Mobile Small RNAs Are Predominately Accumulated via Long-Distance Movement Rather than Local Biogenesis

Abstract Long-distance RNA movement is important for plant growth and environmental responses; however, the extent to which RNAs move between distant tissues, their relative magnitude and functional significance remain to be elucidated on a genomic scale. Using a soybean (Glycine max)/common bean (Phaseolus vulgaris) grafting system, we identified 100 shoot-root mobile miRNAs and 32 shoot-root mobile phasiRNAs, which were predominantly produced in shoots but transported to roots, some of which enabled cleavage of their mRNA targets or their precursors. In contrast, most of the mobile mRNAs were transcribed in both shoots and roots and were truncated fragments, with the transported copies accounting for only a tiny portion of all copies accumulated in the recipient (shoots or roots) tissues. These findings suggest that the regulatory mechanisms for sRNAs movement are different from those for mRNA movement, and that the earlier is more strictly regulated, and likely, more functionally significant than the latter.