Photosynthetic light reactions in C4 photosynthesis.

The most productive wild plants and crops use C4 photosynthesis (Brown, 1999). C4 photosynthesis requires the coordinated functions of two cell types in leaves, namely mesophyll (M) and bundle-sheath (BS) cells (Hatch, 1987). Atmospheric CO2 is initially fixed by phosphoenolpyruvate carboxylase (PEPC) in M cells. The resulting products, C4 acids are transported into BS cells where CO2 is released by decarboxylation of C4 acids and refixed by riburose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) functioning in the Calvin cycle (C3 cycle). Since this process increase CO2 levels at the site of RuBisCO in the BS cells, the oxygenase reaction of RuBisCO is largely reduced and RuBisCO can achieve maximal catalytic activity of CO2 fixation. Therefore, C4 plants have higher potential efficiencies in the use of light, water and nitrogen than C3 plants (Long, 1999). C4 plants have evolved from ancestral C3 plants (Sage et al., 2011). In C4 plants, not only the CO2 metabolism but a manner of light reactions which is a process to produce ATP and NADPH used for CO2 metabolism has been changed. However, mechanism of light reactions and its physiological roles on C4 photosynthesis are not yet fully understood. Here, we introduce latest findings and perspectives of physiological roles of light reactions in C4 photosynthesis.