The circadian clock and Crassulacean acid metabolism in Mesembryanthemum crystallinum

Abstract

For over 40 years, a circadian clock has been known to control the nocturnal fixation of CO2 in Crassulacean acid metabolism (CAM) plants. We now know that the observed rhythms of CO2-fixation are mainly due to circadian control of flux through phosphoenolpyruvate carboxylase (PEPc). Flux through PEPc is regulated by reversible protein phosphorylation, which is largely controlled by circadian fluctuations in the expression and activity of PEPc kinase. In order to obtain a more global picture of how the transcripts of CAM-associated genes vary over the diurnal and circadian cycle, we examined the transcript levels of a range of CAM-associated genes under diurnal and constant conditions in both young C3 and older CAM-induced Mesembryanthemum crystallinum plants. We designed sequence specific primers to a wide variety of CAM-associated genes. These primers were used to perform semi-quantitative RT-PCR analysis on each gene. Samples were collected every 6 h for 54 h from both C3 and CAM-performing plants. These plants were growing in either 12:12 light:dark (LD) or had been transferred to continuous light and constant temperature (LL) at the beginning of the experiment. This expression analysis provides an intriguing picture of how the circadian clock regulates both genes which are known to be CAM-associated and some which are not. Interestingly, several genes which were not found to be clock-controlled in C3 leaves were clearly clock-controlled in CAM-leaves. This data provides a significant insight into how the circadian clock regulates gene expression both before and after CAM-induction.