Specialization of electron transport pathways

The most obvious feature of electron transport in C4 plants is the very low photosystem II activity in the bundle sheath chloroplasts of NADP-ME plants such as sugarcane, sorghum, and maize. Photosystem I-dependent cyclic electron transport leads to ATP formation (Chapman et al 1980, Leegood 1985). NADPH required for the reductive phase of the Calvin cycle is generated by NADP-ME, which is sufficient to reduce half of the glycerate-3-P. the remainder being exported to the mesophyll. The lack of 0:-evolution in the bundle sheath is presumably an adaptation to reduce the O, concentration in the bundle sheath, although export of glycerate-3-Palso achieves integrated regulation of metabolism (see above). However, both NAD-ME-and PEPCK-type C4 plants have normal noncyclic electron flow in the bundle sheath. The second important feature is that the C4 cycle is essentially an ATP-driven CO, pump, requiring 2 ATP per C02 transported. This ATP is required by pyruvate. Pi dikinase (PPDK) in the mesophyll and it is notable that C4 mesophyll chloroplasts readily catalyze cyclic electron transport, presumably to meet this extra ATP demand (Crowther et al 1983), although this might also be met by a Q-cycle mechanism (see von Caemmerer and Furbank 1999 for discussion).

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