Developmental patterning by gradients of mobile small RNAs.

Development of multicellular organisms depends on intercellular communication via mobile signals that provide positional information to coordinate cell fate decisions. In addition to peptide ligands, transcription factors, and hormones, plants use small RNAs as positional instructive signals. The unique patterning properties of small RNA gradients resulting from regulated mobility suggest conceptual similarities to the function of animal morphogens, and provide robustness and precision to the formation of cell fate boundaries. While common principles may underlie the formation, stability, and interpretation of both plant small RNA and animal morphogen gradients, the unique nature of small RNAs with respect to their biogenesis and target regulation imply important differences as well. In this review, we discuss the patterning properties of mobile small RNAs and highlight recent studies that have advanced our understanding of how small RNAs move, and how the graded accumulation that underlies their patterning activity could be created, maintained, and interpreted.