A Plant SMALL RNA-BINDING PROTEIN 1 Family Mediates Cell-to-Cell Trafficking of RNAi Signals

Abstract In plants, RNA interference (RNAi) plays a pivotal role in growth and development, and responses to environmental inputs, including pathogen attack. The intercellular and systemic trafficking of small interfering/micro(si/mi)RNA is a central component in this regulatory pathway. Currently, little is known with regards to the molecular agents involved in the movement of these si/miRNAs. To address this situation, we employed a biochemical approach to identify and characterize a conserved SMALL RNA-BINDING PROTEIN 1 (SRBP1) family that mediates non-cell-autonomous small (s)RNA trafficking. In Arabidopsis, AtSRBP1 is a glycine-rich RNA-binding protein, also known as AtGRP7, which we show binds single-stranded-siRNA. A viral vector, Zucchini yellow mosaic virus (ZYMV), was employed to functionally characterized the AtSRBP1-4 (AtGRP7/2/4/8) RNA recognition motif (RRM) and GR domains. Cellular-based studies revealed the GR domain as being necessary and sufficient for SRBP1 cell-to-cell movement. Our findings provide a foundation for future research into the mechanism and function of mobile sRNA signaling agents in plants.