Photosynthetic Gene Expression in Meristems and during Initial Leaf Development in a C4 Dicotyledonous Plant

Immunolocalization and fluorescent in situ hybridization were used with confocal microscopy to examine patterns of photosynthetic gene expression during initial stages of leaf development in the C4 dicot Amaranthus hypochondriacus. mRNAs encoding the large and small subunit of ribulose-1,5-bisphosphate carboxylase were present in the apical dome and in all cells of the leaf primordia. In contrast, these polypeptides were detected only in cells of the ground meristem, with no accumulation detected in the apical dome or in other leaf primordia cells. The ribulose-1,5-bisphosphate carboxylase transcripts showed very little cell-type specificity as leaf structures began to differentiate, whereas their polypeptides accumulated primarily in bundle-sheath precursor cells. Phosphoenolpyruvate carboxylase and pyruvate orthophosphate dikinase mRNAs were abundant in meristems and leaf primordia, although their corresponding polypeptides did not accumulate in leaves until the leaf vascular system began to differentiate. These polypeptides were mostly restricted to premesophyll cells from their earliest detection, whereas their transcripts remained present in nearly all leaf cells. These findings indicate that individual C4 genes are independently regulated as they become initially localized to their appropriate cell types. Furthermore, posttranscriptional regulation plays a major role in determining early patterns of C4 gene expression.