Over-expression of OsIRT1 leads to increased iron and zinc accumulations in rice

Uptake and translocation of micronutrients are essential for plant growth. These micronutrients are also important food components. We generated transgenic rice plants over-expressing OsIRT1 to evaluate its functional roles in metal homeostasis. Those plants showed enhanced tolerance to iron deficiency at the seedling stage. In paddy fields, this over-expression caused plant architecture to be altered. In addition, those plants were sensitive to excess Zn and Cd, indicating that OsIRT1 also transports those metals. As expected, iron and zinc contents were elevated in the shoots, roots and mature seeds of over-expressing plants. This demonstrates that OsIRT1 can be used for enhancing micronutrient levels in rice grains.     

Nickel: a micronutrient essential for higher plants

Nickel was established as an essential micronutrient for the growth of temperate cereal crops. Grain from barley (Hordeum vulgare L. cv `Onda'; containing 40 to 80 nanograms of Ni per gram dry weight) grown in solution culture with negligible Ni concentrations (< 30 nanograms of Ni per liter) exhibited greatly reduced germination rates (i.e. 50% less than grain from Ni-adequate plants) and seedling vigor of the viable grain was greatly depressed. Grain containing less than 30 nanograms per gram dry weight was inviable. Under Ni-deficient conditions, barley plants fail to produce viable grain because of a disruption of the maternal plant's normal grain-filling and maturation processes that occur following formation of the grain embryo. The observations that (a) barley plants fail to complete their life cycle in the absence of Ni and (b) addition of Ni to the growth medium completely alleviates deficiency symptoms in the maternal plants satisfies the essentiality criteria; thus, Ni should be considered a micronutrient for cereals. Because Ni is required by legumes, and is now established as essential for cereals, we conclude that Ni should be added to the list of micronutrients essential for all higher plant growth.     

Nickel as a micronutrient element for plants

The detrimental effects of excessive Ni on plant growth have been well known for many years. More recent evidence indicates that Ni is required in small amounts for normal plant growth and development. Ni is an essential component of urease in plants and microorganisms. A deficiency of Ni in plants is reported to result in necrotic lesions in leaves in response to toxic accumulations of urea. Urease plays an essential role in mobilization of nitrogenous compounds in plants, a process that is especially important during seed germination and fruit formation when protein reserves are degraded into amino acids. Arginine, an abundant amino acid in plants, when degraded produces urea as a product and urease is needed for urea utilization. Theories of urea formation during allantoin degradation in Glycine max have been recently refuted. In G. max ureides apparently are metabolized via an amidohydrolase reaction with subsequent degradation of ureidoglycine, yielding glyoxylate, NH+4 and CO2. No evidence is available for the formation of urea in this pathway. Nitrogen-fixing symbionts, such as Rhizobium and Bradyrhizobium, contain two known Ni enzymes: urease and hydrogenase. Optimum growth of nodulated legumes and actinorhizal plants may depend on an adequate supply of Ni to meet the requirements of the Ni-requiring enzymes in host plants and endophytes. The seeds of severely Ni-deficient Hordeum are completely inviable, thus providing conclusive evidence for the essentiality of Ni for this species. The evidence indicates that Ni must be added to the list of micronutrient elements generally required by plants.     

Zinc efficiency is correlated with enhanced expression and activity of zinc-requiring enzymes in wheat

Zinc (Zn) is an essential micronutrient for plants. The ability of plants to maintain significant yields under low Zn is termed Zn efficiency (ZE) and its genetic and mechanistic basis is still not well understood. Previously, we showed that root Zn uptake did not play a role in ZE. In the current study, Zn-efficient and -inefficient wheat (Triticum aestivum) genotypes were grown for 13 d in chelate buffer nutrient solutions at low (0.1 pM), sufficient (150 pM), and high (1 microM) Zn(2+) activities and analyzed for root-to-shoot translocation of Zn, subcellular leaf Zn distribution, and activity and expression of the Zn-requiring enzymes in leaves. No correlation between ZE and Zn translocation to the shoot was found. Furthermore, total and water-soluble concentrations of leaf Zn were not associated with ZE, and no differences in subcellular Zn compartmentation were found between Zn-efficient and -inefficient genotypes. However, the expression and activity of the Zn-requiring enzymes copper (Cu)/Zn superoxide dismutase (SOD) and carbonic anhydrase did correlate with differences in ZE. Northern analysis suggested that Cu/ZnSOD gene expression was up-regulated in the Zn-efficient genotype, Kirgiz, but not in inefficient BDME. Under Zn deficiency stress, the very Zn-efficient genotype Kirgiz and moderately Zn-efficient Dagdas exhibited an increased activity of Cu/ZnSOD and carbonic anhydrase when compared with Zn-inefficient BDME. These results suggest that Zn-efficient genotypes may be able to maintain the functioning of Zn-requiring enzymes under low Zn conditions; thus, biochemical Zn utilization may be an important component of ZE in wheat.     

