The tonoplast functioning as the master switch for circadian regulation ofcrassulacean acid metabolism

From the initial discovery of free-running endogenous circadian oscillation s of Crassulacean acid metabolism (CAM) under constant conditions in the li ght and in air, it has been disputed whether the underlying oscillator is e nzymic or biophysical. The hypothesis of a biophysical hysteresis switch or beat oscillator started from osmotic considerations of malate accumulation and remobilisation. indicating a tonoplast tension/relaxation mechanism. I t then advanced to application of non-linear dynamics theory for the analys is of rhythmic and arrhythmic time series of CO2 exchange under the regime of external control parameters, mainly temperature, and the implementation of models for computer simulations of CAM rhythms. This provided strong evi dence for the tonoplast functioning as a master switch for circadian regula tion of CAM. Conversely, the hypothesis of an enzymic beat oscillator stron gly developed on the experimental basis of phosphorylation/dephosphorylatio n of phosphoeno/pyruvate carboxylase (PEPC) regulating the enzyme activity, and hence CO2 fixation and malate synthesis via this enzyme. It was much s upported by the discovery that PEPC-kinase gene-transcription was under cir cadian control. However, biochemical and molecular analysis, as well as mod el simulation, strongly suggests that this is a secondary and not the prima ry oscillator. The synchronisation/desynchronisation of leaf patches has re vealed spatiotemporal characteristics of circadian rhythmicity that may ope n new ways for understanding biological clocks.