Tissue-specific and light-mediated expression of the C₄ photosynthetic NAD-dependent malic enzyme of amaranth mitochondria

In the C₄ dicotyledonous grain plant amaranth (Amaranthus Hypochondriacus), a mitochondrial NAD-dependent malic enzyme (NAD-ME; EC 1.1.1.39) serves a specialized and essential role in photosynthetic carbon fixation. In this study we have examined specialized photosynthetic gene expression patterns for the NAD-ME α subunit. We show here that the a subunit gene is preferentially expressed in leaves and cotyledons (the most photosynthetically active tissues), and this expression is specific to the bundle-sheath cells of these tissues from the earliest stages of development. Synthesis of the α subunit polypeptide and accumulation of its corresponding mRNA are strongly light-dependent, but this regulation is also influenced by seedling development. In addition, light-dependent accumulation of the a subunit mRNA is regulated at transcriptional as well as posttranscriptional levels. Our findings demonstrate that the NAD-ME of amaranth has acquired numerous complex tissue-specific and light-mediated regulation patterns that define its specialized function as a key enzyme in the C₄ photosynthetic pathway.