C-4 GENE-EXPRESSION IN PHOTOSYNTHETIC AND NONPHOTOSYNTHETIC LEAF REGIONS OF AMARANTHUS TRICOLOR

Throughout most of its growth and development, Amaranthus tricolor pro duces fully green leaves. However, near the onset of flowering, unique leaves emerge that consist of three distinct color regions: green api ces, yellow middle regions, and red basal regions. The green apices ar e identical to fully green leaves in terms of pigment composition, pho tosynthetic function, and C-4 gene expression. The yellow and red regi ons possess greatly reduced levels of chlorophyll and they lack photos ynthetic activity. The absence of photosynthetic capacity in the nongr een leaf regions was associated with three distinct alterations in C-4 gene expression. First, there was a reduction in the translation of C -4 polypeptides, and in the yellow regions synthesis of the ribulose-1 ,5-bisphosphate carboxylase small subunit occurred in the absence of l arge subunit synthesis. Second, there was a reduction in the relative transcription rates of two plastid-encoded photosynthetic genes, rbcL and psbA. Third, there was a loss of bundle-sheath cell-specific accum ulation of the rbcL and RbcS mRNAs (but not the polypeptides, which re mained bundle-sheath-specific). This study indicates that alterations in photosynthetic activity or developmental processes responsible for the loss of activity can influence C-4 gene expression at multiple reg ulatory levels.