Iron uptake of etioplasts is independent from photosynthesis but applies the reduction-based strategy

Abstract

IntroductionIron (Fe) is one of themost important cofactors in the photosynthetic apparatus, and its uptake by chloroplasts has also been 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 non-photosynthetic 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.MethodsIn our combined structure, iron content and transcript amount analysis studies, 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.ResultsFoliar 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 that 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 were 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.DiscussionBased on these findings, the reduction-based plastidial Fe acquisition should not be considered exclusively dependent on the photosynthetic functions.

Fluorescence lifetime of plant leaves with sub-nanosecond resolution

“…The first selected test plant type was Savoy cabbage (Brassica oleracea var. sabauda L.), from which layers of leaves were selected based on the method described by Sági-Kazár et al [32] to examine isolated chloroplast and etioplast containing suspension samples. As similar layers were selected, we expected to obtain similar values as previously.…”

“…In a previous work, we investigated plastids isolated from different layers of Savoy cabbage leaves in a cuvette-based system [32] that revealed a gradual increase of fluorescence lifetime starting from samples isolated from the outermost layer 1 towards the innermost layers (till layer 5). While the outermost leaves were exposed to light, the leaves of the innermost layer 5 grew in total darkness representing a natural case of the etiolation syndrome [36,37].…”