Conceptual and Methodological Considerations on mRNA And Proteins as Intercellular and Long-distance Signals.

High throughput studies identified approximately one fifth of Arabidopsis protein encoding transcripts to be graft transmissible and to move over long distances in phloem. In roots, one fifth of transcription factors were annotated as non-cell-autonomous moving between cells. Is this massive transport a way of inter-organ and cell-to-cell communication or does it serve different purposes? On the tissue level, many miRNAs and all siRNAs act non-cell autonomously. Why are these RNAs and proteins not just expressed in cells where they exert their function? Short and long distance transport of these macromolecules raises the question whether all mobile mRNAs and transcription factors could be defined as signalling molecules. Since the answer is not clear yet, we will discuss in this review conceptual approaches to this phenomenon using a single mobile signalling macromolecule, FLOWERING LOCUS T, which has been characterized extensively. We conclude that careful individual studies of mobile macromolecules are necessary to uncover their biological function and the observed massive mobility. To stimulate such studies, we provide a review summarizing the resourceful wealth of experimental approaches to this intriguing question and discuss methodological scopes and limits.