A key item in the developmental agenda of a plant leaf is the establishment of an axis that makes a leaf's top half distinct from its bottom half. This asymmetry is crucial for the leaf's function: it ensures that the leaf develops a flattened blade that is optimized for photosynthesis, with a top surface specialized for light harvesting and a bottom surface containing tiny pores that serve as locales for gas exchange.
For years, plant biologists have known that this top/bottom axis – analogous to the front/back or "dorso-ventral" axis in animals – is established by a signal derived from the meristem, the stem cell-rich growing tip of the plant from which new leaves arise. Other signals that traffic between the upper and lower sides of the leaf are thought to stably maintain this polar axis. Just as a GPS signal tells drivers where they are, these signals give cells positional information about where they are located within the leaf, causing them to acquire their correct identities by switching specific genes "on" or "off."
Associate Professor Marja Timmermans, Ph.D., and her team of scientists at Cold Spring Harbor Laboratory (CSHL) are the first group to uncover the identity of one such positional signal. In a study that appears in the March 1st issue of Genes and Development, they describe a family of mobile small RNAs that patterns the top/bottom axis in leaves. These small RNAs, they discovered, are generated on the upper surface of young leaves but traffic from this source to form a concentration gradient across each leaf. This graded distribution pattern creates discrete regions of gene activity so that cells in each half of a leaf develop a distinct "top" or "bottom" identity.
"We've known that small RNAs produced upon viral infection can move from cell to cell," explains Timmermans. "But this is the first time anyone has shown that small RNAs that are native to the organism are similarly mobile and set up developmental patterns when they move through neighboring cells."
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.
"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.
Hema Bashyam | EurekAlert!
Bolstering fat cells offers potential new leukemia treatment
17.10.2017 | McMaster University
Ocean atmosphere rife with microbes
17.10.2017 | King Abdullah University of Science & Technology (KAUST)
University of Maryland researchers contribute to historic detection of gravitational waves and light created by event
On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...
Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....
Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).
When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...
Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.
How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...
Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...
17.10.2017 | Event News
10.10.2017 | Event News
10.10.2017 | Event News
17.10.2017 | Life Sciences
17.10.2017 | Life Sciences
17.10.2017 | Earth Sciences