Effect of ethylene treatment on the concentration of fructose‐2,6‐bisphosphate and on the activity of phosphofructokinase 2/fructose‐2,6‐bisphosphatase in banana

Abstract

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

Fructose 2,6‐bisphosphate and the climacteric in bananas

“…During ripening our bananas behaved in the same way as those studied by Mertens et al [2]. The latter reported that C 0 2 production rose from 20 to 80 p1 .…”

“…Thus although the amount of Fru(2,6)Pz that we detected in the extracts rose markedly during ripening this rise was very closely paralleled by a corresponding increase in recovery. This was demonstrated specifically for the methods described by Mertens et al [2] and by Beaudry et al [3]. The recoveries that we found for samples extracted in Hepes were, however, measurable and sufficiently consistent for us to be able to use them to correct the observed values for Fru(2,6)Pz to give a truer estimate of the actual amount present in vivo.…”

“…Thus we compared the recoveries obtained with a range of extraction procedures (Table 1). We found that extraction in cold NaOH as described by Mertens et al [2] gave modest recoveries for ripe fruit and recoveries that were too low to measure for pre-climacteric and unripe fruit. Some improvement was obtained for ripe fruit when the method was modified by adding Triton to the cold NaOH and including a subsequent treatment with hot NaOH, but recoveries at the other stages were barely measurable.…”

“…For avocado fruit Bennett et al 111 reported that a rise in Fru(2,6)P2, from 30 to 77 pmol/g paralleled the increase in respiration and suggested that the latter was due to activation of PPi-PFK in the glycolytic direction by Fru(2,6)P2. Mertens et al [2] reported that a rise in Fru(2,6)P2 from 25 to 12.5 pmol/g correlated with the respiratory rise in bananas. They suggested that the rise in Fru(2,6)P2, acting via PPi-PFK, could explain the increase in Fru(1,6)P2 held to be characteristic of banana ripening.…”

“…Recently there have been three claims [l -31 that the climacteric is also accompanied by a dramatic (2-5-fold) rise in the concentration of Fru (2,6)P2. This point is of importance, both in respect of the mechanism of fruit ripening and of the role of PPi-PFK in plant tissues.…”