Diurnal expression of five protein phosphatase type 2C genes in the common ice plant, Mesembryanthemum crystallinum

The common ice plant, Mesembryanthemum crystallinum L., is a eu-halophytic model species with an environmental stress-initiated switch from C₃ photosynthesis to crassulacean acid metabolism (CAM). Phosphoenolpyruvate carboxylase activity in 6-week-old plants exposed to salt stress for 5 days was ~15-fold higher than before stress, indicating the salinity-dependent induction of the C₃ to CAM transition. Five plant protein phosphatase type 2C (PP2C) genes were cloned, representative of five of the 10 plant PP2C sub-families. We measured mRNA levels of these PP2Cs and of myo-inositol 1-phosphate synthase (Inps1) in 6-week-old plants before (C₃) and after (CAM) salt stress. Remarkably, four PP2C genes and Inps1 were expressed with a diurnal fluctuation in plants in C₃ mode. After salt-induced CAM induction, the six genes were expressed with more prominent fluctuations than before stress, suggesting that these PP2C genes may be involved in the diurnal regulation of protein phosphorylation in CAM. Under continuous light treatment the expression of two PP2C genes continued to fluctuate, indicating that their expression is controlled by circadian rhythm.