Three decades of mobile RNA silencing within plants: what have we learnt?

In plant RNA silencing, siRNAs and miRNAs guide ARGONAUTE (AGO) effector proteins to silence sequence-complementary RNA/DNA. This helps regulate developmental patterning, adaptation to stress, antiviral defense, or genome integrity/maintenance. Remarkably, these regulations not only occur intracellularly, but may also manifest in remote tissues. Here, I summarize the evidence that RNA silencing moves from cell to cell and via the phloem, the long-distance extension of the symplasm-the cytosolic connection network between cells through plasmodesmata (PDs). I then illustrate several biological functions linked to movement of RNA silencing. Besides a still largely putative role for mobile virus-derived (v)siRNAs in conferring immunity, several endogenous small RNAs (sRNAs) act as systemic signals orchestrating organismal responses to abiotic stress or symbiosis. Other mobile sRNAs act as morphogens and generate gene expression gradients by moving from cell to cell. If RNA silencing indeed moves symplasmically via PDs, then various processes probably regulate its transport; discovering these processes was expected to shed light on macromolecular trafficking in general. In a final part of this perspective, I describe several forward genetic systems set in Arabidopsis to specifically tackle the above issue. Some were instrumental in revealing hitherto unknown AGO-mediated mechanisms that modulate movement of silencing within silencing-incipient, traversed, or recipient cells. Somewhat disappointingly, however, the systems fell short of identifying factors impacting the silencing cell-to cell-trafficking channels or their regulation. I discuss here plausible reasons for these shortcomings, what could be learnt from them, how they could be remedied, and how a better understanding of their physiological foundations might shed light on so far overlooked aspects of movement of plant RNA silencing.