Miraculous mechanism allows plant cells to specifically distribute the growth hormone auxin

Leiden and Austrian researchers have succeeded in further discovering how a plant cell specifically transmits the growth hormone auxin to the next cell. Three proteins that hang together like a cluster appear to be essential for this transport process. 'This discovery solves an important piece of the puzzle,' says professor Remko Offringa.

The hormone auxin can be seen as the growth engine of a plant. Auxin determines how and where a plant grows, both for the leaves and the roots. But how does that growth hormone end up in the right place in the plant? Until now it was known that transport proteins pass auxin from cell to cell. These proteins are a kind of movable mini channels in the cell membrane on the surface of the cell.

Plant cell as a box
'Suppose you see a plant cell as a rectangular box,' says Remko Offringa, professor of Plant Developmental Genetics. 'Then these transport proteins must collect on one of the sides to specifically pass on auxin to the next cell. That cell then passes it on to the cell next to it in the same way.' And so a plant ensures that auxin is transported from hormone-producing cells to the place where the plant needs to grow.

Offringa had previously discovered that so-called kinase proteins label the transport proteins with phosphate groups. This allows them to determine on which side of the cell the transport proteins collect and therefore in which direction auxin is pumped.

Important chain
Clear story, you would think. Yet something was still missing, says Offringa. 'How does such an accumulation of transport proteins arise on one side of the cell membrane? We have now discovered an important link in this, together with colleagues from the Institute of Science and Technology Austria.' The team published this discovery on March 11 in the renowned journal Current Biology.

Stable complex
The problem lies in the transport proteins. These are located in the cell membrane of the plant cell and can move within it. The proteins can therefore not only be located at the top or bottom of the cell, but also on the sides. 'We have discovered that another group of proteins is involved, namely the MAB proteins. Such a MAB protein interacts with the labeled transport protein and the kinase protein, which then together form a kind of cluster. Because the bunch is quite bulky, it stays neatly on the right side of the cell.'

The researchers showed this by comparing normal plant cells with mutant plant cells that either had no kinase proteins or no MAB proteins. In the last two cases, there appeared to be no accumulation of transport proteins on one side of the cell. The researchers thus proved that both the kinase and MAB proteins are essential for the correct transport of auxin.

Stronger together
'Our publication is a good example of how science can work,' says Offringa. 'I have been working with Jiri Friml, the group leader of our Austrian colleagues, for almost twenty years. So I know him well. But we happened to do this research into MAB proteins independently of each other. We discovered during a conference that we were working on the same topic. It then turned out that our data complemented each other well.' For Offringa, collaboration was the only logical next step. 'You can also choose to publish both yourself, but then we would both have an incomplete story. Now we have a much stronger publication together and everyone still gets the recognition he or she deserves.'

What makes a plant grow towards the light?
Place a plant in the sun and it will grow crooked in no time. But why is that? Ten years ago, Remko Offringa and colleagues mapped this mechanism for the first time at the molecular level. Here too, auxin and transport and kinase proteins appeared to play a crucial role.

 

Source: https://www.universiteitleiden.nl/nieuws/2021/03/wonderlijk-mechanisme-laat-plantencellen-gericht-het-groeihormoon-auxine-verspreiden