Mechanism of phosphorus-induced zinc deficiency in cotton. I. Zinc deficiency-enhanced uptake rate of phosphorus

The effect of withholding Zn on the uptake, translocation and accumulation of P was studied in cotton plants ( Gossypium hirsutum L. cv. Deltapine 15/21) grown in nutrient solutions under controlled environmental conditions. The influence of P on the uptake rate, translocation and distribution of ⁶⁵Zn in the plants was also examined. Increasing the P supply resulted in severe Zn deficiency symptoms (interveinal chlorosis) as well as P toxicity symptoms, which were characterized by leaf puckering and grayish-brown marginal necrosis. Zinc deficiency markedly increased the uptake and translocation rates of P over the whole concentration range tested (5x10^-5 to 1.25x10^-3 M ). Uptake and translocation rates of P increased with both level of P and severity of Zn deficiency. This often caused P toxicity symptoms on Zn-deficient leaves. In contrast to P, the concentrations of K and Mg in the leaves were not affected by Zn deficiency. Similar results were obtained for sunflower ( Helianthus annuus L.) and buckwheat ( Fagopyrum esculentum Moench) plants. Higher P concentrations in Zn-deficient leaves or shoots could not be attributed wholly to reduced shoot growth. This was also evident when Zn deficiency was compared with other micronutrient (Fe, Mn, and Cu) deficiencies. Only Zn-deficient plants showed enhanced uptake and translocation of P. In experiments with ⁶⁵Zn, a high P supply did not depress uptake and translocation of Zn. From the results obtained it is concluded that the P-induced Zn deficiency in cotton, as well as in other species, is primarily caused by enhanced P uptake and translocation and not by inhibition of Zn uptake.     

Micronutrient uptake and distribution in mycorrhizal or phosphorus-fertilized soybeans

Summary Soybean plants were grown in a soil very low in available P. Seedlings were inoculated with two vesicular-arbuscular mycorrhizal (VAM) fungi or were left non-inoculated and fertilized with P. Assimilation and allocation of micronutrients (Fe, Mn, Zn, and Cu) were determined during host development, and the uptake of trace elements in VAM plants was compared to P-fertilized, non-VAM plants of similar weight, growth stage, and P status. Copper and zinc concentrations were always higher in VAM plants, while iron and manganese concentrations were lower than in the equivalent P-fertilized soybeans. Differences in the micronutrient content of fully-mature soybean pods reflected differences in the leaves and roots. Thus, for trace elements, seed quality can be altered by VAM colonization in a fashion not duplicated by P fertilizer. Contribution from the Western Regional Research Center, USDA-ARS (CRIS No. 5325-20580-003).     

植物对硼的吸收、转运及其遗传调控(综述)

硼是植物生长发育必需的一种微量营养元素,缺硼和硼毒害是国内外农业生产中普遍存在的问题,不仅影响产量,而且还会降低品质。近年来,植物的硼吸收和转运机制研究取得较大进展,从而为基因工程改良植物的缺硼和硼毒害奠定了重要基础。本文就植物对硼的吸收和转运机制以及植物对缺硼和硼毒害耐性的遗传调控研究概况进行综述。     

Zeroing in on zinc uptake in yeast and plants.

Zinc is an essential micronutrient. Genes responsible for zinc uptake have now been identified from yeast and plants. These genes belong to an extended family of cation transporters called the ZIP gene family. Zinc efflux genes that belong to another transporter family, the CDF family, have also been identified in yeast and Arabidopsis. It is clear that studies in yeast can greatly aid our understanding of zinc metabolism in plants.     

