Stromal Electron flow through the plastoquinone pool controls the state transition Capacity in Sunechococcus sp. PCC 7942 cells. Effects of iron deficiency

Abstract

Chlorophyll fluorescence measurements have indicated that dark-adapted cyanobacterial cells are usually in State II characterised by low PSII fluorescence yield. Upon illumination, the quenching of PSII was released and fluorescence increased to a much higher level characteristic for cells in State I. In contrast, iron deficient cells exhibited higher PSII fluorescence in dark-adapted state and no fluorescence changes upon illumination were registered. This indicates that the cells are locked in State I under iron deficient conditions. Low temperature (77K) fluorescence measurements confirmed this suggestion. Following the postilumination increase of Fo as a measure of the dark reduction of the PQ pool, it was found that a massive stromal electron flow exists in the control iron sufficient cells. On the contrary, no measurable dark reduction of the PQ pool was detected in iron deficient cells. Interestingly, preventing the HADP dehydrogenase-dependent stromal electron flow to the intersystem chain completely inhibited the light induced increase of PSII fluorescence in control cells. 77K measurements indicated that under these dark-adapted conditions, cells were shifted from State II to State I and the observed 77K fluorescence pattern resembled that of iron deficient cells. Based on these results, it is reasonable to suggest that the state transition phenomena in Synechococcus strongly depend on the redox state of the PQ pool and that the NADP-dependent sromal electron flow is one of the major mechanisms controlling it. Since the NADP-dehydrogenase is one of the most Fe-abundant complexes within the cyanobacterial thylakoid membranes minimal stromal electron flow should be expected. This would prevent the PQ pool from dark reduction and keep the iron deficient cells in state I.