Differential expression and ethylene regulation of β-galactosidase genes and isozymes isolated from avocado ( Persea americana Mill.) fruit

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β-Galactosidases (EC 3.2.1.23; β-Gals) consist of several isoforms which have different activity levels against native and synthetic substrates and play an important role in cell wall metabolism during fruit growth and ripening. In this study, we isolated three new β-Gal cDNA clones, PaGAL2, PaGAL3 and PaGAL4, from the fruit of ripening avocado in addition to the AV-GAL1 clone previously obtained. The expression patterns of these genes during fruit ripening were quite different. The AV-GAL1 transcript, which was solely found in the fruit, accumulated with fruit ripening. PaGAL2 transcript, which was detected in leaves, shoots, roots and fruit, showed a constant level throughout fruit ripening. The level of PaGAL3 transcript in control fruit, which was not detected in root but only in other tissues, increased markedly at 2 days after treatment (DAT) (air treatment) and dropped quickly at 4 DAT in fruit. The transcript was not detectable at 6 DAT and thereafter. The PaGAL4 transcript was detected in all tissues except for the fruit. In order to investigate the role of ethylene, on the regulation of β-Gal expression, pre-ripe fruit were treated with either ethylene or its inhibitor 1-methylcyclopropene (1-MCP). Exogenous ethylene promoted AV-GAL1 expression but severely suppressed PaGAL3 expression. Ethylene also affected the activities of fractionated β-Gal isozymes in a differential manner. Among the three isozymes, the increase in AV-GAL III activity with fruit softening were promoted by exogenous ethylene and delayed by 1-MCP. However, no apparent changes in the activities were observed in the other two isozymes. Based on the results obtained, it seems that AV-GAL1, which may encode AV-GAL III, is important for postharvest fruit softening while PaGAL2, PaGAL3 and PaGAL4 may be involved in galactose metabolism of cells or cell walls during development and ripening.