Evolutionary gain of oligosaccharide hydrolysis and sugar transport enhanced carbohydrate partitioning in sweet watermelon fruits.

The Plant cell 2021 Jul 02

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How raffinose (Raf) family oligosaccharides, the major translocated sugars in the vascular bundle in cucurbits, are hydrolyzed and subsequently partitioned has not been fully elucidated. By performing reciprocal grafting of watermelon (Citrullus lanatus) fruits to branch stems, we observed that Raf was hydrolyzed in the fruit of cultivar watermelons but was backlogged in the fruit of wild ancestor species. Through a genome-wide association study, the alkaline alpha-galactosidase ClAGA2 was identified as the key factor controlling stachyose and Raf hydrolysis, and it was determined to be specifically expressed in the vascular bundle. Analysis of transgenic plants confirmed that ClAGA2 controls fruit Raf hydrolysis and reduces sugar content in fruits. Two single-nucleotide polymorphisms (SNPs) within the ClAGA2 promoter affect the recruitment of the transcription factor ClNF-YC2 (nuclear transcription factor Y subunit C) to regulate ClAGA2 expression. Moreover, this study demonstrates that C. lanatus Sugars Will Eventually Be Exported Transporter 3 (ClSWEET3) and Tonoplast Sugar Transporter (ClTST2) participate in plasma membrane sugar transport and sugar storage in fruit cell vacuoles, respectively. Knocking out ClAGA2, ClSWEET3, and ClTST2 affected fruit sugar accumulation. Genomic signatures indicate that the selection of ClAGA2, ClSWEET3, and ClTST2 for carbohydrate partitioning led to the derivation of modern sweet watermelon from non-sweet ancestors during domestication. © American Society of Plant Biologists 2021. All rights reserved. For permissions, please email: journals.permissions@oup.com.

Citation

Yi Ren, Maoying Li, Shaogui Guo, Honghe Sun, Jianyu Zhao, Jie Zhang, Guangmin Liu, Hongju He, Shouwei Tian, Yongtao Yu, Guoyi Gong, Haiying Zhang, Xiaolan Zhang, Saleh Alseekh, Alisdair R Fernie, Henrik V Scheller, Yong Xu. Evolutionary gain of oligosaccharide hydrolysis and sugar transport enhanced carbohydrate partitioning in sweet watermelon fruits. The Plant cell. 2021 Jul 02;33(5):1554-1573