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

In plant RNA-silencing, small interfering (si)RNAs and micro (mi)RNAs 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 intra-cellularly, 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 RNA-silencing movement. Besides a still largely putative role for mobile virus-derived (v)siRNAs in conferring immunity, several endogenous 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 processes likely regulate its transport; discovering these processes was expected to illuminate 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 silencing movement 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 regulations. 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 illuminate so far overlooked aspects of plant RNA- silencing movement.