Identification of Zn-nicotianamine and Fe-2'-Deoxymugineic acid in the phloem sap from rice plants (Oryza sativa L.).
ABSTRACT In higher plants, the supply of metals such as Zn and Fe via phloem is important for the growth and physiology of young organs. However, little information is available on the speciation (chemical forms) of these metals in the phloem fluids. Because the pH of phloem fluids is slightly alkaline and the concentration of phosphate, which may bind to metals, is high, Zn and Fe in phloem fluids could be precipitated if these metals do not form complexes with some ligand compounds. In the present experiment, we examined the chemical forms of Zn and Fe in phloem sap collected from rice (Oryza sativa L.) by separating the phloem sap using size-exclusion and anion-exchange chromatography, and identifying the contents using electrospray ionization time-of-flight mass spectrometry. The low molecular weight chemical forms of Zn and Fe were identified as Zn-nicotianamine and Fe(III)-2'-deoxymugineic acid complexes, respectively. This report is the first to identify metal-chelate complexes in rice phloem sap.
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ABSTRACT: Biofortification of staple crops with essential micronutrients relies on the efficient, long distance transport of nutrients to the developing seed. The main route of this transport in common wheat (Triticum aestivum) is via the phloem, but due to the reactive nature of some essential micronutrients (specifically Fe and Zn), they need to form ligands with metabolites for transport within the phloem. Current methods available in collecting phloem exudate allows for small volumes (μL or nL) to be collected which limits the breadth of metabolite analysis. We present a technical advance in the measurement of 79 metabolites in as little as 19.5 nL of phloem exudate. This was achieved by using mass spectrometry based, metabolomic techniques.Plant Methods 01/2014; 10:27. · 2.59 Impact Factor
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ABSTRACT: Background During wheat senescence, leaf components are degraded in a coordinated manner, releasing amino acids and micronutrients which are subsequently transported to the developing grain. We have previously shown that the simultaneous downregulation of Grain Protein Content (GPC) transcription factors, GPC1 and GPC2, greatly delays senescence and disrupts nutrient remobilization, and therefore provide a valuable entry point to identify genes involved in micronutrient transport to the wheat grain.ResultsWe generated loss-of-function mutations for GPC1 and GPC2 in tetraploid wheat and showed in field trials that gpc1 mutants exhibit significant delays in senescence and reductions in grain Zn and Fe content, but that mutations in GPC2 had no significant effect on these traits. An RNA-seq study of these mutants at different time points showed a larger proportion of senescence-regulated genes among the GPC1 (64%) than among the GPC2 (37%) regulated genes. Combined, the two GPC genes regulate a subset (21.2%) of the senescence-regulated genes, 76.1% of which are upregulated at 12 days after anthesis, before the appearance of any visible signs of senescence. Taken together, these results demonstrate that GPC1 is a key regulator of nutrient remobilization which acts predominantly during the early stages of senescence. Genes upregulated at this stage include transporters from the ZIP and YSL gene families, which facilitate Zn and Fe export from the cytoplasm to the phloem, and genes involved in the biosynthesis of chelators that facilitate the phloem-based transport of these nutrients to the grains.Conclusions This study provides an overview of the transport mechanisms activated in the wheat flag leaf during monocarpic senescence. It also identifies promising targets to improve nutrient remobilization to the wheat grain, which can help mitigate Zn and Fe deficiencies that afflict many regions of the developing world.BMC Plant Biology 12/2014; 14(1):368. · 4.35 Impact Factor
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ABSTRACT: Increasing nitrogen supply can increase Fe and Zn concentrations in wheat grain, but the underlying mechanisms remain unclear. Size-exclusion chromatography coupled with inductively coupled plasma mass spectrometry was used to determine Fe and Zn speciation in the soluble extracts of grain pearling fractions of two wheat cultivars grown at two N rates (100 and 350 kg of N ha −1). Increasing N supply increased the concentrations of total Fe and Zn and the portions of Fe and Zn unextractable with a Tris−HCl buffer and decreased the concentrations of Tris−HCl-extractable (soluble) Fe and Zn. Within the soluble fraction, Fe and Zn bound to low molecular weight compounds, likely to be Fe−nicotianamine and Fe−deoxymugineic acid or Zn−nicotianamine, were decreased by 5−12% and 4−37%, respectively, by the high N treatment, whereas Fe and Zn bound to soluble high molecular weight or soluble phytate fractions were less affected. The positive effect of N on grain Fe and Zn concentrations was attributed to an increased sink in the grain, probably in the form of water-insoluble proteins. ■ INTRODUCTION Iron and zinc deficiencies are widespread nutritional disorders, affecting over two billion people in the world. 1,2 Insufficient dietary intakes of Fe and Zn and limited dietary diversity are thought to be responsible for human micronutrient deficiencies, especially in developing countries, where high proportions of cereal grains with inherently low concentrations of Fe and Zn, such as wheat and rice, are consumed as staple foods. 1,3 The bioavailability of Fe and Zn in cereal grains is also relatively low due to the presence of antinutritional compounds such as phytic acid and phenolic compounds. 1 Additionally, milling of wheat grain into white flour further results in reduced concentrations of Fe and Zn, because they are enriched in the outer parts of the grains, consisting mainly of the aleurone layer, embryo, pericarp, and testa. 4−6 Therefore, increasing Fe and Zn concentrations and/or their bioavailability in white flour is desirable for tackling the problem of micronutrient malnutrition. Recent studies have shown that nitrogen supply is an important factor affecting the concentrations of Fe and Zn in wheat grain. For example, under both field and glasshouse conditions, increasing N supply generally enhances Fe and Zn concentrations in wheat grain. 5,7−9 It has been reported that N increases Zn uptake by roots, Zn translocation from roots to shoots and Zn remobilization from leaves to grain in wheat, 4,1001/2014;