PHOTOSYNTHETIC GENE-EXPRESSION IN MERISTEMS AND DURING INITIAL LEAF DEVELOPMENT IN A C-4 DICOTYLEDONOUS PLANT

Immunolocalization and fluorescent in situ hybridization were used wit h confocal microscopy to examine patterns of photosynthetic gene expre ssion during initial stages of leaf development in the C-4 dicot Amara nthus hypochondriacus. mRNAs encoding the large and small subunit of r ibulose-1,5-bisphosphate carboxylase were present in the apical dome a nd in all cells of the leaf primordia. In contrast, these polypeptides were detected only in cells of the ground meristem, with no accumulat ion detected in the apical dome or in other leaf primordia cells. The ribulose-1,5-bisphosphate carboxylase transcripts showed very little c ell-type specificity as leaf structures began to differentiate, wherea s their polypeptides accumulated primarily in bundle-sheath precursor cells. Phosphoenolpyruvate carboxylase and pyruvate orthophosphate dik inase mRNAs were abundant in meristems and leaf primordia, although th eir corresponding polypeptides did not accumulate in leaves until the leaf vascular system began to differentiate. These polypeptides were m ostly restricted to premesophyll cells from their earliest detection, whereas their transcripts remained present in nearly all leaf cells. T hese findings indicate that individual C, genes are independently regu lated as they become initially localized to their appropriate cell typ es. Furthermore, posttranscriptional regulation plays a major role in determining early patterns of C-4 gene expression.