Expression of ACC Synthase and ACC Oxidase Genes in Melons Harvested at Different Stages of Maturity

Melon (Cucumis melo L.) fruits vary in ethylene production rates, depending on the cultivar, which can affect the storability of the fruits. To understand these differences at both the biochemical and molecular levels, two popular cultivars, 'Andes' and 'Earl's Favourite' were used ; these cultivars produce relatively high and low amounts of ethylene, respectively. Ethylene biosynthetic activity in the peel, mesocarp and placenta of 'Earl's Favourite' fruits harvested at different stages of maturity remained low even at the commercial harvest maturity (CHM), and ethylene production in the intact fruit was also low. In 'Andes', 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) activity and ACC content in the mesocarp and placenta and ethylene production in intact fruit increased drastically at CHM. Northern blot analysis using mRNA extracted from the respective tissues of fruits at different stages of maturity indicated that CMe-ACS1 mRNA accumulated in the mesocarp and placenta of 'Andes' at CHM, whereas in 'Earl's Favourite' there was no accumulation in any of the tissues at all maturities. In ACC oxidase (ACO) genes, mRNA for ACO1 accumulated in the mesocarp and placenta of both cultivars at CHM, and that for ACO2 was almost constitutively expressed. Ethylene, exogenously applied to fruits at the preclimacteric stage, slightly induced CMe-ACS1 mRNA accumulation in 'Andes' but not in 'Earl's Favourite' fruits, whereas wounding induced CMe-ACS1 transcripts accumulation in both cultivars. The accumulation of ACO1 mRNA in the two cultivars was greatly stimulated by ethylene and wounding treatments. When 'Andes' fruit at climacteric stage were treated with 1-methylcyclopropene (MCP), an inhibitor of ethylene action, transcripts for CMe-ACS1, ACO1 and ACO2 were barely detectable. These results suggest that the difference in the ethylene-forming capability between the two cultivars may result from the expression of CMe-ACS1 mRNA during natural fruit ripening and in response to exogenous ethylene.