Abstract
Zinc deficiency is causing malnutrition for nearly one third of world populations. It is especially relevant in cereal-based diets in which low amounts of mineral and protein are present. In biological systems, Zn is mainly associated with protein. Cereal grains contain the highest Zn concentration during early developmental stage. Although hordeins are the major storage proteins in the mature barley grain and suggested to be involved in Zn binding, very little information is available regarding the Zn fertilization effects of hordein transcripts at early developmental stage and possible incorporation of Zn with hordein protein of matured grain. Zinc fertilization experiments were conducted in a greenhouse with barley cv. Golden Promise. Zn concentration of the matured grain was measured and the results showed that the increasing Zn fertilization increased grain Zn concentration. Quantitative real time PCR showed increased level of total hordein transcripts upon increasing level of Zn fertilization at 10 days after pollination. Among the hordein transcripts the amount of B-hordeins was highly correlated with the Zn concentration of matured grain. In addition, protein content of the matured grain was analysed and a positive linear relationship was found between the percentage of B-hordein and total grain Zn concentration while C-hordein level decreased. Zn sensing dithizone assay was applied to localize Zn in the matured grain. The Zn distribution was not limited to the embryo and aleurone layer but was also present in the outer part of the endosperm (sub-aleurone layers) which known to be rich in proteins including B-hordeins. Increased Zn fertilization enriched Zn even in the endosperm. Therefore, the increased amount of B-hordein and decreased C-hordein content suggested that B-hordein upregulation or difference between B and C hordein could be one of the key factors for Zn biofortification of cereal grains due to the Zn fertilization.
Highlights
Zinc (Zn) is an essential element for plants and animals
Effects of Zn fertilization on the Zn concentration of the matured grain The soil used in our experiments has very little amount of Zn (0.4 g/m3) and the low Zn treated plants received only slight amount of Zn from soil
Foliar and soil Zn fertilization resulted in an increase of grain Zn concentration
Summary
Zinc (Zn) is an essential element for plants and animals. After iron, Zn is the most abundant transition metal in organisms and is present in all six enzyme classes [1]. Bioinformatics searches for known zinc binding motifs identified that the human proteome contains 10–15% zinc binding proteins; and in Arabidopsis a total of 2367 proteins in 181 gene families are identified as Zn-related [1,7,8]. These figures do not reflect the total number of actual zinc binding proteins which might exceed these numbers since a lot of zinc binding motifs are impossible to predict with bioinformatics analyses [9,10]. In addition to undiscovered potential zinc binding motifs, there are intermolecular binding sites (in which Zn ion acts as a bridging ligand between two polypeptides) in the sequences that are extremely difficult to predict in silico [7,11]
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