Distinct But Conserved Functions for Two Chloroplastic NADP-Malic Enzyme Isoforms in C3 and C4 Flaveria Species

Abstract

Abstract In the most common C4 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 C3, C3-C4 intermediate, and C4 types. However, only one of these genes,ChlMe1, encodes the enzyme that functions in the C4 pathway. We compared the expression patterns of theChlMe1 and ChlMe2 genes in developing leaves of Flaveria pringlei (C3) andFlaveria trinervia (C4) and in transgenicFlaveria bidentis (C4).ChlMe1 expression in C4 species increases in leaves with high C4 pathway activity. In the C3species F. pringlei, ChlMe1 expression is transient and limited to early leaf development. In contrast,ChlMe2 is expressed in C3 and C4species 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 ofChlMe1 and ChlMe2 based on these expression patterns.