Vascular plants, unlike bryophytes, have a strong root-shoot dichotomy in which the tissue systems are mutually interdependent; roots are completely dependent on shoots for photosynthetic sugars, and shoots are completely dependent on roots for water and mineral nutrients. Long-distance communication between shoot and root is therefore critical for the growth, development and survival of vascular plants, especially with regard to variable environmental conditions. However, this long-distance signalling does not appear an ancestral feature of land plants, and has likely arisen in vascular plants to service the radical alterations in body-plan seen in this taxon. In this review, we examine the defined hormonal root-to-shoot and shoot-to-root signalling pathways that coordinate the growth of vascular plants, with a particular view to understanding how these pathways may have evolved. We highlight the completely divergent roles of isopentenyl-adenine and trans-zeatin cytokinin species in long-distance signalling, and ask whether cytokinin can really be considered as a single class of hormones in the light of recent research. We also discuss the puzzlingly sparse evidence for auxin as a shoot-to-root signal, the evolutionary re-purposing of strigolactones and gibberellins as hormonal signals, and speculate on the possible role of sugars as long-distance signals. We conclude by discussing the ‘design principles’ of long-distance signalling in vascular plants.

与苔藓植物不同，维管植物具有很强的根茎二分法，其中组织系统相互依存；根完全依赖芽获取光合糖分，而芽完全依赖根获取水分和矿物质营养。因此，枝条和根部之间的长距离通信对于维管植物的生长、发育和存活至关重要，尤其是在可变环境条件方面。然而，这种长距离信号似乎不是陆生植物的祖先特征，而且很可能出现在维管植物中，以服务于该分类群中身体计划的根本改变。在这篇综述中，我们检查了协调维管植物生长的确定的激素根到芽和芽到根信号通路，以特定的视角来了解这些途径可能是如何演变的。我们强调异戊烯基腺嘌呤和长距离信号转导中的反式玉米素细胞分裂素种类，并根据最近的研究询问细胞分裂素是否真的可以被视为单一类别的激素。我们还讨论了生长素作为从芽到根信号的令人费解的稀少证据，独脚金内酯和赤霉素作为激素信号的进化再利用，并推测糖类作为长距离信号的可能作用。我们最后讨论了维管植物中长距离信号传导的“设计原则”。