) were identified, resulting in the difference of some individual aminoacids in duduced aminoacid sequences. These SNPs and the resulted difference of aminoacid sequence may be contribute to the variance of browning in three varieties.Southern blot analysis shows that the litch PPO gene is single copy gene. Real-Time Fluorescent Quantitative PCR analysis shows that, PPO gene had differential expression pattern in different litchi tissues. The expression level was highest in flowers,and lowest in pulp. With the fruit development, the gene expression of PPO was low and changed gently, then expression increased rapidly after postharvest. With the storage time extended, expression of PPO gene from each cultivars jncreased with pericarp browning index and then all decreased at the late stage of storage. Ziniangxi had lower gene expression level in pericarp than Wuhe. Control water loss and low temperature storage reduced PPO gene expression, and fungal infection can improve PPO gene expression.Changes of browning index, gene expression and enzyme activity of PPO showed positive correlation in different varieties. With the storage time extended, pericarp browning index increased. Gene expression and enzyme activity of PPO in pericarp of different cultivars were all first increased and then decreased. Ziniangxi had lower gene express quantity and enzyme activity in pericarp than Wuhe,Changes of browning index, PPO gene expression and enzyme activity in pericarp of fruits suffered different postharvest treatment were positive correlated With the storage time extended, pericarp browning index increased. Gene expression and enzyme activity of PPO in perciarps of different cultivars under different storage conditions were all first increased and then decreased. The gene expression and enzyme activity under control water loss were lower than rapid dehydrate; the variation of gene expression and enzyme activity under low temperature storage were gentler than normal, browning was significantly more longer. The PPO enzymatic activity and gene expression quantity sharply increased afterAnthrax infection.To sum up, with the storage time extended, pericarp browning index increased, pericarp browing was related with PPO gene express quantity and enzyme activity. The ability of storabilities of different varieties was related with diversity of PPO express quantity. |
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