Photosynthetic Gene Expression in Amaranth, an NAD-ME Type C4 Dicot

Abstract

Post-transcriptional regulation determines initial C₄ gene expression patterns in developing leaves of Amaranthus hypochondriacus, an NAD-ME type C₄ dicot. RuBPCase, PEPCase, and PPdK mRNAs are abundant in meristems and in leaf primordia, but are utilised only during specific developmental stages. While each enzyme shows independent patterns of initial mRNA and polypeptide accumulation, cell-specific localisation of the polypeptides occurs prior to cell-specific localisation of the mRNAs. In mature three-coloured leaves of A. tricolor, loss of photosynthetic activity correlates with reductions in the transcription rates of some plastid-encoded genes, reduction and loss of coordination in the translation of RuBPCase polypeptides, and loss of cell-specific accumulation of RuBPCase mRNAs (but not the polypeptides). The mitochondrial NAD-dependent malic enzyme (NAD-ME) provides an example of a basic metabolic enzyme that has acquired specialised gene expression patterns allowing it to function in the C₄ pathway. NAD-ME occurs preferentially in photosynthetic tissues, and is specific to bundle sheath cells throughout development. NAD-ME synthesis is regulated by light and development at transcriptional and post-transcriptional levels. The findings summarised here indicate that C₄ genes are independently regulated by multiple control mechanisms in response to developmental, environmental, and metabolic signals.