Abscisic acid signaling and protein synthesis requirements for phosphoenolpyruvate carboxylase transcript induction in the common ice plant

Crassulacean acid metabolism induction by water deficit, osmotic or ionic stress, or abscisic acid (ABA) treatment is mediated by a calcium-dependent signaling pathway. To understand the relationship between stress signals and ABA and between the downstream signaling events activated by these stimuli, ABA content was determined in detached Mesembryanthemum crystallinum leaves exposed to osmotic stress (dehydration or mannitol), ionic stress (NaCl) or treated with exogenous ABA. Dehydration and exogenous ABA application led to large increases in ABA accumulation, whereas mannitol treatment caused only a small increase in ABA accumulation. NaCl exposure failed to induce a significant change in ABA accumulation, yet resulted in CAM induction as measured by the increased accumulation of transcripts of Ppc1, which encodes a CAM-specific isoform of phosphoenolpyruvate carboxylase (PEPC). Pretreatment of detached leaves with the ABA biosynthesis inhibitor, Fluridone, failed to inhibit Ppc1 transcript accumulation following stress induction. Taken together, these results suggest that stress-induced CAM gene expression may occur via both ABA-dependent and ABA-independent signaling pathways. Additional pharmacological investigations suggest a role for phosphoinositides in the CAM-induction signaling process. Pre-treatment of detached leaves with cycloheximide inhibited Ppc1 transcript accumulation in response to abiotic stresses and ABA application demonstrating that signaling and/or transcriptional activation events leading to the induction of CAM gene expression are dependent upon protein synthesis.