PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Ferritin functions as an iron buffer in the green alga Chlamydomonas reinhardtii. Blandine Rimbault^*,1, Stefan Rensch¹, Michael Hippler^{1, 2}, ¹ Lehrstuhl für Pflanzenphysiologie, FSU, Jena, Germany² Department of Biology, University of Pennsylvania, Philadelphia, PA, USA

ABSTRACT- Ferritin is a key element of iron homeostasis able to store large quantities of iron, which in plants is located in the chloroplast stroma. We present here the first insights on ferritin regulation in response to iron deficiency in the green alga Chlamydomonas reinhardtii. In contrast to land plants, Ferritin is encoded by a single gene and is constitutively expressed in iron-containing media. In conditions where the cells can use either respiration or photosynthesis (photoheterotrophy), the transfer to a medium lacking iron leads to a rapid degradation of the photosynthetic apparatus, and especially the iron-rich photosystem I and its associated light harvesting proteins (PSI/LHCI) (1). There we observed a rapid response of the ferritin mRNA accumulation and a progressive increase of the subunit accumulation. It is of note that PSI/LHCI is almost completely degraded after 5 days in iron-deficiency, but restored within 24 h after iron-repletion at the expense of the accumulated ferritin. Therefore we suggest that the ferritin stored-iron allows fast adjustment of the photosynthetic apparatus in response to iron-availability. When the cells are grown in a medium where photosynthesis is indispensable (photoautrophic growth), there is a delay of at least 5 days before the phenotype associated with iron deficiency starts to occur. Whereas ferritin mRNA accumulation still responds rapidly to the iron stress signal, the protein accumulation is little affected. We conclude that ferritin is used to buffer the iron freed by the degradation of the photosynthetic complexes, and that the physiological status of the cell determines the strategy that is chosen to master the impact of iron depletion. 1. Moseley, J. L., Allinger, et al. (2002) EMBO J 21, 6709-6720

KEY WORDS: iron-availability, Ferritin , regulation