C₄ gene expression in photosynthetic and nonphotosynthetic leaf regions of Amaranthus tricolor

Throughout most of its growth and development, Amaranthus tricolor produces fully green leaves. However, near the onset of flowering, unique leaves emerge that consist of three distinct color regions: green apices, yellow middle regions, and red basal regions. The green apices are identical to fully green leaves in terms of pigment composition, photosynthetic function, and C₄ gene expression. The yellow and red regions possess greatly reduced levels of chlorophyll and they lack photosynthetic activity. The absence of photosynthetic capacity in the nongreen leaf regions was associated with three distinct alterations in C₄ gene expression. First, there was a reduction in the translation of C₄ polypeptides, and in the yellow regions synthesis of the ribulose-1,5-bisphosphate carboxylase small subunit occurred in the absence of large 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 accumulation of the rbcL and RbcS mRNAs (but not the polypeptides, which remained bundle-sheath-specific). This study indicates that alterations in photosynthetic activity or developmental processes responsible for the loss of activity can influence C₄ gene expression at multiple regulatory levels.