PEPCase Transcript Levels in Mesembryanthemum crystallinum Decline Rapidly upon Relief from Salt Stress

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Abstract
Mesembryanthemum crystallinum plants respond to water stress by changing their pathway of carbon assimilation from C3 to Crassulacean acid metabolism (CAM). Stressed plants are characterized by elevated levels of phosphoenolpyruvate carboxylase (PEPCase) mRNA, protein, and enzyme activity. We wanted to determine whether CAM is a reversible response to environmental conditions or a developmentally programmed adaptation that is irreversibly expressed once induced. Plants were osmotically stressed by irrigation with 500 millimolar NaCl for 12 days to elicit CAM. Salt was then thoroughly flushed from the soil and PEPCase protein and transcript levels were monitored. PEPCase mRNA levels dropped by 77% within 2.5 hours after salt removal. PEPCase activity and polypeptide levels declined more slowly, with a half-life of 2 to 3 days. These results show that PEPCase expression in M. crystallinum is a reversible response to stress that is regulated at the level of transcription or stability of the PEPCase mRNA.