Mystery of plant hormone solved

The plant hormone auxin drives almost all the growth processes in most plants. Until now, no one knew how that one substance could cause all those different reactions. By going back in time, professor of Biochemistry Dolf Weijers and his colleagues have cleared up this mystery.
Tessa Louwerens

The researchers studied the effect of the plant hormone auxin in species of moss that have existed for hundreds of millions of years. ©Hirotaka Kato

Every species of plant reacts to auxin in its own way. This is because in every cell it activates unique ‘switches’, which switch different genes on and off. Weijers: ‘Although the hormone was discovered 100 years ago, we still don’t know very much about how it works, particularly how it is possible for such a simple molecule to drive so many different processes.’ With funding from NWO – Weijers has a Vici grant – he and his team studied how this switch system in plants came about and evolved. They published their findings in eLife.

Previously, scientists have usually studied the reaction to auxin in model plants such as Arabidopsis thaliana (thale cress). On the basis of these studies, researchers know that the ‘switches’ with which plants react to auxin consist of three different kinds of protein. Different species of plant make slightly different variants of those three proteins, making a range of combinations possible which determine how the plant reacts to the hormone. In Arabidopsis, for example, more than 4000 combinations of the three kinds of protein are possible. Weijers: ‘We wrestled with that complexity in Arabidopsis for a very long time, trying to pinpoint how the hormone worked. The disadvantage is that you make assumptions that may only apply to those relatively young species of plant. That is just like discovering something about mice and then saying it applies to all animal species.’

Genome archaeology

Weijers and his colleagues took a different approach: they looked at the genetic material of over 1000 plant species, including ancient species such as algae and seaweeds. Working like genome archaeologists, they mapped out the evolution of the switches step by step. Weijers: ‘Green algae developed the first bit of the switch about 800 million years ago but we come across the complete system for the first time in terrestrial plants.’

The researchers then went on to test the reactions of algae, mosses and ferns – which represent various stages of evolution – to auxin and proved that the reaction to auxin got more and more complex in the course of evolution, so that more and more genes are driven by it. They also looked at the genes of liverworts, plants which are like living fossils in that they have changed very little over the past 500 million years. Here they saw that even though the reaction to auxin had become more complex over time, the steering system worked in a similar way throughout that time.

More precise

Weijers sees this study as a breakthrough for plant biology and a nice example of how bio-informatics, evolution biology, biochemistry and genetics can reinforce each other. ‘Every system, including proteins, came about through evolution ultimately. It is crucial to see them in that context. You could call it evolutionary biochemistry.’

With this knowledge, Weijers believes it will be possible to control plant growth more precisely. In agriculture and horticulture, for instance, auxin is used to get cuttings to form new roots. But it can also be used in some plants to deal with weeds: auxin makes the weed literally grow until they die . Weijers: ‘These applications have been developed by trial and error: you throw auxin at them and see what happens. But nature has conducted a lot of these experiments in the last few billion years too. Now that we understand the rules of the game, we can get more control over it. You might be able to create aubergines without any seeds, for instance, or ensure the plant puts down more roots where you want it to. But we haven’t got that far yet.’

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