C4 Photosynthetic Gene Expression in Light- and Dark-Grown Amaranth Cotyledons

The patterns of expression for genes encoding several C4 photosynthetic enzymes were examined in light-grown and dark-grown (etiolated) cotyledons of amaranth (Amaranthus hypochondriacus), a dicotyledonous C4 plant. The large subunit and small subunit of ribulose-1,5-bisphosphate carboxylase (RuBPCase), phosphoenolpyruvate carboxylase (PEPCase), and pyruvate orthophosphate dikinase (PPdK) were all present in the cotyledons by d 2 after planting when the seedlings first emerged from the seed coat. Kranz anatomy was apparent in light-grown cotyledons throughout development, and the overall patterns of C4 gene expression were similar to those recently described for developing amaranth leaves (J.L. Wang, D.F. Klessig, J.O. Berry [1992] Plant Cell 4: 173-184). RuBPCase mRNA and proteins were present in both bundle sheath and mesophyll cells in a C3-like pattern during early development and became progressively more localized to bundle sheath cells in the C4-type pattern as the cotyledons expanded over 2 to 7 d. PEPCase and PPdK polypeptides were localized to mesophyll cells throughout development, even though PEPCase transcripts were detected in both bundle sheath and mesophyll cells. Kranz anatomy also developed in cotyledons grown in complete darkness. In 7-d-old dark-grown cotyledons, RuBPCase, PPdK, and PEPCase were all localized to the appropriate cell types, although at somewhat lower levels than in light-grown cotyledons. These findings demonstrate that the leaves and postembryonic cotyledons of amaranth undergo common developmental programs of C4 gene expression during maturation. Furthermore, light is not required for the cell-type-specific expression of genes encoding RuBPCase and other photosynthetic enzymes in this dicotyledonous C4 plant.