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

Abstract

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. Copyright © 1992, Wiley Blackwell. All rights reserved