Shapely leaves: Auxin explains

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  • Published: Apr 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Shapely leaves: Auxin explains

Auxin networks

New crystallographic details reveal details about how the plant hormone auxin networks biochemically within a growing plant and controls the size and shape of its leaves, according to a new study. Coriander photo David Bradley

New crystallographic details reveal details about how the plant hormone auxin networks biochemically within a growing plant and controls the size and shape of its leaves, according to a new study.

Botanists have almost as many words for describing the shapes of leaves as the mythical word count for types of snow. Indeed, Wikipedia lists 65 adjectives that are commonly used either in English or derived from Latin terms: lance-shaped, spear-shaped, kidney-shaped, diamond shaped, arrow-head-shaped, egg-shaped, circular, spoon-shaped , heart-shaped, tear-drop-shaped or sickle-shaped…the list goes on.

Until the discovery of plant hormones, and specifically, the chemical auxin, it was difficult, if not impossible, to explain how plants "know" which shape to adopt. What was certain was that outlandish explanations did not resonate, morphically or otherwise. The distribution of auxin as the plant grows controls the rate at which plant cells divide, lengthen and undergo programmed cell death. But, even with such an explanation, how does a single hormone give rise to such variety. The answer lies in the large chemical network of proteins that themselves control the activity or latency of auxin.

However, the complexity of this protein system has grown unwieldy and what was originally an explanation has become an opaque tangle of biomolecular signalling machinery. What was needed was a discovery of a key, a protein, that would unlock the system. Now, researchers at Washington University in St. Louis have made a discovery about one of the proteins in the auxin signalling network that could well be this elusive key.

Active domains and chains

Writing in the Proceedings of the National Academy of Sciences the team explains how they crystallized a key transcription factor and determined its structure. They found that the active domain of this protein folds into a flat paddle with a positively charged face and a negatively charged face, which allows the proteins to snap together forming long chain oligomers.

The team has some evidence that proteins form chains in plant cells as well as in solution. 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 [coriander] leaf, they suggest.

Sculpting leaves is just one of many roles auxin plays in plants. Among other things the hormone helps make plants bend toward the light, allows roots to grow downwards and shoots to grow up, it tells the plant when to develop fruit and when that fruit should drop.

"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 adds. Of course, whole families of proteins intervene between auxin and genes that respond to auxin by making other proteins. In the model plant Aribidopsis thaliana these include five 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," explains Strader. "On top of that, some combinations interact more strongly than others and some of the transcription factors also interact with one another." Team member David Korasick assumed that if interactions involved just one protein and worked out that were some 3828 possible combinations of the auxin-related Arabidopsis proteins. If multiple protein interactions are possible then that number could grow exponentially.

In protein chemistry, function follows form, or folds more specifically. And, so a crystal structure determination of ARF7, the transcription factor involved in Arabidopsis growth towards the light was essential. The team worked with Joseph Jez, Corey Westfall and Soon Goo Lee, removed extraneous parts of the protein that were precluding crystallization and homed in on the active site that interacts with repressor molecules. The diffraction experiments were carried out at the Advanced Photon Source at the Argonne National Laboratory near Chicago. "We used Beamline 19-ID and in addition, we used single anomalous dispersion (SAD) from SeMet-substituted protein for the experiment," Strader told SpectroscopyNOW.

The previous best model for repressor to transcription factor interaction had been around for fifteen years. In it, the repressor ay flat on the transcription factor, two domains on the repressor matching up with the corresponding two domains on the transcription factor. Korasick's revised model based on the new diffraction data showed that the two domains fold together to form a single domain, the common PB1 domain found in plants, animals and also fungi.

Transcription, the name of the game

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 adhering to both faces of the transcription factor's PB1 domain which inhibits auxin activity. This means the model, which pairs a single repressor protein with a single transcription factor, is incorrect. Moreover, there might not just be two interactions either, there could be hundreds. In the crystallography five ARF7 PB1 domains stick together forming a pentamer, although that may be an artefact of their being in the solid state, that is not necessarily their composition in the living plant. There are evolutionary hints that this is indeed a realistic picture, nevertheless, a simple plant like the moss Physcomitrella patens has fewer signalling proteins than a complicated plant like soybean, Strader points out.

"Probably what that's saying is that it's really, really important for a plant to be able to modulate auxin signalling, to have the right amount in each cell, to balance positive and negative growth," Korasick explains. Plants have rigid cell walls, unlike animals, so when a plant cell "decides" to divide or elongate that's a permanent decision, which is why it has to be so tightly controlled.

"Our next step for this project is to determine whether these PB1 domain-containing transcription factors and repressors form multimers in planta (it is clear from our PNAS data that dimerization is insufficient for repressor activity). How big a complex can these make? What is the biological role for forming multimers? Ultimately, we feel that understanding the molecular details of auxin response control will be helpful in understanding how plants regulate growth," Strader told us.

Related Links

Proc Natl Acad Sci, 2014, online: "Molecular basis for AUXIN RESPONSE FACTOR protein interaction and the control of auxin response repression"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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