REGULATION OF THE EXPRESSION OF THE GLYCINE DECARBOXYLASE COMPLEX DURING PEA LEAF DEVELOPMENT

The expression of the genes encoding the four proteins (P, H, T, and L ) of glycine decarboxylase, a multienzymatic complex involved in the m itochondrial step of the photorespiration pathway, was examined during pea (Pisum sativum) leaf development in comparison with ribulose-1,5- bisphosphate carboxylase/oxygenase. Mitochondria from the primary leaf were isolated at several well-defined stages of development. Their ca pacity to oxidize glycine was negligible during the earlier stages but increased dramatically once the leaflet opened. This was correlated w ith the accumulation of the glycine decarboxylase complex (CDC) protei ns, which was shown to occur in preexisting mitochondria, producing an increase in their density. The transcription of the CDC genes was coo rdinated and occurred early, with a peak at 7 d, a stage at which mito chondria are unable to oxidize glycine. This implies the existence of posttranscriptional control of gene expression. The comparison of the expression patterns of the genes encoding specific proteins of CDC wit h that of rbcS genes suggests a common regulation scheme that is relat ed to light induction. However, ribulose-1,5-bisphosphate carboxylase/ oxygenase is present in the chloroplast well before GDC fills the mito chondria, suggesting that the setup of photorespiration occurs in cell s already engaged in active photosynthesis.