Zhou Yixuan, Li Kexin, Wen Suying, Yang Dong, Gao Jun, Wang Ziwei, Zhu Peilu, Bie Zhilong, Cheng Jintao
National Key Laboratory for Germplasm Innovation and Utilization for Fruit and Vegetable Horticultural Crops, College of Horticulture and Forestry Sciences, Huazhong Agricultural University, Wuhan 430070, Hubei Province, China.
Hortic Res. 2023 Jul 4;10(8):uhad123. doi: 10.1093/hr/uhad123. eCollection 2023 Aug.
Melon ( L.) has a long history of cultivation worldwide. During cultivation, domestication, and selection breeding, the sugar content of mature melon fruits has been significantly increased. Compared with unsweet melon and wild melon, rapid sucrose accumulation can occur in the middle and late stages of sweet melon fruit development. The phloem unloading pathway during the evolution and development of melon fruit has not been identified and analyzed. In this study, the phloem unloading pathway and the function of related sugar transporters in cultivated and wild melon fruits were analyzed by CFDA [5(6)-carbofluorescein diacetate] and esculin tracing, cytological pathway observation, qRT-PCR, and gene function analysis, etc. Results show that the phloem unloading pathway of wild melon fruit is largely symplastic, whereas the phloem unloading pathway of cultivated melon fruit shifts from symplastic to apoplasmic during development. According to a fruit grafting experiment, the fruit sink accumulates sugars independently. Correlation analysis showed that the expression amounts of several sucrose transporter genes were positively correlated with the sucrose content of melon fruit. Furthermore, CmSWEET10 was proved to be a sucrose transporter located on the plasma membrane of the phloem and highly expressed in the premature stage of sweet melon fruits, which means it may be involved in phloem apoplast unloading and sucrose accumulation in sweet melon fruits. Finally, we summarize a functional model of related enzymes and sugar transporters involved in the apoplast unloading of sweet melon fruits during enlargement and sucrose accumulation.
甜瓜（Cucumis melo L.）在全球范围内有着悠久的种植历史。在种植、驯化和选育过程中，成熟甜瓜果实的含糖量显著提高。与不甜的甜瓜和野生甜瓜相比，甜瓜果实在发育中后期会快速积累蔗糖。甜瓜果实进化发育过程中的韧皮部卸载途径尚未得到鉴定和分析。在本研究中，通过羧基荧光素二乙酸酯（CFDA）和七叶苷示踪、细胞学途径观察、qRT-PCR以及基因功能分析等方法，对栽培甜瓜和野生甜瓜果实中的韧皮部卸载途径及相关糖转运蛋白的功能进行了分析。结果表明，野生甜瓜果实的韧皮部卸载途径主要是共质体途径，而栽培甜瓜果实的韧皮部卸载途径在发育过程中从共质体途径转变为质外体途径。根据果实嫁接实验，果实库独立积累糖分。相关性分析表明，几个蔗糖转运蛋白基因的表达量与甜瓜果实的蔗糖含量呈正相关。此外，CmSWEET10被证明是一种位于韧皮部质膜上的蔗糖转运蛋白，在甜瓜果实发育早期高度表达，这意味着它可能参与了甜瓜果实质外体卸载和蔗糖积累过程。最后，我们总结了一个在甜瓜果实膨大及蔗糖积累过程中参与质外体卸载的相关酶和糖转运蛋白的功能模型。
