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Title: A tonoplast sugar transporter underlies a sugar accumulation QTL in watermelon

Author
item REN, YI - National Engineering Research Center For Information Technology In Agriculture
item GUO, SHAOGUI - National Engineering Research Center For Information Technology In Agriculture
item ZHANG, JIE - National Engineering Research Center For Information Technology In Agriculture
item HE, HONGJU - National Engineering Research Center For Information Technology In Agriculture
item SUN, HONGHE - National Engineering Research Center For Information Technology In Agriculture
item TIAN, SHOUWEI - National Engineering Research Center For Information Technology In Agriculture
item GONG, GUOYI - National Engineering Research Center For Information Technology In Agriculture
item ZHANG, HAIYING - National Engineering Research Center For Information Technology In Agriculture
item Levi, Amnon
item TADMOR, YAAKOV - National Engineering Research Center For Information Technology In Agriculture
item XU, YONG - National Engineering Research Center For Information Technology In Agriculture

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2017
Publication Date: 11/8/2017
Citation: Ren, Y., Guo, S., Zhang, J., He, H., Sun, H., Tian, S., Gong, G., Zhang, H., Levi, A., Tadmor, Y., Xu, Y. 2017. A tonoplast sugar transporter underlies a sugar accumulation QTL in watermelon. Plant Physiology. 176(1):836-850. https://doi:10.1104/pp.17.01290.
DOI: https://doi.org/10.1104/pp.17.01290

Interpretive Summary: Watermelon is an important vegetable crop in the United States and throughout the world. Africa is the center of origin of watermelon. There, it exists in wild forms and the watermelon fruits have low sugar levels (0.5-3.0%). In contrast, the modern watermelon contains high levels (10-13%) of soluble sugars, like glucose, fructose and sucrose that grant its sweetness. There is limited knowledge on the genetics and genes controlling sugar accumulation in watermelon. In this study, ARS scientists have collaborated with scientists in China and Israel, on using advanced genomic technologies to construct a detailed genetic map and identify a sugar transporter gene which is responsible for sugar accumulation in watermelon. The researchers concluded that this sugar transporter gene may play an important role in the domestication of the sweet-dessert watermelon. The knowledge gained in this study should be useful for researchers and plant breeders interested in developing breeding program strategies to improve sweetness quality in watermelon cultivars.

Technical Abstract: The molecular mechanism controlling accumulation of soluble sugars in watermelon (Citrullus lanatus) fruit, a trait associated with sweet-dessert watermelon domestication, is still unknown. We re-sequenced 96 recombinant inbred lines, derived from a cross between sweet and unsweet watermelon accessions, to generate an ultrahigh-density single nucleotide polymorphisms (SNP)-based genetic map with 0.948 million segregating SNP. We used the assembled bin-map to narrow down a previously discovered sugar content major quantitative trait locus on chromosome 2 that contains a Tonoplast Sugar Transporter gene (ClTST2). Molecular analysis and biochemical assays indicated that ClTST2 encodes for a vacuolar membrane protein, whose expression increases tonoplast uptake and accumulation of sucrose, fructose and glucose in watermelon fruit flesh cells. Further molecular analyses identified a watermelon sugar induced transcription factor (SUSIWM1), which binds to a sugar responsive cis element in the ClTST2 promoter to activate its expression in sweet watermelon. Large-scale population analysis strongly suggests increased expression of ClTST2 as a major molecular event in dessert watermelon domestication. The functional characterization of ClTST2 and its expression regulation by SUSIWM1 provide novel tools to increase sugar sink potency in watermelon and possibly in other crops.