Ta-siRNAs behave like morphogens
These native, or endogenous, small RNAs are called trans-acting small interfering RNAs (ta-siRNAs). Like microRNAs and endogenous siRNAs present in other organisms, they regulate gene activity via a mechanism called RNA interference. Because of their newly discovered properties in leaf patterning, Timmermans likens the ta-siRNAs to "morphogens" or form-generating substances. Morphogens have been well studied in animals, although to date, scientists have only discovered protein and hormone morphogens. These molecules operate as positional signals whose effect on target cells is concentration-dependent. Secreted at a defined location, their movement establishes a concentration gradient that patterns a developing tissue such that cells closest to the morphogen's point of origin become distinct from cells that are farther away.
The CSHL team has now found that in plant leaves, ta-siRNAs similarly generate a concentration gradient that divides the developing leaf into a top and bottom half with different specialized cell types.
A ta-siRNA 'gradient' determines the ups and downs of developing leaves
The function of these ta-siRNAs is to specifically block the activity of a gene called ARF3. This gene defines the identities of cells found in the bottom half of leaves. For the correct leaf pattern to develop, it is therefore crucial that ARF3 is switched "on" in the right cells those at the leaf's lower side and turned "off" everywhere else.
"Without ta-siRNAs, leaves look like needles, because they lack an upper side," Timmermans says. "But we didn't understand how they set up patterning." This raised the question, in other words, of why ta-siRNAs only switch off ARF3 on the upper side of leaves. The CSHL team's finding that these RNA molecules seem to act as morphogens now solves the puzzle.
|Contact: Hema Bashyam|
Cold Spring Harbor Laboratory