Iron deficiency is among the most common nutritional disorders in plants. To cope with low iron supply, plants with the exception of the Gramineae increase the solubility and uptake of iron by inducing physiological and developmental alterations including iron reduction, soil acidification, Fe(II) transport and root-hair proliferation (strategy I). The chlorotic tomato fer mutant fails to activate the strategy I. It was shown previously that the fer gene is required in the root. Here, we show that fer plants exhibit root developmental phenotypes after low and sufficient iron nutrition indicating that FER acts irrespective of iron supply. Mutant fer roots displayed lower Leirt1 expression than wild-type roots. We isolated the fer gene by map-based cloning and demonstrate that it encodes a protein containing a basic helix-loop-helix domain. fer is expressed in a cell-specific pattern at the root tip independently from iron supply. Our results suggest that FER may control root physiology and development at a transcriptional level in response to iron supply and thus may be the first identified regulator for iron nutrition in plants.

Metal transporters regulated by iron can transport a variety of divalent metals, suggesting that iron regulation is important for specificity of iron transport. In plants, the iron-regulated broad-range metal transporter IRT1 is required for uptake of iron into the root epidermis. Functions of other iron-regulated plant metal transporters are not yet established. To deduce novel plant iron transport functions we studied the regulation of four tomato metal transporter genes belonging to the nramp and irt families with respect to environmental and genetic factors influencing iron uptake. We isolated Lenramp1 and Lenramp3 from tomato and demonstrate that these genes encode functional NRAMP metal transporters in yeast, where they were iron-regulated and localized mainly to intracellular vesicles. Lenramp1 and Leirt1 revealed both root-specific expression and up-regulation by iron deficiency, respectively, in contrast to Leirt2 and Lenramp3. Lenramp1 and Leirt1, but not Lenramp3 and Leirt2, were down-regulated in the roots of fer mutant plants deficient in a bHLH gene regulating iron uptake. In chloronerva mutant plants lacking the functional enzyme for synthesis of the plant-specific metal chelator nicotianamine Leirt1 and Lenramp1 were up-regulated despite sufficient iron supply independent of a functional fer gene. Lenramp1 was expressed in the vascular root parenchyma in a similar cellular pattern as the fer gene. However, the fer gene was not sufficient for inducing Lenramp1 and Leirt1 when ectopically expressed. Based on our results, we suggest a novel function for NRAMP1 in mobilizing iron in the vascular parenchyma upon iron deficiency in plants. We discuss fer/nicotianamine synthase-dependent and -independent regulatory pathways for metal transporter gene regulation.

Arabidopsis (Arabidopsis thaliana) and tomato (Lycopersicon esculentum) show similar physiological responses to iron deficiency, suggesting that homologous genes are involved. Essential gene functions are generally considered to be carried out by orthologs that have remained conserved in sequence and map position in evolutionarily related species. This assumption has not yet been proven for plant genomes that underwent large genome rearrangements. We addressed this question in an attempt to deduce functional gene pairs for iron reduction, iron transport, and iron regulation between Arabidopsis and tomato. Iron uptake processes are essential for plant growth. We investigated iron uptake gene pairs from tomato and Arabidopsis, namely sequence, conserved gene content of the regions containing iron uptake homologs based on conserved orthologous set marker analysis, gene expression patterns, and, in two cases, genetic data. Compared to tomato, the Arabidopsis genome revealed more and larger gene families coding for the iron uptake functions. The number of possible homologous pairs was reduced if functional expression data were taken into account in addition to sequence and map position. We predict novel homologous as well as partially redundant functions of ferric reductase-like and iron-regulated transporter-like genes in Arabidopsis and tomato. Arabidopsis nicotianamine synthase genes encode a partially redundant family. In this study, Arabidopsis gene redundancy generally reflected the presumed genome duplication structure. In some cases, statistical analysis of conserved gene regions between tomato and Arabidopsis suggested a common evolutionary origin. Although involvement of conserved genes in iron uptake was found, these essential genes seem to be of paralogous rather than orthologous origin in tomato and Arabidopsis.

