Abstract
During C4 photosynthesis, CO2 is first fixed into C4 acid by phosphoenolpyuvate carboxylase in the mesophyll cell. The C4 acid is transported to bundle sheath cells (BSC), where it is decarboxylated, released CO2 being refixed by Rubisco. Since this CO2 concentrating mechanism decreases the Rubisco oxygenase activity, C4 photosynthesis can be advantageous under water stress or high temperature. However, C4 metabolism requires additional ATP compared to C3 metabolism. It is assumed that additional ATP can be produced by cyclic electron transport around photosystem I which can increase ΔpH across the thylakoid membrane. In Arabidopsis, two cyclic pathways, PGR5-dependent pathway and NDH-dependent pathway, have been shown to be functional. To elucidate the role of the two cyclic pathways during C4 photosynthesis, we investigated localization of PGR5 and NDH in leaf tissues of Flaveria C3 and C4 species. Results suggested that both cyclic pathways are up-regulated in C4 species, especially in BSC.
