Distinct RNA motifs mediate systemic RNA trafficking.

Systemic RNA trafficking is emerging as a new paradigm in gene regulation at the whole plant level. The mechanisms that ensure delivery of all sorts of RNAs to their respective cellular destinations remain poorly understood. Several lines of research suggest that specific sequence/structural motif(s) within an RNA directs its trafficking. The first motif was identified in Potato spindle tuber viroid (PSTVd), which mediates trafficking from bundle sheath to mesophyll in young tobacco leaves. Our recent work identified a tertiary structural motif in PSTVd that is required for trafficking from bundle sheath to phloem in Nicotiana benthamiana to initiate long-distance trafficking. This motif consists of a U/C cis Watson-Crick/Watson-Crick base pair with water insertion to widen the minor groove. Similar motifs that are structurally conserved in rRNAs serve as protein-binding sites. These data suggest that distinct RNA motifs interact with specific cellular factors for trafficking across various cellular boundaries to achieve systemic trafficking.