Plant small RNAs as morphogens

RNA interference (RNAi) in plants has long been known to produce a non-cell autonomous signal capable of silencing target genes over great cellular distances. However, only recently have RNAi-derived small RNAs been formally shown to comprise that mobile signal. Interestingly, some of these mobile small RNAs play critical roles in plant development, forming gradients that regulate the activity of their targets in a dosage-dependent manner. These properties resemble features of morphogens in animals, leading us to postulate that such cell-fate-defining small RNAs employ similar principles for the generation, stabilization and interpretation of their expression gradients. Here we review our understanding of small RNA mobility in plants, evaluate their potential as morphogen-like signals, and consider how the graded accumulation patterns that underlie their patterning/biological activity could be created and maintained.