Light Induction of Cell Type Differentiation and Cell-Type-Specific Gene Expression in Cotyledons of a C₄ Plant, Flaveria trinervia

In Flaveria trinervia (Asteraceae) seedlings, light-induced signals are required for differentiation of cotyledon bundle sheath cells and mesophyll cells and for cell-type-specific expression of Rubisco small subunit genes (bundle sheath cell specific) and the genes that encode pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase (mesophyll cell specific). Both cell type differentiation and cell-type-specific gene expression were complete by d 7 in light-grown seedlings, but were arrested beyond d 4 in dark-grown seedlings. Our results contrast with those found for another C4 dicot, Amaranthus hypochondriacus, in which light was not required for either process. The differences between the two C4 dicot species in cotyledon cell differentiation may arise from differences in embryonic and post-embryonic cotyledon development. Our results illustrate that a common C4 photosynthetic mechanism can be established through different developmental pathways in different species, and provide evidence for independent evolutionary origins of C4 photosynthetic mechanisms within dicotyledonous plants.