High molecular weight glutenin subunit (HMW-GS) 1Dx5 is concentrated in small protein bodies when overexpressed in wheat starchy endosperm

The high molecular weight glutenin subunits (HMW-GSs) are key determinants of the processing properties of wheat and over-expression of HMW-GS by transgenesis has therefore been widely studied as a strategy to improve grain quality. Two routes of protein body (PB) formation operate in the developing endosperm, with glutenin-rich PBs being formed by direct accumulation within the lumen of the ER and most gliadins being transported via the Golgi apparatus and deposited within the vacuole. However, nothing is known about the effects of HMW-GS over-expression on the operation of these routes. In this study, light microscopy was used to determine the morphology and distribution of protein bodies in developing and mature grains of transgenic wheat with high over-expression of HMW-GS 1Dx5. Many small ER-derived PBs were observed in the cytoplasm of endosperm cells in the transgenic line, but very few in control grains. Immunofluorescence microscopy showed that these smaller PBs contained mainly HMW-GSs. Image analysis confirmed that the number of the smallest PBs was significantly higher in sub-aleurone cells. The segregation of the HMW-GS in ER-derived PBs may affect grain processing quality as over-expressions of HMW-GS 1Dx5 is known to result in the formation of insoluble polymers and an over-strong dough phenotype.