Characterization and expression of a NADP-malic enzyme cDNA induced by salt stress from the facultative crassulacean acid metabolism plant, Mesembryanthemum crystallinum

The facultative halophyte and crassulacean acid-metabolism plant, Mesembryanthemum crystallinum shifts from C₃ photosynthesis to crassulacean acid metabolism when exposed to high-salt or drought conditions. To study the molecular basis of this metabolic transition, the expression of NADP⁺-dependent malic enzyme (NADP-ME), which catalyzes the decarboxylation of malate to release pyruvate and CO₂, has been investigated. The complete nucleotide sequence of a full-length cDNA clone was determined and found to contain a single open reading frame encoding a 585-amino-acid polypeptide of 64284 Da. The ice plant (M. crystallinum) NADP-ME shares amino acid identities in the range 72.5–79.0% when compared to other higher-plant enzymes and is more closely related to C₃ rather than C₄ forms of the enzyme. Genomic Southern-blot analysis of ice-plant DNA indicates that NADP-ME is encoded by a small gene family. Steady-state transcript levels increase 8–10-fold in response to salt stress in the leaves. Transcript levels in roots are extremely low and are unaffected by salt-stress treatment. Nuclear run-on experiments, using isolated nuclei from leaf tissue, confirm that the accumulation of NADP-ME transcripts is, in part, the result of increased transcription of this gene during salt stress.