miRNAs are useful molecules for plants. These small parcels of RNA pass along signals between cells. They move through plasmodesmata, microscopic channels through cell walls, and through the phloem to help cope with stresses. In a study published in Nature Communications Damianos Skopelitis and colleagues show that miRNA mobility is precisely regulated through a gating mechanism polarised at defined cell–cell interfaces. By polarising the miRNA, the plant cells know which side of the cell is facing toward the stem and which side is facing away.
The gating plays an important role in miRNA signalling. The authors state: “The gating of miRNA mobility allows for selectivity in long-distance signalling. The polarised regulation of miRNA mobility at the phloem of the central vasculature generates directional cell-to-cell movement, with miRNAs moving out but not into the phloem poles. Consequently, miRNAs not produced or amplified in phloem tissues of the shoot are restricted from moving long distance into the root. Thus, the gate-keeping mechanism regulating miRNA mobility at the central vasculature creates a movement barrier that ensures some small RNA-mediated signalling responses are contained while permitting others to be propagated systemically.”
This selectivity means that small RNAs can be used as local positional signals or to aid signals throughout an organism.