Alcohol dehydrogenase expression and alcohol production during pear ripening

Abstract

Regulation of alcohol dehydrogenase (ADH), activity of pyruvate decarboxylase (PDC) and accumulation of acetaldehyde and ethanol in 'Packham's Triumph' pears (Pyrus communis, L.) subsequent to different storage regimes were investigated. Pears were stored for two months at -1 °C either in air (Air) or under hypoxia at 3 kPa O2 (Hyp) and subsequently warmed and allowed to ripen in air at 20 °C. One set of fruit stored in air at -1 °C was subjected to 3 days of hypoxia at -1 °C (Air+Hyp) before ripening in air. Acetaldehyde, ethanol and methanol levels increased in all fruit in a similar fashion during ripening and did not reflect differences in storage treatments. During ripening, ADH activities in posthypoxic samples were generally twice that of air samples. PDC activities increased for ≃6 days during ripening then declined slightly but did not differ significantly among treatments. Upon transfer to 20 °C in air, slightly higher levels of Adh mRNA were observed in samples treated with hypoxia than in air controls. Over the following 2 days at 20 °C, the Adh transcription was markedly induced in Air and Air+Hyp samples. Although all Adh mRNAs returned to control levels within 4 days, ADH activities remained higher in hypoxia-treated fruit than in controls for up to 18 days. These results suggest that, in ripening pears, ADH does not limit ethanol production, and that the expression of this enzyme comprises post-transcriptional regulations. GenBank accession numbers of the Adh cDNAs are AFO 31899 and AFO 31900.