engleski

Electron partitioning beyond photosystem I involves TROL-FNR interaction and reveals alternative pathways

In photosynthesis, final electron transfer from ferredoxin (Fd) to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR) 1. FNR is recruited to thylakoid membranes via integral membrane thylakoid rodanase-like protein TROL 2. Here, we address the fate of electrons downstream of photosystem I (PSI) when TROL is absent. We have employed electron paramagnetic resonance (EPR) to quantify free radical formation and electron partitioning in TROL-depleted chloroplasts. DMPO (5, 5-dimethyl-1-pyrroline-N-oxide) was used to quantify superoxide O2.- anione formation, while the propagation of other free radicals was monitored by Tiron (1, 2-dihydroxybenzene-3, 5-disulphonate. Trol plants grown in different light regimes and measured in both dark and light conditions, consistently exhibited diminished O2.- accumulation. Propagation of other oxygen radicals was elevated in trol plants in all tested conditions, except for the plants adapted to high-light. Strikingly, trol plants were resilient to O2.- propagation induced by the inclusion of methyl viologen. These findings implicate that in the absence of the specific FNR membrane attachment, photogenerated electrons do not preferentially spill over to O2, as it could be expected from the present models of photosynthetic electron transport 3, or the O2.- is efficiently scavenged. Further, it is possible that in the absence of TROL PSI photogenerated electrons are not passed to O2 in the first place, but reduced Fd distributes them efficiently to any of the alternative pathways. Here we have to point out that TROL deficiency does not alter cyclic electron transfer 2. This would imply that FNR-driven production of NADPH in linear electron transfer is actually a rate limiting process, and that some other Fd-dependent pathway(s) is (are) more efficient electron sink(s). This could explain the need for TROL as an FNR sequestration site in vascular plant thylakoids. Hence, FNR has to be docked in the vicinity of Fd reduction site to prevent Fd from transferring electrons to alternative pathways. Finally, it has been shown that Fd-reduced, bound FNR cannot catalyze the photoreduction of O2. We conclude that TROL-mediated sequestration of FNR to thylakoid membranes is important for electron partitioning between electron-conserving and -dissipating pathways. Furthermore, activation of the pseudo-cyclic pathway is likely preceded by efficient scavenging mechanism(s) which depends on membrane-detached of the FNR.

photosystem I; cyclic electron transport; linear electron transport; ROS

nije evidentirano