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Phloem Unloading Strategies and Mechanisms in Crop Fruits

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Abstract

Carbohydrate produced by photosynthesis is loaded into phloem via collection phloem, translocated via the transport phloem, and unloaded by release phloem into sink organs. Phloem unloading strategy is an important physiological process and plays a key role in regulating the distribution of photoassimilate and the growth and development of crops. Here, the detailed unloading strategies and mechanisms in three types of sugar transporting-plants, sucrose, sugar alcohol, and Raffinose family of oligosaccharides (RFOs), are summarized and discussed. Symplastic and apoplastic unloading are the main phloem unloading strategies, and these strategies could shift from symplastic to apoplastic (such as grape), or otherwise (such as potato); some crops shift more than one time (jujube and cotton) during fruit development, depending on the type, structure, sugar component, and developmental stages of fruit and a series of genes involved in unloading process, such as transporters, α-galactosidase, invertase, and sucrose synthase. The diversity of the unloading strategies results in the difference in size, yield, quality, and flavor of crop fruit. Why are there a variety of phloem unloading strategies in crop fruit? What is the mechanism of the unloading strategy shift? These issues are summarized, discussed, and put forward a research agenda for the near future.

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Acknowledgements

We thank Drs. Robert Turgeon (Cornell University) and Yong-ling Ruan (The University of Newcastle) for critical reading of the article and constructive comments. This work was supported by the National Natural Science Foundation of China (Grant No. 31471876 to Z.Z.), the earmarked fund for Modern Agro-industry Technology Research System in China (CARS-25-C-12), a project (Grant No. 2013ZX08009 to Z.Z.) from the Ministry of Agriculture of China for transgenic research, and the Beijing Innovation Consortium of Agriculture Research System (Grant No. BAIC01-2018 to X.S.).

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ZZ and XS designed update; SM, YL, ZZ, XS, and XL collected and analyzed data; and ZZ, SM, and YL wrote the manuscript.

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Correspondence to Xiaolei Sui or Zhenxian Zhang.

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Ma, S., Li, Y., Li, X. et al. Phloem Unloading Strategies and Mechanisms in Crop Fruits. J Plant Growth Regul 38, 494–500 (2019). https://doi.org/10.1007/s00344-018-9864-1

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