Long distance RNA movement

Contents Summary 29 I. Introduction 29 II. Phloem as a conduit for macromolecules 30 III. Classes of phloem transported RNAs and their function 32 IV. Mode of RNA transport 35 V. Conclusions 37 Acknowledgements 37 References 37 Summary In higher plants, small noncoding RNA s and large messenger RNA ( mRNA ) molecules are transported between cells and over long distances via the phloem. These large macromolecules are thought to get access to the sugar‐conducting phloem vessels via specialized plasmodesmata ( PD ). Analyses of the phloem exudate suggest that all classes of RNA molecules, including silencing‐induced RNA s (si RNA s), micro RNA s (mi RNA s), transfer RNA s ( tRNA s), ribosomal RNA ( rRNA s) and mRNA s, are transported via the vasculature to distant tissues. Although the functions of mobile si RNA s and mi RNA s as signalling molecules are well established, we lack a profound understanding of mobile mRNA function(s) in recipient cells and tissues, and how they are selected for transport. A surprisingly high number of up to thousands of mRNA s were described in diverse plant species such as cucumber, pumpkin, Arabidopsis and grapevine to move long distances over graft junctions to distinct body parts. In this review, we present an overview of the classes of mobile RNA s, the potential mechanisms facilitating RNA long‐distance transport, and the roles of mobile RNA s in regulating transcription and translation. Furthermore, we address potential function(s) of mobile protein‐encoding mRNA s with respect to their characteristics and evolutionary constraints.