Mature wheat seeds are physiologically devoted to storing nutrients, in which mineral nutrients are unevenly distributed in different parts, reflecting their storage mechanisms, structural characteristics and physiological functions. Therefore, it is important to study the distribution and bioavailability of mineral nutrients in the wheat grains.
Based on Nano-XRF (X-ray fluorescence mapping with the nano-sized spatial resolution) imaging technology, this study characterized the distribution of vital elements, such as zinc (Zn), iron (Fe), calcium(Ca), sulfur (S), phosphorus (P) etc., in the wheat grain during the period of development at cellular and subcellular levels.
Detailed images of a large amount of nutrient storage were shown directly in the scutellum. While some specific elements, such as Zn, was sequestrated in the embryo. Moreover, the granular accumulation patterns of micronutrients such as Zn, Fe and manganese (Mn) were demonstrated as phytates in the aleurone layer, explaining the limited mobility of them; Ca also existed in the starchy endosperm as hot spots, which were possibly presented as calcium oxalate. These results indicated that the immobilization of intracellular metals affected the bioavailability. In addition to physiological functions, Ca is distributed in the cell wall of pericarp cells, revealing the maintenance of the cell structures. Sulfur distribution was special, contained in the protein storage vacuole of endosperm cells. It plays a key role in the storage mechanism of proteins in the endosperm of wheat grain.
Nano-XRF imaging technology, a high spatial technique, highlight how in situ analysis can provide direct evidence for the complexity of element distribution in wheat grains and generate new insights into structural characteristics. Physiological researches are combined with the further development of imaging technology to better understand the mechanism of element distribution and storage in wheat grains. The significantly heterogeneous and distinctive accumulation patterns of these elements signify different transport process and storage capacities among different cell types. These studies will provide basic knowledge for improving nutritional values and agronomic practices for biofortification.