Cloning two members of PG gene family from precocious sandy pear Cuiguan and the relationship between their expression and fruit softening during shelf life

Abstract：PpPG1 and PpPG2, two numbers of the polygalacturonase gene family were cloned from preco- cious sandy pear Cuiguan （Pyrus pyrifolia Nakai.） fruit by RT-PCR and RACE methods. The results showed that PpPG1 cDNA sequence length is 1 793 bp, which has an open reading frame from 287 to 1 666 nucleotides and encodes a polypeptide including 459 residues. At the same time, PpPG1 DNA se- quence consists of eight exons and seven introns. Predicted isoelectric point （pI） and relative molecular mass of PpPG1 protein were 8.47 and 49.45 ku, respectively. PpPG2 cDNA sequence length is 1 671 bp, which has an open reading frame from 120 to 1 316 nucleotides and encodes a polypeptide including 398 residues. At the same time, PpPG2 DNA sequence consists of three exons and two introns. Predicted iso- electric point （pI） and relative molecular mass of PpPG2 protein were 7.34 and 41.14 ku, respectively. Both PpEG1 and PpEG2 had glucoside hydrolase glyco-hydro-28 superfamily and polygalacturonase conserved domains. The phylogenetic tree analysis showed that PpPG1 and PpPG2 located at two different clades. The semi-quantitative RT-PCR results showed that both PpPG1 and PpPG2 gene expression abun- dance increased and then decreased during summer shelf life （28 to 32 ~C ）. Their expression peaks ap- peared at 16 and 8 days after harvest, respectively. After 1-MCP treatment, PpPG1 expression at postharvest 8 days was inhibited but its expression pattern was not affected. However, PpPG2 expression peak was delayed to postharvest 16 days but its expression abundant was not changed. The fruit characteristics changed after 1-MCP treatment, such as polygalacturonase activity increase speed alleviated, ethylene re- lease rate decreased and fruit firmness decline speed slowed down. It was concluded that PpPG1 and PpPG2 were different numbers of polygalacturonase gene family and their expressions were regulated by ethylene. Both of them played roles in Cuiguan fruit softening during summer shelf life （28 to 32℃ ）.