Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Scientists Find a Molecular Clue to the Complex Mystery of Auxin Signaling in Plants

25.03.2014

Interaction domain on proteins that modulate this potent hormone allows them to stack back-to-front like button magnets

Wikipedia lists 65 adjectives that botanists use to describe the shapes of plant leaves. In English (rather than Latin) they mean the leaf is lance-shaped, spear-shaped, kidney-shaped, diamond shaped, arrow-head-shaped, egg-shaped, circular, spoon-shaped , heart-shaped, tear-drop-shaped or sickle-shaped — among other possibilities.


Strader Lab

Graduate student David Korasick commuted between the Strader Lab, which specializes in genetics, and the Jez Lab, which has expertise in structural biology, to learn how plants control the effects of the master hormone auxin.

How does the plant “know” how to make these shapes? The answer is by controlling the distribution of a plant hormone called auxin, which determines the rate at which plant cells divide and lengthen.

But how can one molecule make so many different patterns? Because the hormone’s effects are mediated by the interplay between large families of proteins that either step on the gas or put on the brake when auxin is around.

In recent years as more and more of these proteins were discovered, the auxin signaling machinery began to seem baroque to the point of being unintelligible.

Now the Strader and Jez labs at Washington University in St. Louis have made a discovery about one of the proteins in the auxin signaling network that may prove key to understanding the entire network.

In the March 24 issue of the Proceedings of the National Academy of Sciences they explain that they were able to crystallize a key protein called a transcription factor and work out its structure. The interaction domain of the protein, they learned, folds into a flat paddle with a positively charged face and a negatively charged face. These faces allow the proteins to snap together like magnets, forming long chains, or oligomers.

We have some evidence that proteins chain in plant cells as well as in solution, said senior author Lucia Strader, PhD, assistant professor of biology and an auxin expert. By varying the length of these chains, plants may fine-tune the response of individual cells to auxin to produce detailed patterns such as the toothed lobes of the cilantro leaf.

Combinatorial explosion
Sculpting leaves is just one of many roles auxin plays in plants. Among other things the hormone helps make plants bend toward the light, roots grow down and shoots grow up, fruits develop and fruits fall off.

“The most potent form of the hormone is indole-3-acetic acid, abbreviated IAA, and my lab members joke that IAA really stands for Involved in Almost Everything,” Strader said.

The backstory here is that whole families of proteins intervene between auxin and genes that respond to auxin by making proteins. In the model plant Aribidopsis thaliana these include 5 transcription factors that activate genes when auxin is present (called ARFs) and 29 repressor proteins that block the transcription factors by binding to them (Aux/IAA proteins). A third family marks repressors for destruction.

“Different combinations of these proteins are present in each cell,” said Strader. “On top of that, some combinations interact more strongly than others and some of the transcription factors also interact with one another.”

In an idle moment David Korasick, a graduate fellow in the Strader and Jez labs and first author on the PNAS article, did a back-of-the-envelope calculation to put a number on the complexity of the system they were trying to understand. From a strictly mathematical point of view there are 3,828 possible combinations of the auxin-related Arabidopsis proteins. That is assuming interactions involve only one of each type of protein; if multiples are possible, the number, of course, explodes.

To make any headway, Strader said, we had a better understanding of how these proteins interact. The rule in protein chemistry is the opposite of the one in design: instead of form following function, function follows form.

So to figure out a protein’s form — the way it folds in space — they turned to the Jez lab, which specializes in protein crystallography, essentially a form of high-resolution microscopy that allows protein structures to be visualized at the atomic level.

Korasick had the job of crystallizing ARF7, a transcription factor that helps, Arabidopsis bend toward the light. With the help of Joseph Jez, PhD, associate professor of biology, Corey Westfall, and Soon Goo Lee), Korasick cut “floppy bits” off the protein that might have made it hard to crystallize, leaving just the part of the protein where it interacts with repressor molecules.

After he had that construct, crystallization was remarkably fast. He set up his first drops in solution wells on the 4th of July. The protein crystallized with a fuss, and he ran the crystals up to the Advanced Photon Source at the Argonne National Laboratory outside Chicago. By August 1 he had the diffraction data he needed to solve the protein’s structure.

Surprise, surprise
The previous model for the interaction between a repressor and a transcription factor – a model that had stood for 15 years, Strader said– was that the repressor lay flat on the transcription factor, two domains on the repressor matching up with the corresponding two domains on the transcription factor.

The structural model Korasick developed showed that the two domains fold together to form a single domain, called a PB1 domain. A PB1 domain is a protein interaction module that can be found in animals and fungi as well as plants.

The repressor proteins, which are predicted to have PB1 domains identical to that of the ARF transcription factor, then stick to one or the other side of the transcription factor’s PB1 domain, preventing it from doing its job. Experiments showed that there had to be a repressor protein stuck to both faces of the transcription factor’s PB1 domain to repress the activity of auxin.

This means the model, which pairs a single repressor protein with a single transcription factor, is wrong, Strader said.

“Nor can we limit the interactions to just two,” she said. “It could be hundreds for all we know.“

In Korasick’s crystal five of the ARF7 PB1 domains stuck to one another, forming a pentamer.

“I like to think of the PB1 domains as magnets, “ Strader said. “Like magnets, they can stick together, back-to-front, to form long chains.”

“But we have to put an asterisk next to that,” Korasick said, “because it’s possible it’s an artifact of crystallography and doesn’t work that way in living plants.“

But both Strader and Korasick suspect that it does. Strader points out that the complexity of the auxin signaling system has increased over evolutionary time as plants became fancier. A simple plant like the moss Physcomitrella patens has fewer signaling proteins than a complicated plant like soybean.

“Probably what that’s saying is that it’s really, really important for a plant to be able to modulate auxin signaling, to have the right amount in each cell, to balance positive and negative growth,” Korasick said.

“The difference between plants and animals,” said Strader, “is that plants have rigid cell walls. So when a plant cell decides to divide itself or length itself, that’s a permanent decision, which is why it’s so tightly controlled.“

Diana Lutz | newswise
Further information:
http://www.wustl.edu

Further reports about: Auxin IAA Molecular Mystery Plants Signaling domains factors hormone proteins transcription

More articles from Life Sciences:

nachricht New technology offers fast peptide synthesis
28.02.2017 | Massachusetts Institute of Technology

nachricht Biofuel produced by microalgae
28.02.2017 | Tokyo Institute of Technology

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Safe glide at total engine failure with ELA-inside

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded after a glide flight with an Airbus A320 in ditching on the Hudson River. All 155 people on board were saved.

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded...

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

New technology offers fast peptide synthesis

28.02.2017 | Life Sciences

WSU research advances energy savings for oil, gas industries

28.02.2017 | Power and Electrical Engineering

Who can find the fish that makes the best sound?

28.02.2017 | Information Technology

VideoLinks
B2B-VideoLinks
More VideoLinks >>>