PHOTOSYNTHETIC GENE-EXPRESSION IN AMARANTH, AN NAD-ME TYPE C-4 DICOT

Post-transcriptional regulation determines initial C-4 gene expression patterns in developing leaves of Amaranthus hypochondriacus, an NAD-M E type C-4 dicot. RuBPCase, PEPCase, and PPdK mRNAs are abundant in me ristems and in leaf primordia, but are utilised only during specific d evelopmental stages. While each enzyme shows independent patterns of i nitial mRNA and polypeptide accumulation, cell-specific localisation o f the polypeptides occurs prior to cell-specific localisation of the m RNAs. In mature three-coloured leaves of A. tricolor, loss of photosyn thetic activity correlates with reductions in the transcription rates of some plastid-encoded genes, reduction and loss of coordination in t he translation of RuBPCase polypeptides, and loss of cell-specific acc umulation of RuBPCase mRNAs (but not the polypeptides). The mitochondr ial NAD-dependent malic enzyme (NAD-ME) provides an example of a basic metabolic enzyme that has acquired specialised gene expression patter ns allowing it to function in the C-4 pathway. NAD-ME occurs preferent ially in photosynthetic tissues, and is specific to bundle sheath cell s throughout development. NAD-ME synthesis is regulated by light and d evelopment at transcriptional and post-transcriptional levels. The fin dings summarised here indicate that C-4 genes are independently regula ted by multiple control mechanisms in response to developmental, envir onmental, and metabolic signals.