Identification of a tri-iron(III), tri-citrate complex in the xylem sap of iron-deficient tomato resupplied with iron: new insights into plant iron long-distance transport

Chloroplastic and mitochondrial metal homeostasis

Transition metal deficiency has a strong impact on the growth and survival of an organism. Indeed, transition metals, such as iron, copper, manganese and zinc, constitute essential cofactors for many key cellular functions. Both photosynthesis and respiration rely on metal cofactor-mediated electron transport chains. Chloroplasts and mitochondria are, therefore, organelles with high metal ion demand and represent essential components of the metal homeostasis network in photosynthetic cells. In this review, we describe the metal requirements of chloroplasts and mitochondria, the acclimation of their functions to metal deficiency and recent advances in our understanding of their contributions to cellular metal homeostasis, the control of the cellular redox status and the synthesis of metal cofactors.

Towards a knowledge-based correction of iron chlorosis

Iron (Fe) deficiency-induced chlorosis is a major nutritional disorder in crops growing in calcareous soils. Iron deficiency in fruit tree crops causes chlorosis, decreases in vegetative growth and marked fruit yield and quality losses. Therefore, Fe fertilizers, either applied to the soil or delivered to the foliage, are used every year to control Fe deficiency in these crops. On the other hand, a substantial body of knowledge is available on the fundamentals of Fe uptake, long and short distance Fe transport and subcellular Fe allocation in plants. Most of this basic knowledge, however, applies only to Fe deficiency, with studies involving Fe fertilization (i.e., with Fe-deficient plants resupplied with Fe) being still scarce. This paper reviews recent developments in Fe-fertilizer research and the state-of-the-art of the knowledge on Fe acquisition, transport and utilization in plants. Also, the effects of Fe-fertilization on the plant responses to Fe deficiency are reviewed. Agronomical Fe-fertilization practices should benefit from the basic knowledge on plant Fe homeostasis already available; this should be considered as a long-term goal that can optimize fertilizer inputs, reduce grower's costs and minimize the environmental impact of fertilization.

Transporters contributing to iron trafficking in plants

This review will discuss recent progress in understanding the many roles of transporters in the whole-plant physiological processes that maintain iron (Fe) homeostasis. These processes include uptake from the soil via roots, control of transport from roots to above-ground parts of the plant, unloading of Fe from the xylem in above-ground parts, loading of Fe into mitochondria and plastids, transport of Fe to reproductive parts of the plant, and Fe mobilization during seed germination. In addition, we will discuss the mechanisms that plants use to cope with an apparently unintended consequence of Fe acquisition: the uptake of toxic heavy metals via Fe transporters. Rapid progress has been made in understanding the transport processes involved in each of these areas in the last 5 years and this review will focus on this recent progress. We will also highlight the key questions regarding transport steps that remain to be elucidated.

TOMATOMA: a novel tomato mutant database distributing Micro-Tom mutant collections

The tomato is an excellent model for studies of plants bearing berry-type fruits and for experimental studies of the Solanaceae family of plants due to its conserved genetic organization. In this study, a comprehensive mutant tomato population was generated in the background of Micro-Tom, a dwarf, rapid-growth variety. In this and previous studies, a family including 8,598 and 6,422 M(2) mutagenized lines was produced by ethylmethane sulfonate (EMS) mutagenesis and γ-ray irradiation, and this study developed and investigated these M(2) plants for alteration of visible phenotypes. A total of 9,183 independent M(2) families comprising 91,830 M(2) plants were inspected for phenotypic alteration, and 1,048 individual mutants were isolated. Subsequently, the observed mutant phenotypes were classified into 15 major categories and 48 subcategories. Overall, 1,819 phenotypic categories were found in 1,048 mutants. Of these mutants, 549 were pleiotropic, whereas 499 were non-pleiotropic. Multiple different mutant alleles per locus were found in the mutant libraries, suggesting that the mutagenized populations were nearly saturated. Additionally, genetic analysis of backcrosses indicated the successful inheritance of the mutations in BC(1)F(2) populations, confirming the reproducibility in the morphological phenotyping of the M(2) plants. To integrate and manage the visible phenotypes of mutants and other associated data, we developed the in silico database TOMATOMA, a relational system interfacing modules between mutant line names and phenotypic categories. TOMATOMA is a freely accessible database, and these mutant recourses are available through the TOMATOMA (http://tomatoma.nbrp.jp/index.jsp).

