Phloem Unloading Strategies and Mechanisms in Crop Fruits

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, -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.