The FRD3-mediated efflux of citrate into the root vasculature is necessary for efficient iron translocation

Iron, despite being an essential micronutrient, becomes toxic if present at high levels. As a result, plants possess carefully regulated mechanisms to acquire iron from the soil. The ferric reductase defective3 (frd3) mutant of Arabidopsis (Arabidopsis thaliana) is chlorotic and exhibits constitutive expression of its iron uptake responses. Consequently, frd3 mutants overaccumulate iron; yet, paradoxically, the frd3 phenotypes are due to a reduction in the amount of iron present inside frd3 leaf cells. The FRD3 protein belongs to the multidrug and toxin efflux family, members of which are known to export low-M(r) organic molecules. We therefore hypothesized that FRD3 loads an iron chelator necessary for the correct distribution of iron throughout the plant into the xylem. One such potential chelator is citrate. Xylem exudate from frd3 plants contains significantly less citrate and iron than the exudate from wild-type plants. Additionally, supplementation of growth media with citrate rescues the frd3 phenotypes. The ectopic expression of FRD3-GFP results in enhanced tolerance to aluminum in Arabidopsis roots, a hallmark of organic acid exudation. Consistent with this result, approximately 3 times more citrate was detected in root exudate from plants ectopically expressing FRD3-GFP. Finally, heterologous studies in Xenopus laevis oocytes reveal that FRD3 mediates the transport of citrate. These results all strongly support the hypothesis that FRD3 effluxes citrate into the root vasculature, a process important for the translocation of iron to the leaves, as well as confirm previous reports suggesting that iron moves through the xylem as a ferric-citrate complex. Our results provide additional answers to long-standing questions about iron chelation in the vasculature and organic acid transport.     

Beryllium in the environment: Whether fatal for plant growth?

Beryllium could be a threatening heavy metal pollutant in the agro-ecosystem that may severely affect the performance of crops. Beryllium is used in various industries to make nuclear weapons and reactors, aircraft and space vehicle structures, instruments, and X-ray machines, and its entry into the environment is alarming for the productivity and sustainability of the ecosystem. In this review, we present a contemporary synthesis of the existing data regarding the toxic effects of beryllium on toxicity on biochemical and physiological processes in plants. Moreover, uptake, translocation, and assimilation of beryllium and its interaction with some essential mineral elements are also discussed. Although limited data are available regarding biochemical responses of plants to beryllium toxicity, we tried to clarify some basic physiological and biochemical steps that can be hampered by beryllium in plants. We linked our hypothetical concepts with previous evidence and provide a comprehensive summary of all possible remediation strategies that can be used for plants. Overall, we hope this review will be beneficial due to its practical implications and research directions.     

Micronutrient transport in mycorrhizal symbiosis; zinc steals the show

Mycorrhizas are mutually beneficial associations between soil-borne fungi and plant roots. Mycorrhizal fungi provide their host plant with essential nutrients in exchange for sugars and/or lipids. Traditionally, transport and translocation of macronutrients, including nitrogen and phosphorus, throughout the fungal mycelium and towards the host plant are well studied. However, the regulation of nutrient exchange and their contribution in the morphogenesis and development of mycorrhizas remains unclear. In this Opinion, we argue that adding micronutrients in the current models of symbiotic transport is essential to fully understand the establishment, maintenance, and functioning of mycorrhizal associations. Homeostatic mechanisms at the cellular level and the first transport proteins involved have been recently documented for zinc (Zn) in arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal fungi. Mycorrhizal plants benefit from an improved Zn status in control conditions and are better protected when environmental Zn availability fluctuates. These recent progresses are paving the way for a better understanding of micronutrient allocation in mycorrhizas. Revising our vision on the role of micronutrients, particularly of Zn, in these interactions will allow a better use of mycorrhizal fungi in sustainable agriculture and forestry, and will increase management practices in waste land, as well as in agricultural and natural ecosystems.     

Nutrition of Cosmarium turpinii

A defined medium for Cosmarium turpinii Bréb. was established. The optimal concentrations of major elements (Ca, N, P, S, and Mg) were detemiined, with the calcium level found to be most demanding. Iron, boron, and manganese were found to be essential micronutrient elements. Vitamin B(12) is also required, eitber as the planar group of cyanocobalamin alone or with adenine or benziniizole derivatives. Survival of cells during plateau pbase of growth can be extended by tbe addition of eitber 10(-3) M urea, isoleucine, or glycine.     

Magnesium ions alleviate the negative effect of manganese on Glomus claroideum BEG23

The phytotoxicity of excessive levels of manganese (Mn), an essential micronutrient, can be alleviated significantly by a high supply of magnesium (Mg) ions to plants. A similar interaction of these two elements in the development of arbuscular mycorrhizal (AM) fungi has been verified in two experimental systems. In in vitro experiments, an outgrowth of hyphae from excised, surface-disinfected root segments colonised with Glomus claroideum BEG23 was measured after 5 days incubation in liquid media. When only Mn ions were present in the media at higher concentrations (>0.05 mM), the growth of hyphae from root segments was reduced significantly. Addition of magnesium sulphate to the incubation solution reduced the inhibitory effects of Mn on hyphal growth. Alleviation of Mn toxicity by Mg ions observed in in vitro experiments was verified also for the symbiotic association between G. claroideum and maize as a host plant in a hydroponics sand culture experiment.     

