Uptake and incorporation of iron in sugar beet chloroplasts

Chloroplasts contain 80–90% of iron taken up by plant cells. Though some iron transport-related envelope proteins were identified recently, the mechanism of iron uptake into chloroplasts remained unresolved. To shed more light on the process of chloroplast iron uptake, trials were performed with isolated intact chloroplasts of sugar beet (Beta vulgaris). Iron uptake was followed by measuring the iron content of chloroplasts in the form of ferrous-bathophenantroline–disulphonate complex after solubilising the chloroplasts in reducing environment. Ferric citrate was preferred to ferrous citrate as substrate for chloroplasts. Strong dependency of ferric citrate uptake on photosynthetic electron transport activity suggests that ferric chelate reductase uses NADPH, and is localised in the inner envelope membrane. The Km for iron uptake from ferric-citrate pool was 14.65 ± 3.13 μM Fe(III)-citrate. The relatively fast incorporation of 57Fe isotope into Fe-S clusters/heme, detected by Mössbauer spectroscopy, showed the efficiency of the biosynthetic machinery of these cofactors in isolated chloroplasts. The negative correlation between the chloroplast iron concentration and the rate of iron uptake refers to a strong feedback regulation of the uptake. ► Chloroplast iron acquisition prefers ferric to ferrous iron complexes. ► The release of free ferrous iron ions by reduction is obligatory for uptake. ► Km value of iron uptake from ferric iron pool is 14.65 ± 3.13 μM Fe(III)-citrate. ► Iron is preferentially incorporating into Fe-S and/or heme cofactors.