Phloem unloading represents a series of cell-to-cell transport steps transferring phloem-mobile constituents from phloem to sink tissues/organs to fuel their development or resource storage. Our analysis focuses on unloading of two major phloem-mobile constituents, sugars and water. Their unloading can occur across phloem plasma membranes (apoplasmic unloading), through plasmodesmata interconnecting phloem and sink cells (symplasmic unloading) or predominately symplasmically with an intervening post-phloem apoplasmic step. In planta studies of phloem unloading encounter substantial technical challenges in accessing phloem within a meshwork of vascular/ground tissues. Thus, current understanding of phloem-unloading mechanisms largely has been deduced from indirect experimental measures or modelling. Here we highlight recent advances in understanding phloem unloading mechanisms and identify where important knowledge gaps remain.

韧皮部的卸载代表了一系列的细胞间移动步骤，将韧皮部移动成分从韧皮部转移到沉没的组织/器官，以促进其发育或资源储存。我们的分析重点是卸载两种主要韧皮部流动成分糖和水。它们的卸载可发生在韧皮部质膜上（无质子性卸载），通过连接韧皮部和沉没细胞的胞浆线虫（对称性卸载）或主要与中间的韧皮部后无性质步骤同质。在植物中韧皮部卸载的研究在获取血管/地面组织网状结构中的韧皮部方面遇到了重大的技术挑战。因此，目前对韧皮部卸载机制的了解主要是通过间接实验方法或模型得出的。在这里，我们重点介绍了在理解韧皮部卸载机制方面的最新进展，并确定了哪些地方仍存在重要的知识空白。