RNA in the phloem: A crisis or a return on investment?

It has long been known that the phloem facilitates long-distance transport of sugars, hormones and amino acids in plants. In the last decade, messenger RNAs (mRNA) and small interfering RNAs (siRNA)/micro-RNAs (miRNA) were found to use the same systemic source-sink pathway. It is thought that these RNA macromolecules are selectively delivered via plasmodesmata from companion cells to sieve tubes, which are devoid of nuclei. A number of these phloem-delivered RNAs were shown to have a regulatory function in differentiation and adaptation to environmental conditions. For example, a phloem-delivered FLOWERING LOCUS T (FT) gene product produced in leaves turns on the flower differentiation program in shoot apices. However, it is still not resolved whether FT mRNA or protein, or both, could act as signals. Stressing the importance of further inspection of the phloem content, recent -omics studies revealed that the phloem sap contains the basic components for a functional mRNA translation machinery. This short review discusses the function of phloem-delivered RNA molecules, the advances and difficulties analyzing the phloem-mediated mRNA signaling mechanism, and the likelihood of a functional phloem-based mRNA translation system. (C) 2009 Elsevier Ireland Ltd. All rights reserved.