Non-photosynthetic 'malic enzyme' from maize: a constituvely expressed enzyme that responds to plant defence inducers

The characterization of a non-photosynthetic isoform of NADP - malic enzyme (NADP-ME) from maize roots, which represents nearly 7% of the total solubl e protein of this tissue, was performed. The molecular properties of the pu rified protein, as well as the kinetic parameters determined, indicate that the NADP-ME isoform present in maize roots differs from the photosynthetic enzyme implicated in the C-4 cycle, but is similar, or identical, to the e nzyme previously characterized from etiolated maize leaves (Maurino, Drinco vich and Andreo, Biochem. Mol. Biol. Int. 38 (1996) 239-250). A full-length ORF encoding a plastidic NADP-ME (almost identical to the maize root NADP- ME, GenBank accession number U39958) was cloned from a root cDNA library as well as isolated by reverse transcription (RT)-PCR using green leaves mRNA as template. These results indicate that root NADP-ME does not constitute a root-specific isoform, but represents a protein with a constitutive patte rn of expression in plastids of the C-4 plant maize. The amount of NADP-ME measured by activity, western and northern blot was modified when different stress conditions (including treatments with cellulase, fungal elicitors, jasmonate and hypoxic treatment) were applied to maize roots, indicating th at the enzyme from maize roots is under transcriptional or post-transcripti onal regulation by effectors related to plant defence responses. It is dedu ced that the induction of housekeeping genes, like non-photosynthetic NADP- ME, whose constitutive role may be the provision of reductive power in non- photosynthetic plastids, is likely to accompany the defence response.