Differential electron flow around photosystem I by two C-4-photosynthetic-cell-specific ferredoxins

In the C-4 plant maize (Zea mays L.), two ferredoxin isoproteins, Fd I and Fd II, are expressed specifically in mesophyll and bundle-sheath cells, res pectively. cDNAs for these ferredoxins were introduced separately into the cyanobacterium Plectonema boryanum with a disrupted endogenous ferredoxin g ene, yielding TM202 and KM2-9 strains expressing Fd I and Fd II. The growth of TM202 was retarded under high light (130 mu mol/m(2)/s), whereas KM2-9 grew at a normal rate but exhibited a nitrogen-deficient phenotype, Measure ment of photosynthetic O-2 evolution revealed that the reducing power was n ot efficiently partitioned into nitrogen assimilation in KM2-9, After starv ation of the cells in darkness, the P700 oxidation level under far-red illu mination increased significantly in TM202, However, it remained low in KM2- 9, indicating an active cyclic electron flow. In accordance with this, the cellular ratio of ATP/ADP increased and that of NADPH/NADP(+) decreased in KM2-9 as compared with TM202. These results demonstrated that the two cell type-specific ferredoxins differentially modulate electron flow around phot osystem I.