Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO3−, OH−, H+), gaseous molecules (CO2, H2), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N2-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N2 is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N2 reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH− and HCO3− to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.

Root exudates as mediators of mineral acquisition in low-nutrient environments

Iron deficiency afflicts more than three billion people worldwide, and plants are the principal source of iron in most diets. Low availability of iron often limits plant growth because iron forms insoluble ferric oxides, leaving only a small, organically complexed fraction in soil solutions. The enzyme ferric-chelate reductase is required for most plants to acquire soluble iron. Here we report the isolation of the FRO2 gene, which is expressed in iron-deficient roots of Arabidopsis. FRO2 belongs to a superfamily of flavocytochromes that transport electrons across membranes. It possesses intramembranous binding sites for haem and cytoplasmic binding sites for nucleotide cofactors that donate and transfer electrons. We show that FRO2 is allelic to the frd1 mutations that impair the activity of ferric-chelate reductase. There is a nonsense mutation within the first exon of FRO2 in frd1-1 and a missense mutation within FRO2 in frd1-3. Introduction of functional FRO2 complements the frd1-1 phenotype in transgenic plants. The isolation of FRO2 has implications for the generation of crops with improved nutritional quality and increased growth in iron-deficient soils.

A ferric-chelate reductase for iron uptake from soils

Fe(I1) and Fe(II1) are relatively small ions with a marked propensity to form six-coordinate complexes with ligands containing O, N, and S. This property, combined with the remarkable range of redox potentials covered by ironcontaining enzymes, accounts for the role of iron in such fundamental reactions as ribonucleotide and dinitrogen reduction as well as in the energy-yielding electron transfer reactions of respiration and photosynthesis. At the same time, the chemical properties of iron place limitations on the cellular accumulation of this element. First, Fe(I1) and Fe(II1) can act catalytically to generate hydroxyl radicals that are the most potent oxidizing agents known (Table I). Because of the potential of iron for wreaking cellular havoc, organisms generally regulate its uptake; as well, they store iron in the form of ferritin, a multimeric protein consisting of a 24-subunit shell that can house up to 4500 atoms of iron in its central cavity (Theil, 1987). Iron stored in this manner is nontoxic and is readily available to the cell. The second limit to iron acquisition is the fact that iron is found in nature mostly as a constituent of insoluble oxyhydroxide polymers of the general composition FeOOH (Table I). These Fe(II1) oxides (e.g. goethite, hematite) are produced by the weathering of rock. Because Fe(II1) oxides are quite stable and not very soluble at neutra1 pH, free Fe(II1) in an aerobic, aqueous environment is limited to an equilibrium concentration of approximately 10-17 M, a value far below that required for the optimal growth of plants or microbes (Table I). Thus, the problem that soil-based organisms have with iron is not one of abundance, since iron ranks fourth among a11 elements on the surface of the earth, but rather one of availability in aerobic environments at biological pH. Iron deficiency can be particularly pronounced in plants grown on calcareous soils, which cover approximately onethird of the earth's surface. Iron deficiency is usually recognized by chlorotic or yellowed interveinal areas in new leaves and, if severe, can lead to reduction in crop yields and even complete crop failure. Chemically speaking, organisms have three means at their disposal to dissolve Fe(II1) oxides: protonation, chelation, and reduction. To compete successfully for iron, organisms have thus evolved specific mechanisms to acquire iron that are based on these chemical processes. In many organisms, in-

http://www.plantphysiol.org/content/plantphysiol/104/3/815.full.pdf

Iron: Nutritious, Noxious, and Not Readily Available.

Enzymic mechanisms of superoxide production.

Iron toxicity is a syndrome of disorder associated with large concentrations of reduced iron (Fe2+) in the soil solution. It only occurs in flooded soils and hence affects primarily the production of lowland rice. The appearance of iron toxicity symptoms in rice involves an excessive uptake of Fe2+ by the rice roots and its acropetal translocation into the leaves where an elevated production of toxic oxygen radicals can damage cell structural components and impair physiological processes. The typical visual symptom associated with these processes is the “bronzing” of the rice leaves and substantial associated yield losses.



The circumstances of iron toxicity are quite well established. Thus, the geochemistry, soil microbial processes, and the physiological effects of Fe2+ within the plant or cell are documented in a number of reviews and book chapters. However, despite our current knowledge of the processes and mechanisms involved, iron toxicity remains an important constraint to rice production, and together with Zn deficiency, it is the most commonly observed micronutrient disorder in wetland rice. Reported yield losses in farmers' fields usually range between 15% and 30%, but can also reach the level of complete crop failure.



