Pattern of iron distribution in maternal and filial tissues in wheat grains with contrasting levels of iron

Abstract

Iron insufficiency is a worldwide problem in human diets. In cereals like wheat, the bran layer of the grains is an important source of iron. However, the dietary availability of iron in wheat flour is limited due to the loss of the iron-rich bran during milling and processing and the presence of anti-nutrients like phytic acid that keep iron strongly chelated in the grain. The present study investigated the localization of iron and phosphorus in grain tissues of wheat genotypes with contrasting grain iron content using synchrotron-based micro-X-ray fluorescence (micro-XRF) and micro-proton-induced X-ray emission (micro-PIXE). X-ray absorption near-edge spectroscopy (XANES) was employed to determine the proportion of divalent and trivalent forms of Fe in the grains. It revealed the abundance of oxygen, phosphorus, and sulphur in the local chemical environment of Fe in grains, as Fe-O-P-R and Fe-O-S-R coordination. Contrasting differences were noticed in tissue-specific relative localization of Fe, P, and S among the different genotypes, suggesting a possible effect of localization pattern on iron bioavailability. The current study reports the shift in iron distribution from maternal to filial tissues of grains during the evolution of wheat from its wild relatives to the present-day cultivated varieties, and thus suggests the value of detailed physical localization studies in varietal improvement programmes for food crops.

Comparative Transcriptional Profiling of Two Wheat Genotypes, with Contrasting Levels of Minerals in Grains, Shows Expression Differences during Grain Filling

“…This is in agreement with previous reports showing significant correlation between grain protein and micronutrient content [15], [70], [71]. The results also coincide with our observation of proportional association between elemental images of sulphur (possibly sulphur-rich peptides) and iron in cross-sections of the two contrasting genotypes [10].…”

“…Fe, Zn and Mn) in grain tissues than WL711. Furthermore, comparatively higher occurrence of nutritionally important minerals was observed in the endosperm of IITR26 grains, than WL711 [10], [11]. Differential expression profiling of genotypes with contrasting characters is an efficient tool for understanding the molecular basis of such phenotypic differences [13].…”

Micro-PIXE mapping of mineral distribution in mature grain of two pearl millet cultivars

“…S and Fe were also noted to be present at relatively high levels in the outer grain layers. It therefore seems plausible that Fe and S may be closely linked, particularly as part of a metalloprotein complex [8,47].…”

“…To support this drive for biofortification, there is a need to understand the spatial distribution of minerals in different tissues within the grain. The mapping of minerals in seed tissues has been achieved in several notable studies, which include work on important cereals, such as wheat [8,9], barley [10] and rice [11,12]; as well as certain pseudocereals [13][14][15] and legumes [16,17]. To our knowledge, the only studies reporting on the spatial distribution of mineral elements in millet, include a recent study using micro-proton-induced Xray emission (micro-PIXE) for finger millet [18] and a much earlier study on pearl millet, using energy dispersive X-ray analysis (EDX) [19].…”

Visualisation of the distribution of minerals in red non-tannin finger millet using PIXE microanalysis

“…To our knowledge, this quantitative elemental map of finger millet by PIXE is the first of an entire cereal kernel. While there are PIXE elemental maps of parts of some cereal species, these maps are not quantitative (Singh et al, 2013). Apparently, the only PIXE quantitative elemental map, of a whole edible grain, is that of the pseudocereal buckwheat (Pongrac et al, 2012).…”