Preclimacteric bananas fruits were treated for 12 h with ethylene to induce the climacteric rise in respiration. One day after the end of the hormonal treatment, the two activities of the bifunctional enzyme, phosphofructokinase 2/fructose-2,6-bisphosphatase started to increase to reach fourfold their initial value 6 days later. By contrast, the activities of the pyrophosphate-dependent and of the ATP-dependent 6-phosphofructo-1 -kinases remained constant during the whole experimental period, the first one being fourfold greater than the second. The concentrations of fructose 2,6-bisphosphate and of fructose 1,6-bisphosphate increased in parallel during 4 days and then slowly decreased, the second one being always about 100-fold greater than the first. The change in fructose 2,6-bisphosphate concentration can be partly explained by the rise of the bifunctional enzyme, but also by an early increase in the concentration of fructose 6-phosphate, the substrate of all phosphofructokinases, and also by the decrease in the concentration of glycerate 3-phosphate, a potent inhibitor of phosphofructokinase 2. The burst in fructose 2,6-bisphosphate and the activity of the pyrophosphate-dependent phosphofructokinase, which is in banana the only enzyme known to be sensitive to fructose 2,6-bisphosphate, can explain the well-known increase in fructose 1,6-bisphosphate which occurs during ripening.In ripening fruits, the presence of ethylene has been associated with a large increase in respiratory metabolism, particularly in so-called climacteric fruits in which low concentrations of exogenous ethylene induce autocatalytic ethylene production and accelerate the ripening process [1, 21. During this ethylene-induced burst in respiration, glycolysis is activated as indicated by the 10 -20-fold increase in Fru(l,6)Pz concentration [3-51. The mechanism which allows the increase of the glycolytic flux is not clear; it was therefore of interest to investigate its relationship to Fru(2,6)Pz, a new regulator of glycolysis discovered 6 years ago [6]. Fru(2,6)P2 is a regulatory molecule which promotes glycolysis and restricts gluconeogenesis in very different types of eukaryotic cells (reviewed in [7]). In higher plants, Fru(2,6)Pz activates PP1-PFK and inhibits cytoplasmic FBPase 1 [7]. A dramatic increase in its concentration has been observed in various situations in which a resumption of metabolic activity occurs [8-101.Climacteric fruits, such as banana, are good models to reevaluate the link between ethylene and glycolysis. The aim of the present work was to determine whether Fru(2,6)Pz
P. 1988. Regulation by CO, of 1-aminocyclopropane-I-carboxylic acid conversion to ethylene in climateric fruits. -Physiol. Plant. 72: 535-540.A high CO, concentration (20%) at 20°C rapidly and strongly inhibited the development of the climacteric ethylene burst in apple (Malus domestica Borkh. cv. Granny Smith) and avocado (Persea americana Mill. cv. Fuerte) fruits and did not change 1-aminocyclopropane-1-carboxylic acid (ACC) content. Treatment with 20% CO, markedly decreased ACC-dependent ethylene biosynthesis at 20°C in climacteric pericarp tissues. It is suggested, therefore, that high CO, levels inhibit conversion of ACC to ethylene. Synthesis of the ethylene forming enzyme (EFE) was enhanced when intact preclimacteric apples or early climacteric avocados were pretreated for 40 h with 10 pl I-' ethylene. When CO, (20%) and ethylene were both applied, a reduced stimulatory effect of ethylene on EFE synthesis was observed. A high CO, concentration enhanced EFE acivity in excised tissues of apples and avocados incubated with ACC (2 mM) and cycloheximide (1 mM) or 2-Snorbornadiene (5 ml I-'). In the autocatalytic process, 20% CO, antagonized the stimulation of EFE synthesis by ethylene, but promoted EFE activity.
The succinate oxidation capacities of mitochondria isolated from mango fruits (Mangifera indica L.) stored at 4, 8, 12, and 20 C were investigated during storage. In normally ripening fruits (at 12 and 20 C) the oxidative capacities increased during the first 10 days and then decreased slowly. At lower temperatures (4 and 8 C), the fruits showed chilling injury symptoms, after about 10 days of storage and the succinate oxidation capacities of mitochondria decreased progressively. Plots of succinate oxidation capacities as against storage temperature showed a marked discontinuity between 12 and 8 C, only when chilling injury was observed on fruits stored at low temperature.The variations of mitochondrial fatty acid composition during the storage of fruits at different temperatures were also investigated. A marked decrease of the molar ratio palmitoleic acid/palmitic acid, the predominant fatty acids in mitochondrial lipids, was observed to accompany both the succinate oxidation decrease and the induction of chilling injury.
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