Studies on plant ferritin functions and regulation of their synthesis revealed strong links between these proteins and protection against oxidative stress. In contrast, their putative iron-storage function to furnish iron during various development processes is unlikely to be essential. Ferritins, by buffering iron, exert a fine tuning of the quantity of metal required for metabolic purposes, and help plants to cope with adverse situations, the deleterious effects of which would be amplified if no system had evolved to take care of free reactive iron.

The Arabidopsis basic helix-loop-helix (bHLH) proteins INDEHISCENT (IND) and ALCATRAZ (ALC) specify tissues required for fruit opening that have major roles in seed dispersal and plant domestication. Here, we show that synthesis of the phytohormone gibberellin is a direct and necessary target of IND, and that ALC interacts directly with DELLA repressors, which antagonize ALC function but are destabilized by gibberellin. Thus, the gibberellin/DELLA pathway has a key role in patterning the Arabidopsis fruit, and the interaction between DELLA and bHLH proteins, previously shown to connect gibberellin and light responses, is a versatile regulatory module also used in tissue patterning.Supplemental material is available at http://www.genesdev.org.

Ferritins play an essential role in iron homeostasis by sequestering iron in a bioavailable and non-toxic form. In plants, ferritin mRNAs are highly and quickly accumulated in response to iron overload. Such accumulation leads to a subsequent ferritin protein synthesis and iron storage, thus avoiding oxidative stress to take place. By combining pharmacological and imaging approaches in an Arabidopsis cell culture system, we have identified several elements in the signal transduction pathway leading to the increase of AtFer1 transcript level after iron treatment. Nitric oxide quickly accumulates in the plastids after iron treatment. This compound acts downstream of iron and upstream of a PP2A-type phosphatase to promote an increase of AtFer1 mRNA level. The AtFer1 gene transcription has been previously shown to be repressed under low iron conditions with the involvement of the cis-acting element iron-dependent regulatory sequence identified within the AtFer1 promoter sequence. We show here that the repressor is unlikely a transcription factor directly bound to the iron-dependent regulatory sequence; such a repressor is ubiquitinated upon iron treatment and subsequently degraded through a 26 S proteasome-dependent pathway.As the major cofactor of proteins involved in essential processes like photosynthesis, respiration, DNA replication, or nitrogen fixation, iron is an essential element for life. Nonetheless, in the free ionic form, iron is toxic as it can catalyze the formation of reactive oxygen species through the Fenton reaction. These reactive oxygen species damage the cell membranes, DNA, and proteins (1, 2). Thus, iron homeostasis has to be tightly regulated, to avoid starvation that impairs the metabolism, and to avoid excess that may lead to cell death. Iron homeostasis is strongly dependent on ferritins, which are ironstorage proteins, found in bacteria, animals, and plants. Plant and animal ferritin structures are very similar, and are formed by 24 subunits arranged to form a hollow sphere able to sequester iron in a non-toxic and bioavailable form (3).In animals, ferritin synthesis is mainly regulated at the posttranscriptional level (3, 4). Ferritin mRNAs contain iron-responsive elements in their 5Ј-untranslated regions that function as binding sites for two related trans-acting factors, namely iron regulatory proteins IRP1 and IRP2. When bound to the iron-responsive element in the ferritin mRNA, the IRP inhibit translation of the transcript (4). IRP1 is a bifunctional protein that when iron is abundant possesses a 4Fe-4S cluster and acts as cytoplasmic aconitase. When iron levels are low, the 4Fe-4S cluster disassembles and the apoprotein acquires IRP 3 activity, thus repressing ferritin translation. High levels of iron lead to the 4Fe-4S cluster reconstitution and therefore the protein aconitase activity. In contrast to IRP1, IRP2 cannot assemble a iron-sulfur cluster and lacks aconitase activity. IRP2 shares about 60% amino acid sequence identity with IRP1, but differs only in having a 73-amino aci...

SUMMARYThe evolution of plant reproductive strategies has led to a remarkable diversity of structures, especially within the flower, a structure characteristic of the angiosperms. In flowering plants, sexual reproduction depends notably on the development of the gynoecium that produces and protects the ovules. In Arabidopsis thaliana, ovule initiation is promoted by the concerted action of auxin with CUC1 (CUP-SHAPED COTY-LEDON1) and CUC2, two genes that encode transcription factors of the NAC family (NAM/ATAF1,2/CUC). Here we highlight an additional role for CUC2 and CUC3 in Arabidopsis thaliana ovule separation. While CUC1 and CUC2 are broadly expressed in the medial tissue of the gynoecium, CUC2 and CUC3 are expressed in the placental tissue between developing ovules. Consistent with the partial overlap between CUC1, CUC2 and CUC3 expression patterns, we show that CUC proteins can physically interact, both in yeast cells and in planta. We found that the cuc2;cuc3 double mutant specifically harbours defects in ovule separation, producing fused seeds that share the seed coat, and suggesting that CUC2 and CUC3 promote ovule separation in a partially redundant manner. Functional analyses show that CUC transcription factors are also involved in ovule development in Cardamine hirsuta. Additionally we show a conserved expression pattern of CUC orthologues between ovule primordia in other phylogenetically distant species with different gynoecium architectures. Taken together these results suggest an ancient role for CUC transcription factors in ovule separation, and shed light on the conservation of mechanisms involved in the development of innovative structures.