Identification of Candidate Genes Responsible for Sugar Accumulation and Regulation in Watermelon Fruit

Sugar accumulation in fruit flesh is the most noticeable characteristic of watermelon. Sugar transporters are necessary for sugar transmembrane transportation and partitioning. Transcriptome expression profiling indicated that 11 of the 76 sugar transporters from genome annotation differentially expressed between leaf (source organ) and fruit (sink organ) during fruit development. The detailed functions and expression patterns of the 11 genes were confirmed by further phylogenetic analysis and Real-time PCR. Considering the fact that over fifty percent of the major storage sugar in fruit flesh of watermelon is sucrose, the ability to transport sucrose of these 11 genes were evaluated by the sucrose uptake-deficient yeast mutant strain SUSY7/ura3 (Saccharomyces cerevisiae). The complementation tests indicated that two candidates, named as ClSUT1and ClSUT2, were able to restore the ability of yeast mutant strain SUSY7/ura3 to utilize external sucrose. This evidence proved the potential biological function to transmembrane transport sucrose of ClSUT1and ClSUT2 in fruit flesh of watermelon. A search for cis-regulatory elements in the promoter sequences of the two genes revealed that light and plant hormones, including ethylene and abscisic acid, may regulate their expressions. Using yeast one-hybrid library screening, we found that a bZIP gene Cla014572, which contains a sucrose-controlled upstream open reading frame (SC-uORF), can bind to the abscisic acid-responsive element (ABRE) motif in the promoter region of ClSUT1and ClSUT2. Our preliminary finding provides a novel insight to understand the mechanism of sugar accumulation and regulation in fruit flesh of watermelon.