Secondary transport of metal-citrate complexes: the CitMHS family

Primary and secondary transport of citrate has been extensively studied in pathogenic and non-pathogenic bacteria. Primary transporters of citrate complexed with metal ions, particularly Fe, have also garnered attention, with the fec system of E. coli being a classic example. In contrast, little is known about secondary transporters of metal-citrate complexes. Recently, a family of proteins responsible for secondary metal-citrate transport in bacteria was discovered and designated as the CitMHS transporter family. Several members have been functionally characterized to date and serve as the foundation for understanding this family. Three subfamilies have been categorized, depending on the main metal ion transported. These subfamilies are the Mg(2+)-citrate transporter, the Ca(2+)-citrate transporter, and the Fe(3+)-citrate transporter. Each subfamily is believed to be substrate-selective due to the metal-citrate complexes being abundantly present in their environment and/or the ability of the complex to be metabolized by the organism. The implication of this family in the pathogenic access to Fe, information about transcriptional control, putative structure, predicted family members, members characterized to date and potential use in bioremediation are discussed.

Metabolite profile changes in xylem sap and leaf extracts of strategy I plants in response to iron deficiency and resupply

The metabolite profile changes induced by Fe deficiency in leaves and xylem sap of several Strategy I plant species have been characterized. We have confirmed that Fe deficiency causes consistent changes both in the xylem sap and leaf metabolite profiles. The main changes in the xylem sap metabolite profile in response to Fe deficiency include consistent decreases in amino acids, N-related metabolites and carbohydrates, and increases in TCA cycle metabolites. In tomato, Fe resupply causes a transitory flush of xylem sap carboxylates, but within 1 day the metabolite profile of the xylem sap from Fe-deficient plants becomes similar to that of Fe-sufficient controls. The main changes in the metabolite profile of leaf extracts in response to Fe deficiency include consistent increases in amino acids and N-related metabolites, carbohydrates and TCA cycle metabolites. In leaves, selected pairs of amino acids and TCA cycle metabolites show high correlations, with the sign depending of the Fe status. These data suggest that in low photosynthesis, C-starved Fe-deficient plants anaplerotic reactions involving amino acids can be crucial for short-term survival.

Proteomic characterization of iron deficiency responses in Cucumis sativus L. roots

BACKGROUND

Iron deficiency induces in Strategy I plants physiological, biochemical and molecular modifications capable to increase iron uptake from the rhizosphere. This effort needs a reorganization of metabolic pathways to efficiently sustain activities linked to the acquisition of iron; in fact, carbohydrates and the energetic metabolism has been shown to be involved in these responses. The aim of this work was to find both a confirmation of the already expected change in the enzyme concentrations induced in cucumber root tissue in response to iron deficiency as well as to find new insights on the involvement of other pathways.

RESULTS

The proteome pattern of soluble cytosolic proteins extracted from roots was obtained by 2-DE. Of about two thousand spots found, only those showing at least a two-fold increase or decrease in the concentration were considered for subsequent identification by mass spectrometry. Fifty-seven proteins showed significant changes, and 44 of them were identified. Twenty-one of them were increased in quantity, whereas 23 were decreased in quantity. Most of the increased proteins belong to glycolysis and nitrogen metabolism in agreement with the biochemical evidence. On the other hand, the proteins being decreased belong to the metabolism of sucrose and complex structural carbohydrates and to structural proteins.

CONCLUSIONS

The new available techniques allow to cast new light on the mechanisms involved in the changes occurring in plants under iron deficiency. The data obtained from this proteomic study confirm the metabolic changes occurring in cucumber as a response to Fe deficiency. Two main conclusions may be drawn. The first one is the confirmation of the increase in the glycolytic flux and in the anaerobic metabolism to sustain the energetic effort the Fe-deficient plants must undertake. The second conclusion is, on one hand, the decrease in the amount of enzymes linked to the biosynthesis of complex carbohydrates of the cell wall, and, on the other hand, the increase in enzymes linked to the turnover of proteins.

Recent progress in plant nutrition research: cross-talk between nutrients, plant physiology and soil microorganisms

Mineral nutrients taken up from the soil become incorporated into a variety of important compounds with structural and physiological roles in plants. We summarize how plant nutrients are linked to many metabolic pathways, plant hormones and other biological processes. We also focus on nutrient uptake, describing plant-microbe interactions, plant exudates, root architecture, transporters and their applications. Plants need to survive in soils with mineral concentrations that vary widely. Describing the relationships between nutrients and biological processes will enable us to understand the molecular basis for signaling, physiological damage and responses to mineral stresses.