Distinct but conserved functions for two chloroplastic NADP-malic enzyme isoforms in C-3 and C-4 Flaveria species

In the most common C-4 pathway for carbon fixation, an NADP-malic enzyme (NADP-ME) decarboxylates malate in the chloroplasts of bundle sheath cells. Isoforms of plastidic NADP-ME are encoded by two genes in all species of Flaveria, including C-3, C-3-C-4 intermediate, and C, types. However, only one of these genes, ChlMe1, encodes the enzyme that functions in the C, pathway. We compared the expression patterns of the ChlMe1 and ChlMe2 genes in developing leaves of Flaveria pringlei (C-3) and Flaveria trinervia (C-4) and in transgenic Flaveria bidentis (C-4). ChlMe1 expression in C4 species increases in leaves with high C4 pathway activity. In the C, species F. pringlei, ChlMe1 expression is transient and limited to early leaf development. In contrast, ChlMe2 Is expressed in C, and C4 species concurrent with stages in chloroplast biogenesis. Because previous studies suggest that NADP-ME activities generally reflect the level of its mRNA abundance, we discuss possible roles of ChlMe1 and ChlMe2 based on these expression patterns.