Fructose 2,6-Bisphosphate, Carbohydrate Partitioning, and Crassulacean Acid Metabolism

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Abstract
Fructose 2,6-bisphosphate (F 2,6-P2) was detected in the CAM species, Ananas comosus and Bryophyllum tubiflorum, and in C3- and CAM-Mesembryanthemum crystallinum. In both Mesembryanthemum tissues, F 2,6-P2 was located outside the chloroplast. The levels of F 2,6-P2, malate, starch, or soluble sugars were measured during various periods during the day-night cycle in the leaves of Ananas, a species which stores carbohydrate in an extrachloroplastic compartment, and in Bryophyllum, a species which stores carbon as starch in the chloroplast. In both species, the levels of F 2,6-P2 were correlated with the stages of the day-night CAM cycle. Immediately following the dark-light transition the F 2,6-P2 levels exhibited a rapid transient increase followed by a decrease. F 2,6-P2 reached a daily minimum soon after the onset of deacidification and remained low until the malic acid pools approached their daily minima; the levels of F 2,6-P2 then began a slow increase which accelerated during the period of afternoon CO2 uptake. Immediately following the light-dark transition F 2,6-P2 levels fluctuated. The levels were usually low after the fluctuations had ceased. The pools then increased as the rate of malate synthesis increased, remained at relatively constant high levels when the rates of malate synthesis were constant, and decreased as malate synthesis decreased towards the end of the dark period. The absolute levels of F 2,6-P2 were always higher in Ananas than in Bryophyllum. The ratios of the activity of pyrophosphate fructose-6-phosphate 1-phosphotransferase to cytoplasmic fructose 1,6-bisphosphatase and to phosphofructokinase were also far higher in Ananas than in Bryophyllum or in C3- or CAM-Mesembryanthemum.