Iron (Fe) is one of the most important cofactors in the photosynthetic apparatus,
and its uptake by chloroplasts have been also associated with the operation of the
photosynthetic electron transport chain during reduction-based plastidial Fe uptake.
Therefore, plastidial Fe uptake was considered not to be operational in the absence
of the photosynthetic activity. Nevertheless, Fe is also required for enzymatic functions
unrelated to photosynthesis, highlighting the importance of Fe acquisition by nonphotosynthetic
plastids. Yet, it remains unclear how these plastids acquire Fe in the absence of
photosynthetic function. Furthermore, plastids of etiolated tissues should already
possess the ability to acquire Fe, since the biosynthesis of thylakoid membrane complexes
requires a massive amount of readily available Fe. Thus, we aimed to investigate whether
the reduction based plastidial Fe uptake solely relies on the functioning photosynthetic
apparatus. We used Savoy cabbage plant as a model, which develops natural etiolation
in the inner leaves of the heads due to the shading of the outer leaf layers. Foliar
and plastidial Fe content of Savoy cabbage leaves decreased towards the inner leaf
layers. The leaves of the innermost leaf layers proved to be etiolated, containing
etioplasts which lacked the photosynthetic machinery and thus were photosynthetically
inactive. However, we discovered that these etioplasts contained, and were able to
take up Fe. Although, the relative transcript abundance of genes associated with plastidial
Fe uptake and homeostasis decreased towards the inner leaf layers, both ferric chelate
reductase FRO7 transcripts and activity was detected in the innermost leaf layer.
Additionally, a significant NADP(H) pool and NAD(P)H dehydrogenase activity was detected
in the etioplasts of the innermost leaf layer, indicating the presence of the reducing
capacity that likely supports the reduction based Fe uptake of etioplasts. Based on
these findings, the reduction based
plastidial Fe acquisition should not be considered exclusively dependent on the photosynthetic
functions.
