Differential screening of a cDNA library made from RNA extracted from avocado (Persea americana
Mill cv. Hass) fruit stored at low temperature (7 °C) gave 23 cDNA clones grouped into 10 families,
6 of which showed increased expression during cold storage and normal ripening. Partial DNA sequencing
was carried out for representative clones. Database searches found homologies with a polygalacturonase
(PG), endochitinase, cysteine proteinase inhibitor and several stress-related proteins. No
homologies were detected for clones from six families and their biological role remains to be elucidated.
A full-length cDNA sequence for avocado PG was obtained and the predicted amino acid sequence
compared with those from other PGs. mRNA encoding PG increased markedly during normal ripening,
slightly later than mRNAs for cellulase and ethylene-forming enzyme (EFE). Low-temperature
storage delayed ripening and retarded the appearance of mRNAs for enzymes known to be involved in
cell wall metabolism and ethylene synthesis, such as cellulase, PG and EFE, and also other mRNAs
of unknown function. The removal of ethylene from the atmosphere surrounding stored fruit delayed the
appearance of the mRNAs encoding cellulase and PG more than the cold storage itself, although it hardly
affected the expression of the EFE mRNA or the accumulation of mRNAs homologous to some other
unidentified clones.
