The phloem pathway: New issues and old debates

The phloem is a central actor in plant development and nutrition, providing nutrients and energy to sink organs and integrating interorgan communication. A comprehensive picture of the molecules trafficking in phloem sap is being made available, with recent surveys of proteins, RNAs, sugars, and other metabolites, some of which are potentially acting as signals. In this review, we focus on recent breakthroughs on phloem transport and signalling. A case study was phloem loading of sucrose, acting both as a nutrient and as a signal, whose activity was shown to be tightly regulated. Recent advances also described actors of macromolecular trafficking in sieve elements, including chaperones and RNA binding proteins, involved potentially in the formation of ribonucleoprotein complexes. Likewise, long distance signalling appeared to integrate electrical potential waves, calcium bursts and potentially the generation of reactive oxygen species. The ubiquitin–proteasome system was also proposed to be on action in sieve elements for signalling and protein turnover. Surprisingly, several basic processes of phloem physiology are still under debate. Hence, the absence in phloem sap of reducing sugar species, such as hexoses, was recently challenged with observations based on an analysis of the sap from Ranunculaceae and Papaveraceae. The possibility that protein synthesis might occur in sieve elements was again questioned with the identification of components of the translational machinery in Pumpkin phloem sap. Altogether, these new findings strengthen the idea that phloem is playing a central role in interorgan nutrient exchanges and communication and demonstrate that the ways by which this is achieved can obey various patterns among species.