A range of agronomic management interventions have been advocated to reduce the Fe2+ concentration in the soil or to foster the rice plants' ability to cope with excess iron in either soil or the plant. In addition, the available rice germplasm contains numerous accessions and cultivars which are reportedly tolerant to excess Fe2+. However, none of those options is universally applicable or efficient under the diverse environmental conditions where Fe toxicity is expressed. Based on the available literature, this paper categorizes iron-toxic environments, the steps involved in toxicity expression in rice, and the current knowledge of crop adaptation mechanisms in view of establishing a conceptual framework for future constraint analysis, research approaches, and the targeting of technical options.



Eisentoxizitat in Reis – Bedingungen und Managementkonzepte



Eisentoxizitat ist ein multiples Stresssyndrom, das mit hohen Konzentrationen reduzierten Eisens (Fe2+) in der Bodenlosung einhergeht. Da es nur in uberstauten Boden auftritt, beeintrachtigt es in erster Linie die Erzeugung von Nassreis. Das Auftreten von Eisentoxizitat setzt eine ubermasige Aufnahme von Fe2+ durch die Reiswurzel und dessen akropetalen Transport in die Blatter voraus. Dort bedingen die zweiwertigen Eisenionen die Bildung von toxischen Radikalen, die strukturelle Zellbestandteile zerstoren und physiologische Prozesse beeintrachtigen konnen. Das typischerweise damit verbundene makroskopische Symptombild ist eine Kupferfarbung der Blatter (“bronzing“). Damit verbunden sind erhebliche Ertragseinbusen.



Die Bedingungen, die zum Auftreten von Eisentoxizitat fuhren, sind relativ gut untersucht. So sind zum Beispiel die Geochemie, bodenmikrobiologische Prozesse und die physiologischen Effekte von Fe2+ in Geweben und Zellen in zahlreichen Artikeln dokumentiert. Allerdings bleibt die Eisentoxizitat trotz unseres derzeitigen Wissenstandes auch weiterhin ein entscheidender Begrenzungsfaktor fur den Reisanbau in zahlreichen tropischen Gebieten und stellt neben Zinkmangel das wichtigste Mikronahrstoffungleichgewicht im uberstauten Nassreis dar. Ertragsverluste belaufen sich in der Regel auf 15–30 %, allerdings kann es bei massiver Toxizitat auch zu einem totalen Ernteausfall kommen.



Diverse pflanzenbauliche Masnahmen verringern den Fe2+-Gehalt der Bodenlosung oder starken die Abwehrmechanismen von Reis gegenuber Fe2+. Ferner liegen zahlreiche Sorten und Zuchtlinien vor, die eine gewisse Toleranz gegenuber erhohten Fe-Gehalten aufweisen. Allerdings reichen offensichtlich weder die Anbaumasnahmen noch die Sortenwahl aus, um die schadigende Wirkung von Fe2+ an den unterschiedlichen eisentoxischen Standorten und Klima- und Anbausituationen zu kompensieren. Auf der Grundlage der publizierten Fachliteratur kategorisiert der vorliegende Beitrag die Bedingungen, unter denen Eisentoxizitat auftritt, die Wirkungsweise von Fe2+ und die Symptomauspragung an Nassreis sowie die derzeit bekannten agronomischen Managementoptionen und pflanzlichen Anpassungsmechanismen. Ziel dieser Ubersichtsarbeit ist die Erarbeitung eines Konzepts fur die zukunftige Problemanalyse und Schwerpunktsetzung in der Forschung sowie den zielgerichteten Einsatz von Technologien fur ein verbessertes Management von Eisentoxizitat im tropischen Nassreisanbau.

/pdf/iron-toxicity-in-rice-conditions-and-management-concepts-574bsnytek.pdf

Iron toxicity in rice--conditions and management concepts.

Plants take up iron as ferric chelates or, after reduction, as ferrous ions. Ferric reduction takes place at the plasma membrane of the root epidermis cells by a transmembrane redox system, which can be activated when iron is getting short. It is proposed that this inducible system, with NADPH as electron donor, is separate from a system, presumably present in all plant cells, which transports electrons from NADH or NADPH to ferricyanide, or,in vivo, oygen.

Regulated redox processes at the plasmalemma of plant root cells and their function in iron uptake.

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Regulated redox processes at the plasmalemma of plant root cells and their function in iron uptake.