New insights into iron transport from maternal tissues to endosperm in mature wheat seed using synchrotron radiation

Abstract

Iron deficiency is estimated to affect over half of the world’s population (Zheng et al., 2010). Wheat flour is the main ingredient of bread, and is a staple food for many civilizations. In wheat grains, iron is located in the outer layer, called bran and is lost substantially during milling and processing (Zhang et al., 2010). Thus, the flour is almost devoid of iron. Wheat grain with iron-enriched endosperm is of great interest. It is desirable to understand the bottlenecks which prevent iron translocation from the outer bran layers into the endosperm in cereals. Synchrotron radiation can be useful in identifying the transport and state of metals in plant tissues. We generated element localization maps in grains of contrasting genotypes of wheat by using synchrotron powered beam line, VESPERS (very sensitive elemental and structural probe employing radiation), at Canadian Light Source, Saskatoon, Canada. The X-ray Fluorescence analysis was focused at crease, aleurone and endosperm of mature seeds from Triticum aestivum and Aegilops kotschyi. In high iron genotypes, iron was present at
higher concentration in the vascular cells than the aleurone. In contrast, in the low iron genotypes, most of the Fe was mobilized from vascular cells to aleurone cells. To further validate this result and to get accurate quantification of Fe in different tissues, microPIXE (Particle induced x-ray emission) analysis of the same samples is in progress at the Biotechnical Faculty and Jozef Stefan Institute, Ljubljana, Slovenia. We recorded XANES (X-ray Absorption Near Edge Structure) spectra at different points of the crease and aleurone layer. The oxidation state of iron in all cases was Fe3+. The pre-edge structures showed one bump, suggesting tetraedric or octaedric Fe-oxygen coordination. To know the chemical coordination environment around iron centre in the cells of different tissues (such as vascular cells, transfer cells, aleurone cells, and endosperm cells), EXAFS (extended X-ray absorption fine structure) is being run on the wheat grain samples.