Regulation of ferritin expression in Arabidopsis

Iron may bind to heme groups, iron-sulphur clusters, or directly associate with proteins and is essential for many cellular functions. Nonetheless, in the free ionic form iron is toxic as it can catalyze the formation of reactive oxygen species (ROS) through the Haber-Weiss reaction. These ROS in turn damage cell membranes, DNA, and proteins. Iron homeostasis is strongly dependent on ferritins, which are iron-storage proteins present in bacteria, plant and animal cells. Plant and animal ferritins have very similar structures and are formed by 24 subunits arranged to form a protein coat able to sequester up to 4500 iron atoms in a non-noxious form. Knowledge of ferritin regulatory pathways can contribute to the engineering of Fe-deficiency tolerant plants as well as to the production of plants with high Fe content. Our goal is to identify molecules involved in the signalling pathway leading to Arabidopsis ferritin accumulation as well as the physio-pathological conditions regulating that accumulation. Nitric oxide (NO), a signaling molecule implicated in plant growth, development, and defense also mediates iron induced Atfer1 ferritin accumulation. The infiltration of the NO-donor sodium nitroprusside (SNP) in Arabidopsis leaves induces accumulation of ferritin both at mRNA and protein level. Iron is not necessary for this NO-mediated Atfer1 transcript accumulation, since SNP is still able to induce the accumulation of ferritin transcript in Arabidopsis suspension cultures pretreated with the iron chelants DFO or ferrozine. However, NO is required for iron-induced Atfer1 accumulation, as the NO scavenger CPTIO prevents Atfer1 transcript accumulation in Arabidopsis suspension cultures treated with iron. The pathway is ser/thr phosphatase-dependent, necessitates protein synthesis and is apparently cGMP independent as 8 Br-cGMP, a cGMP analogous able to permeate cells, is not sufficient to either induce ferritin accumulation nor to act synergistically with iron or SNP in Arabidopsis suspension cultures and ODQ, an inhibitor of NO-dependent guanilate cyclase is not able to repress iron- nor SNP-induced ferritin accumulation. NO+ is the active NO redox form which mediates ferritin accumulation. Moreover, NO mediates ferritin regulation through the IDRS sequence of the Atfer1 promoter responsible for transcriptional repression under low iron supply. IDRS is not the only regulatory element in the Atfer1 promoter as senescence strongly activates Atfer1 promoter, through a IDRS-independent mechanisms.