Pycnidial fungi belonging to the genus Ampelomyces are common intracellular mycoparasites of powdery mildews worldwide. Some strains have already been developed as commercial biocontrol agents (BCAs) of Erysiphe necator and other powdery mildew species infecting important crops. One of the basic, and still debated, questions concerning the tritrophic relationships between host plants, powdery mildew fungi, and Ampelomyces mycoparasites is whether Ampelomyces strains isolated from certain species of the Erysiphales are narrowly specialized to their original mycohosts or are generalist mycoparasites of many powdery mildew fungi. This is also important for the use of Ampelomyces strains as BCAs. To understand this relationship, the nuclear ribosomal DNA internal transcribed spacer (ITS) and partial actin gene (act1) sequences of 55 Ampelomyces strains from E. necator were analyzed together with those of 47 strains isolated from other powdery mildew species. These phylogenetic analyses distinguished five major clades and strains from E. necator that were present in all but one clade. This work was supplemented with the selection of nine inter-simple sequence repeat (ISSR) markers for strain-specific identification of Ampelomyces mycoparasites to monitor the environmental fate of strains applied as BCAs. The genetic distances among strains calculated based on ISSR patterns have also highlighted the genetic diversity of Ampelomyces mycoparasites naturally occurring in grapevine powdery mildew. Overall, this work showed that Ampelomyces strains isolated from E. necator are genetically diverse and there is no indication of strict mycohost associations in these strains. However, these results cannot rule out a certain degree of quantitative association between at least some of the Ampelomyces lineages identified in this work and their original mycohosts.

-Iron uptake is strictly controlled in all organisms. Iron deficiency induces a well-defined series of physiological events, either based on iron reduction (strategy I, dicots and monots except grasses) or iron chelation (strategy II, grasses). Genes encoding the structural components of the strategies I and II have been described recently using molecular, genetic and biochemical techniques. The major participants of the strategy I are the ferric reductase and the Fe II transporter encoded by Atfro2 and Atirt1 homologs, respectively. The strategy II comprises the enzymes for phytosiderophore biosynthesis encoded by the genes for nicotianamine synthase, nicotianamine aminotransferase and others. Transcriptional and post-transcriptional regulatory mechanisms control the strategy I and II genes. The tomato fer gene is among the first identified regulator genes for iron uptake. The identification and characterisation of the regulatory mechanisms is an important step towards designing new strategies for improving the iron content of crop plants for human health. iron uptake / strategy I genes / strategy II genes / regulation / fer Résumé -Les réseaux de gènes impliqués dans les stratégies d'absorption de fer par les plantes. L'absorption de fer est contrôlée de manière rigoureuse dans tous les organismes. La carence en fer entraîne des séries bien connues d'incidents physiologiques, qu'ils soient basés sur la réduction de fer (stratégie I, dicotylédones et monocotylédones à l'exception des plantes herbacées) ou la chélation de fer (stratégie II, plantes herbacées). Les gènes codant pour les composants structuraux des stratégies I et II ont été décrits récemment en utilisant des techniques moléculaires, génétiques et biochimiques. Les principaux participants de la stratégie I sont la réductase ferrique et le transporteur Fe II codés respectivement par les homologues de Atfro2 et Atirt1. La stratégie II comporte les enzymes pour la biosynthèse de phytosidérophore codées par les gènes de la nicotianamine synthase, de la nicotianamine aminotransférase et d'autres. Les mécanismes de régulation transcriptionels et post-transcriptionels contrôlent les gènes des stratégies I et II. Le gène fer de la tomate figure parmi les premiers gènes régulateurs pour l'absorption de fer à avoir été identifiés. L'identification et la caractérisation des mécanismes régulateurs est une avancée considérable pour envisager de nouvelles stratégies visant à améliorer le contenu en fer des plantes cultivées pour la santé humaine. absorption de fer / stratégie I gènes / stratégie II gènes / régulation / fer