Expression characteristics of seven members of the β-galactosidase gene family in 'La France' pear (Pyrus communis L.) fruit during growth and their regulation by 1-methylcyclopropene during postharvest ripening

We investigated the role of β-galactosidase (EC 3.2.1.23; β-Gal) in 'La France' pear (Pyrus communis L.) fruit growth and softening at both biochemical and molecular levels. Northern hybridization was carried out using probes prepared from Japanese pear β-galactosidase (PpGAL) cDNA clones, designated PpGAL1, PpGAL2, PpGAL3, PpGAL4, PpGAL5, PpGAL6 and PpGAL7. β-Galactosidase activity and accumulation of the transcripts hybridizing with the PpGAL genes were investigated during fruit growth and in relation to fruit softening and in response to propylene and 1-methylcyclopropene (1-MCP) treatments during postharvest ripening. β-Galactosidase activity was highest in very young fruit, decreased to low levels in later stages of fruit expansion and then increased during postharvest ripening. The increase in β-Gal activity during fruit ripening paralleled the decrease in fruit firmness. Transcripts hybridizing with PpGAL1 and PpGAL4 were not detected during fruit growth but were detected in the ripening fruit. On the contrary, the abundance of transcripts hybridizing with PpGAL2, PpGAL3, PpGAL5, PpGAL6 and PpGAL7 was highest in the early stages of fruit development, decreased towards fruit maturity and except for PpGAL2 were detected in the ripening fruit, albeit at low levels. Propylene treatment resulted in increased ethylene production, decreased fruit firmness and increased β-Gal activity. Besides, propylene stimulated the accumulation of transcripts hybridizing with PpGAL1 and PpGAL4 and suppressed the accumulation of transcripts hybridizing with PpGAL6 and PpGAL7 and had no effect on transcripts hybridizing with PpGAL2, PpGAL3 and PpGAL5. The converse was true for fruit treated with 1-MCP, although the inhibition of the accumulation of transcripts hybridizing with PpGAL1 and PpGAL4 by 1-MCP was not complete. These results indicate that PpGAL1 and PpGAL4 play a crucial role in 'La France' pear fruit softening and the increase in their expression at the onset of fruit ripening is in part due to up-regulation by ethylene. The decrease in transcripts hybridizing with PpGAL6 and PpGAL7 in ripe fruit may in part be due to down-regulation by ethylene whereas that of transcripts hybridizing with PpGAL2, PpGAL3 and PpGAL5 may be due to another mechanism(s) unrelated to ethylene action. The divergent members of the β-Gal gene family in 'La France' pear fruit, therefore, have differential developmental and hormonal regulation characteristics.