NaBC1 is a ubiquitous electrogenic Na+ -coupled borate transporter essential for cellular boron homeostasis and cell growth and proliferation

Boron is a vital micronutrient in plants and may be essential for animal growth and development. Whereas the role of boron in the life cycle of plants is well documented, nothing is known about boron homeostasis and function in animal cells. NaBC1, the mammalian homolog of AtBor1, is a borate transporter. In the absence of borate, NaBC1 conducts Na(+) and OH(-) (H(+)), while in the presence of borate, NaBC1 functions as an electrogenic, voltage-regulated, Na(+)-coupled B(OH)(4)(-) transporter. At low concentrations, borate activated the MAPK pathway to stimulate cell growth and proliferation, and at high concentrations, it was toxic. Accordingly, overexpression of NaBC1 shifted both effects of borate to the left, whereas knockdown of NaBC1 halted cell growth and proliferation. These findings may reveal a previously unrecognized role for NaBC1 in borate homeostasis and open the way to better understanding of the many presumed physiological roles of borate in animals.     

Boron uptake by ectomycorrhizas of silver birch

Boron (B) is an essential micronutrient for plants but it is thought not to be essential for fungi. We studied whether the extraradical mycelia of Paxillus involutus in symbiosis with silver birch (Betula pendula) take up B and transport it to the host plant. We grew mycorrhizal plants in flat microcosms with a partitioning wall, below which there was only extraradical mycelium. A boric acid solution enriched in ¹⁰B was applied to these mycelia. Increased ¹⁰B/¹¹B isotope ratios were subsequently measured in birch leaves, stems, and roots plus mycorrhizas in the upper compartment. Boron was therefore taken up by the mycorrhizal mycelia and transported to the host plant in this species combination.     

The use of transgenic plants in the bioremediation of soils contaminated with trace elements

The use of plants to clean-up soils contaminated with trace elements could provide a cheap and sustainable technology for bioremediation. Field trials suggested that the rate of contaminant removal using conventional plants and growth conditions is insufficient. The introduction of novel traits into high biomass plants in a transgenic approach is a promising strategy for the development of effective phytoremediation technologies. This has been exemplified by generating plants able to convert organic and ionic forms of mercury into the less toxic, volatile, elemental mercury, a trait that occurs naturally only in some bacteria and not at all in plants. The engineering of a phytoremediator plant requires the optimization of a number of processes, including trace element mobilization in the soil, uptake into the root, detoxification and allocation within the plant. A number of transgenic plants have been generated in an attempt to modify the tolerance, uptake or homeostasis of trace elements. The phenotypes of these plants provide important insights for the improvement of engineering strategies. A better understanding, both of micronutrient acquisition and homeostasis, and of the genetic, biochemical and physiological basis of metal hyperaccumulation in plants, will be of key importance for the success of phytoremediation.     

Boron requirement in the Discaria trinervis (Rhamnaceae) and Frankia symbiotic relationship. Its essentiality for Frankia BCU110501 growth and nitrogen fixation

The essentiality of boron (B) for nitrogen fixation in heterocystous cyanobacteria and rhizobial symbioses has been widely established. However, nothing is known about the possible involvement of the micronutrient in actinorhizal symbioses. Therefore, the effect of boron (B) deficiency on the establishment of the Discaria trinervis-Frankia BCU110501 symbiosis was investigated. Nodulation was diminished in B-deficient D. trinervis or in plants inoculated with Frankia grown in the absence of B. These poorly nodulated plants showed a reduction of shoot and root weight and small size. Because depletion of the micronutrient during growth of the actinomycete altered the infection capacity of Frankia , we also studied growth, structure and nitrogen fixation of free-living Frankia BCU110501. Growth was delayed in B-deficient BAP media (+N cultures), and completely inhibited in B-deprived N-free BAP media (–N cultures), suggesting that B is required to enhance growth of Frankia and essential for the development of nitrogen fixing activity. Ultrastructural study of B-deficient cells showed an alteration of filament walls both in +N and especially in –N cultures, indicating a possible role of the microelement in the maintenance of these structures. Moreover, the stability of vesicle envelopes was impaired in the absence of B and, hence, nitrogenase occurrence and nitrogen fixation were totally absent. The results show that B is required for both partners to establish an effective symbiosis.