Drought stress-induced upregulation of components involved in ferredoxin-dependent cyclic electron transfer

Linear photosynthetic electron transfer, consisting of both Photosystem (PS) II and PSI, converts light energy into the chemical forms ATP and NADPH, whereas PSI cyclic electron transfer (CET) is exclusively involved in ATP synthesis. In the chloroplasts of higher plants, there are two partially redundant CET routes. The ferredoxin (FD) or ferredoxin-plastoquinone reductase (FQR)-dependent route cycles electrons from PSI to plastoquinone via ferredoxin (FD), while in the NDH-dependent route, NADPH donates electrons to the NDH-complex for reduction of the plastoquinone pool. In the present study, we show that drought stress induces transcriptional and translational upregulation of the PGR5 and PGRL1 genes, which are the only characterized components of the FQR-dependent CET thus far. In contrast, the expression of the NDH-H gene, a representative of the NDH-complex, did not differ between the drought-stressed and the control plants. The overall expression level of the ferredoxin-NADP+-oxidoreductase (FNR) genes increased upon drought stress, with a concomitant release of FNR from the thylakoid membrane. Moreover, drought stress accelerated the rate of P700+ re-reduction, which may indicate induction of CET. Responses of the PSAE, FD and PSAD gene families upon drought stress are also described.