Suppression of Ripening-Associated Gene Expression in Tomato Fruits Subjected to a High CO2 Concentration

High concentrations of CO2 block or delay the ripening of fruits. In this study we investigated the effects of high CO2 on ripening and on the expression of stress- and ripening-inducible genes in cherry tomato (Lycopersicon esculentum Mill.) fruit. Mature-green tomato fruits were submitted to a high CO2 concentration (20%) for 3 d and then transferred to air. These conditions effectively inhibited ripening-associated color changes and ethylene production, and reduced the protein content. No clear-cut effect was observed on the expression of two proteolysis-related genes, encoding polyubiquitin and ubiquitin-conjugating enzyme E2, respectively. Exposure of fruit to high CO2 also resulted in the strong induction of two genes encoding stress-related proteins: a ripening-regulated heat-shock protein and glutamate decarboxylase. Induction of these two genes indicated that high CO2 had a stress effect, most likely through cytosolic acidification. In addition, high CO2 blocked the accumulation of mRNAs for genes involved in the main ripening-related changes: ethylene synthesis (1-aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase), color (phytoene synthase), firmness (polygalacturonase), and sugar accumulation (acid invertase). The expression of ripening-specific genes was affected by CO2 regardless of whether their induction was ethylene- or development-dependent. It is proposed that the inhibition of tomato fruit ripening by high CO2 is due, in part, to the suppression of the expression of ripening-associated genes, which is probably related to the stress effect exerted by high CO2.