Acidic roots
While growing, roots find their way through the soil. This growth is a result of both cell division and the stretching of root cells. Something that is fascinating researchers for years, because how does a plant precisely regulate this growth is still a question. One of the factors that has a role in this process is the changing acidity. Now Czech and German researchers discovered how auxin accumulation locally increases the acidity.
You can divide a root roughly in 5 zones. All the way at the tip of the root is the root cap. Which is protecting the growth region against the obstacles that a growing root comes across. This is followed by a growth zone. The growth region is located here, and here it is that root cells divide. A transition zone connects the growth zone with the elongation zone, where like the name suggest cells stretch out long. The last zone is the maturation zone, where the cells start specializing. Because the cells are all connected to each other, after each cell division all the cells are pushed into the direction of the maturation zone.
To discover how a plant regulates the acidity of its cell walls, and how this is influencing root growth, the researchers made use of a novel dye that visualises the acidity of the cell wall. This dye showed that the cell walls of the root cap and the maturation zone are more acidic than those of the cells in the transition zone. This, it turned out, was not as expected due to a reduction or activity of the AHA-proton pumps.
CNGC14 appears to be the proton-pump regulated via AFB1 that is influencing the acidity
Raising the question, what is regulating this acidity? In order to answer that the researchers studied the effect of another acidity influencer: auxin. When the researchers applied extra auxin to the roots, then the size of the transition zone increased, and the acidity and root growth reduced. Suggesting auxin is accumulating in the transition zone during normal circumstances. To be sure that this was indeed the case, the researchers studied root acidity in a plant whose cells can’t take up auxin. They noticed that the acidity hardly changed over the length of the root.
To discover which protein is responsible to translate the accumulation of auxin into a lower acidity the researchers studied the effect of different auxin receptors. They noticed that plants without the auxin receptor AFB1 did not have a lower acidity in their transition zone. They also discovered that the transition zone of plants without the calcium channel protein CNGC14 did not have a lower acidity compared to the neighbouring zones. CNGC14 appears to be the proton-pump regulated via AFB1 that is influencing the acidity. In addition, it turned out that the roots of plants without AFB1 or CNGC14 had more difficulty finding a way through the soil.
Cells in the transition zone that are reading themselves to be stretched have a lower acidity. This is the result of an accumulation of auxin, which is observed through AFB1, who is via a presently unknown way is telling this to CNGC14. In turn, CNGC14 lowers the acidity of the transition zone. The lower acidity not only is preventing the stretching of the cells in the transition zone. But is also causing an acidity gradient between the transition zone and elongation zone. And it appears, that in particular, this gradient relaxes the cell walls in such a way that the cells can stretch themselves.
Literature
Nelson BC Serre, Daša Wernerová, Pruthvi Vittal, Shiv Mani Dubey, Eva Medvecká, Adriana Jelínková, Jan Petrášek, Guido Grossmann, and Matyáš Fendrych (2023) The AUX1-AFB1-CNGC14 module establishes a longitudinal root surface pH profile. eLife 12:e85193. https://doi.org/10.7554/eLife.85193
Plant en Zo 