Watermelon fruit contains a high percentage of amino acid citrulline (Cit) and arginine (Arg). Cit and Arg accumulation in watermelon fruit are most likely mediated by both de novo
synthesis from other amino acids within fruits and direct import from source tissues (leaves) through the phloem. The amino acid transporters involved in the import of Cit, Arg, and their precursors into developing fruits of watermelon have not been reported. In this study, we have compiled the list of putative amino acid transporters in watermelon and characterized transporters that are expressed in the early stage of fruit development. Using the yeast complementation study, we characterized ClAAP3 (Cla023187) and ClAAP6 (Cla023090) as functional amino acid transporters belonging to the family of amino acid permease (AAP) genes. The yeast growth and uptake assays of radiolabeled amino acid suggested that ClAAP3 and ClAAP6 can transport a broad spectrum of amino acids. Expression of translational fusion proteins with a GFP reporter in Nicotiana benthamiana
leaves confirmed the ER- and plasma membrane-specific localization, suggesting the role of ClAAP proteins in the cellular import of amino acids. Based on the gene expression profiles and functional characterization, ClAAP3 and ClAAP6 are expected to play a major role in regulation of amino acid import into developing watermelon fruits.
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