SummaryFerritin protein nanocages are the main iron store in mammals. They have been predicted to fulfil the same function in plants but direct evidence was lacking. To address this, a loss-of-function approach was developed in Arabidopsis. We present evidence that ferritins do not constitute the major iron pool either in seeds for seedling development or in leaves for proper functioning of the photosynthetic apparatus. Loss of ferritins in vegetative and reproductive organs resulted in sensitivity to excess iron, as shown by reduced growth and strong defects in flower development. Furthermore, the absence of ferritin led to a strong deregulation of expression of several metal transporters genes in the stalk, over-accumulation of iron in reproductive organs, and a decrease in fertility. Finally, we show that, in the absence of ferritin, plants have higher levels of reactive oxygen species, and increased activity of enzymes involved in their detoxification. Seed germination also showed higher sensitivity to pro-oxidant treatments. Arabidopsis ferritins are therefore essential to protect cells against oxidative damage.

(J.M.W.) A combined bioinformatic and experimental approach is being used to uncover the functions of a novel family of cation/H 1 exchanger (CHX) genes in plants using Arabidopsis as a model. The predicted protein (85-95 kD) of 28 AtCHX genes after revision consists of an amino-terminal domain with 10 to 12 transmembrane spans (approximately 440 residues) and a hydrophilic domain of approximately 360 residues at the carboxyl end, which is proposed to have regulatory roles. The hydrophobic, but not the hydrophilic, domain of plant CHX is remarkably similar to monovalent cation/proton antiporter-2 (CPA2) proteins, especially yeast (Saccharomyces cerevisiae) KHA1 and Synechocystis NhaS4. Reports of characterized fungal and prokaryotic CPA2 indicate that they have various transport modes, including K 1 /H 1 (KHA1), Na, and ligand-gated ion channel (KefC). The expression pattern of AtCHX genes was determined by reverse transcription PCR, promoter-driven b-glucuronidase expression in transgenic plants, and Affymetrix ATH1 genome arrays. Results show that 18 genes are specifically or preferentially expressed in the male gametophyte, and six genes are highly expressed in sporophytic tissues. Microarray data revealed that several AtCHX genes were developmentally regulated during microgametogenesis. An exciting idea is that CHX proteins allow osmotic adjustment and K 1 homeostasis as mature pollen desiccates and then rehydrates at germination. The multiplicity of CHX-like genes is conserved in higher plants but is not found in animals. Only 17 genes, OsCHX01 to OsCHX17, were identified in rice (Oryza sativa) subsp. japonica, suggesting diversification of CHX in Arabidopsis. These results reveal a novel CHX gene family in flowering plants with potential functions in pollen development, germination, and tube growth.The ability to complete the plant life cycle depends not only on uptake of essential minerals, but also on the distribution and sorting of each ion to specific tissues, cell types, and organelles at all developmental stages. How plants achieve this under environments containing widely different levels of mineral nutrients is still poorly understood. This resilience can be attributed in part to a large number of transporters with varying ion specificities and affinities, and signal transduction networks that modulate the activities of each transporter. In spite of the remarkable advances since the discovery of the essential nutrients by Hoagland (1944), until recently we had no idea about the total number or types of transporters required to complete the plant life cycle.The completed Arabidopsis genome revealed more than 800 predicted transporters, of which most are secondary active transporters (.65%; Arabidopsis Genome Initiative, 2000). Most cotransporters depend on the proton electrochemical gradient generated by primary proton pumps and have been classified based on both phylogeny and function as transporters for cation, anion, and C-and N-containing compounds, including sugars, amino acids, drugs, and ...