Ferritin-mediated transient iron sequestration facilitates chloroplast iron recycling during leaf senescence

Chloroplasts require a significant amount of iron to build up the photosynthetic apparatus. Upon developmental senescence, chloroplasts iron is subjected to remobilisation. Processes that enable iron removal from the chloroplasts have not been clarified in detail yet. Ferritins are primary iron storage proteins. Although chloroplast ferritins accumulate, in part during leaf senescence, their role in the removal of chloroplast iron has not been revealed in detail yet. Using Arabidopsis thaliana Col-0 model, we have studied the accumulation and the form of iron at the initiation and the progressing of senescence. Senescence status was characterised by the expression of Oresara 1 and Senescence Associated Gene 12. Physiological parameters, iron content and localization together with transcript abundance information were collected from the same leaf individuals. At senescence initiation, the accumulation of iron in the chloroplasts together with Ferritin transcripts and (apo)proteins rose, whereas under progressing senescence, chloroplast iron accumulation decreased. Low-energy X-ray fluorescence microscopy confirmed this increase in the iron signal at chloroplast sites. Ferritin signal in ⁵⁷Fe Mössbauer spectra (indicator of the major iron species population) was absent. Together with the stochastic presence of ferritin particles in chloroplasts this suggest that iron accumulation is a transient event involved in the iron remobilisation. Thus, ferritins do not serve as permanent storages, rather carriers that deliver iron for recycling during developmental senescence.