Effects of temperature on circadian rhythm of crassulacean acid metabolism

Abstract

Crassulacean acid metabolism (CAM) is known to exhibit endogenous circadian rhythm under continuous light conditions. The rhythm, however, disappears at higher temperatures above approximately 30°C. We investigated effects of the temperature on the rhythm of phosphorylation of phosphoenolpyruvate carboxylase (PEP-C), the key enzyme in the regulation of CAM, with a CAM plant Graptopetalum paraguayense. We also analyzed expression of chlorophyll a/b binding protein gene (CAB) as an indicator of the state of the central oscillator, because CCA1 is a transcription factor of CAB in Arabidopsis thaliana and is thought to be a component of the oscillator or a component closely associated with it. The expression of CAB is expected to be under direct control of the clock. Under the continuous light condition, the rhythm in the phosphorylation of PEP-C, which was estimated by measuring the inhibition by 2 mM malate of the activity in leaf extract, did not persist at 32°C. The rhythm in the mRNA level of CAB, which persisted at 20°C, also disappeared at 32°C, suggesting that the circadian clock stops at high temperatures. Decreasing the temperature from 32°C to 25°C reset the rhythm of the phosphrylation of PEP-C. On the other hand, temperature shift from 27°C, at which the rhythm persisted, to 20°C had no effect on the phase of the rhythm.