Shoot-root carbon allocation, sugar signalling and their coupling with nitrogen uptake and assimilation.

Roots and shoots are distantly located but functionally interdependent. The growth and development of these two organ systems compete for energy and nutrient resource, and yet, they keep a dynamic balance with each other for growth and development. The success of such a relationship depends on efficient root-shoot communication. Aside from the well-known signalling processes mediated by hormones such as auxin and cytokinin, sugars have recently been shown to act as a rapid signal to co-ordinate root and shoot development in response to endogenous and exogenous clues, in parallel to their function as carbon and energy resources for biomass production. New findings from studies on vascular fluids have provided molecular insights into the role of sugars in long-distance communications between shoot and root. In this review, we discussed phloem- and xylem- translocation of sugars and the impacts of sugar allocation and signalling on balancing root-shoot development. Also, we have taken the shoot-root carbon-nitrogen allocation as an example to illustrate the communication between the two organs through multi-layer root-shoot-root signalling circuits, comprising sugar, nitrogen, cytokinin, auxin and vascular small peptide signals.