"Establishment of a gradient of mobile small RNAs can create profound differences between neighboring cells by altering their gene activity patterns," Timmermans says. "This is a neat way of dividing a cluster of cells into distinct sections with sharply drawn boundaries."
The top-to-bottom, abundant-to-rare distribution, or "concentration gradient," of ta-siRNAs ensures that the activity of ARF3 is strongly inhibited in the leaf's top half, but mildly or hardly affected at the bottom, thus creating a sharp boundary between leaf sections with different fates.
Ta-siRNA biogenesis is spatially controlled
In addition to mobility, the team attributes the unique distribution pattern of these small RNAs to the way they are produced within the leaf a biochemical process involving several complicated steps.
The small ta-siRNAs are generated from larger RNA strands called precursors that are snipped at specific sites. Two cellular ingredients ensure that the cuts occur in the right place: a microRNA molecule called miR390 that specifies the location of the first cut, and an enzyme called ARGONAUTE7 (AGO7) that ferries miR390 to this location and creates the cut.
The CSHL team found that although miR390 is present in all cells of the leaf, the precursors and ARGONAUTE7 are strictly restricted to only the cells in the two uppermost layers. The ta-siRNAs are therefore generated exclusively in these upper cell layers, from where they move to the lower side of the leaf, accumulating as a gradient.
Thus, besides identifying the first example of a morphogen-like small RNA signal, Timmermans and her team have also shown that the location of the various biochemical ingredients required for small RNA activity can impact pattern formation. Together, their discoveries explain how mobile small RNAs can generate patterns during development.
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory