Overexpressing the universal stress protein gene IbUSP17 increases starch content without a yield penalty in sweetpotato

The universal stress proteins (USPs) play important roles not only in abiotic stress tolerance but also in plant growth and development. However, the role of USPs in regulating starch biosynthesis has not been reported. In this study, the IbUSP17 gene was isolated from the sweetpotato line H283 with a high starch content. The IbUSP17 protein was localized in the nucleus. IbUSP17 was highly expressed in lines with high starch contents and during the rapid thickening and starch accumulation period of storage roots. Overexpressing IbUSP17 increased the storage root starch content, especially the amylopectin proportion, without a storage root yield penalty in sweetpotato. Overexpression of IbUSP17 up-regulated the genes involved in starch biosynthesis and increased the activities of enzymes related to amylopectin biosynthesis. The contents of components related to starch biosynthesis were also increased in IbUSP17-overexpressing plants. Silencing this gene produced the opposite effects. These results suggest that overexpression of IbUSP17 increases the starch content by up-regulating the genes involved in starch biosynthesis and increasing the activities of enzymes related to starch biosynthesis, especially amylopectin biosynthesis. The role of the USP gene in starch biosynthesis is revealed for the first time in this study. This gene is expected to be used to increase the starch yield and improve starch quality in sweetpotato.