Changes In Cell Wall Substances Metabolism During Ripening Of ‘Starkrimson’ Pear Fruits And Screening Analyses Of The Related Genes

To better characterize the mechanism regulating the rapid softening of ‘Starkrimson’fruits, non-softening ‘Dangshansuli’ and softening ‘Starkrimson’, ‘Bartlett’ and ‘Abbe Fetel’pears were sampled, differences in changes of flesh firmness, cell wall pectin and cellulose contents and expression of the cell wall degrading genes among the four cultivars were compared during fruits storage at room temperature, and the key cell wall substance and degrading genes related to the rapid softening of ‘Starkrimson’ fruits were hoped to find out through both physiological and molecular perspectives. In addition, effects of 1-methylcyclopropene(1-MCP) on fruit ripening of ‘Starkrimson’ pears were analyzed. The main results are as follows:1. No significant changes in flesh firmness and cell wall pectin and cellulose contents were observed in the non-softening ‘Dangshansuli’ fruits, and water-soluble pectin(WSP)were increased and acid-soluble pectin(ASP) were decreased along with the flesh firmness loss in the three softening pear cultivars-‘Starkrimson’, ‘Bartlett’ and ‘Abbe Fetel’, while the change patterns of cellulose varied from each cultivar, suggesting that pectin degradation may play a vital role in pear flesh firmness loss and fruit softening; furthermore, the degree of pectin degrading was more obvious in the rapid-softened ‘Starkrimson’ fruits.2. Twenty-six genes including five pectin methylesterases(PMEs), six polygalacturonases(PGs), eight cellulases(Cels), six beta-galactosidases(?-GALs) and one alpha-L-arabinofuranosidase(?-L-ARF) were screened when using the protein family domains and the nucleotide or amino acid sequences of the homologous genes as probes to search in the referential Pear Genome Database, and fifteen genes including four PMEs, two PGs, four Cels, four ?-GALs and one ?-L-ARF were finally identified in the pear flesh tissue.And the phylogenetic analyses show that each gene are close to the homologous genes that have been identified in tomato, apple, peach, strawberry, grape and also other pear cultivars.3. Expression analyses of the fifteen genes showed that, each gene were differentially expressed in the four pear cultivars, and they played different roles in each cultivar. Among these genes, PG2, one member of the PG gene family, exhibited the significant changes in expression between the different pear cultivars with different softening characteristics, whilethe other member of PG gene family, PG1 was only highly expressed in ‘Starkrimson’ fruits.All of these results suggesting that PG genes, especially PG2, play a crucial role in cell wall pectin degradation and softening of pear fruits, and PG1 has a considerable role in the much bigger softening rates of ‘Starkrimson’ fruits than those of ‘Bartlett’ and ‘Abbe Fetel’.4. 1-MCP can delay pectin degradation and flesh firmness loss and thus prolonged the shelf-life when applied in ‘Starkrimson’ fruits. Gene expression analyses reveal that accumulation of the main pectin degrading genes-PG1 and PG2 can be inhibited by 1-MCP,with PG2 was inhibited significantly.