Heavy traffic in the fast lane: long‐distance signalling by macromolecules

Summary The two major vascular conduits in plants, the xylem and phloem, theoretically provide opportunities for the long‐distance translocation of almost any type of water‐borne molecule. This review focuses on the signalling functions conveyed by the movement of macromolecules. Here, a signal is defined as the communication of information from source to destination, where it modifies development, physiology or defence through altered gene expression or by direct influences on other cellular processes. Xylem and phloem sap both contain diverse classes of proteins; in addition, phloem contains many full‐length and small RNA species. Only a few of these mobile molecules have proven functions in signalling. The transduction of signals typically depends on connection to appropriate signalling pathways. Incoming protein signals require specific detection systems, generally via receptors. Mobile RNA s require either the translation or presence of a homologous target. Given that phloem sieve elements are enucleate and lack translation machinery, RNA function requires subsequent unloading at least into adjacent companion cells. The binding of RNA by proteins in ribonucleoprotein complexes enables the translocation of some signals, with evidence for both sequence‐specific and size‐specific binding. Several examples of long‐distance macromolecular signalling are highlighted, including the FT protein signal which regulates flowering time and other developmental switches.ContentsSummary 33 I. Introduction – plant communication systems 33 II. The importance of method selection and experimental design 34 III. Macromolecules in xylem and phloem 37 IV. Plasmodesmata – complex intercellular exchange junctions enabling short‐ and long‐range signalling 39 V. Phloem signalling: regulation 40 VI. Phloem signalling: specific examples 43 VII. Conclusions and prospects 45Acknowledgements 46References 46