Skewing roots
Sometimes a study takes a different route than expected. Like the just in New Phytologist published study about phosphate and root skewing, that started as study towards tropism.
Tropism is the actively growing towards or away from a source. The clearest you see this for plants in relation to gravity. Place them on their sides, and they will adapt their growth so that their shoots grow upwards and their roots downwards. Salt- and phototropism also influence plant growth. The researchers decided to study the effect of phosphate, an essential nutrient for plants, on root growth. Hoping to answering the question if tropism also played a role.
With the first experiments this appeared to be the case. Growing Arabidopsis, tale cress, on a medium with a phosphate gradient, the roots would grow towards the highest concentration of phosphate, that was located left from the plant. Only when they switched the phosphate gradient around, so that the phosphate source was now on the right hand side, the roots were still growing towards the left. So apparently no phosphate tropism.
Is it effect really due to phosphate? To investigate this the researchers studied plants in which the uptake and the signalling reaction to phosphate is not working well. The roots of these plants growing at different phosphate concentrations were growing straight down. Subsequently the researchers studied a variant whose roots grow naturally skewed. The effect of phosphate on root skewing was even greater in these plants. Also how higher the phosphate, hoe stronger the so called phosphate-dependent skewing.
By influencing microtubulin phosphate gives roots a twits
But then what was going on? To investigate how the root growth skewered, the researchers studied the roots underneath a microscope. Normally the cells of the roots form neat files next to each other. But not in roots growing at high phosphate concentrations. There the root, and in line with that the root cells as well, turn anti clockwise. The cells are skewed, overlapping half the line next to them.
The twisting of the root cells can have multiple origins, but one of them lies in the cytoskeleton of the cell. To investigate if the cytoskeleton indeed has a role in this, the researchers blocked the breakdown of microtubules, one of the parts of the cytoskeleton. The roots of plants from which the breakdown of microtubules was blocked showed more leftward skewing. This effect was even more pronounced at higher phosphate concentrations.
How exactly phosphate is influencing microtubulin is not clear jet. But it is known that microtubulin consist out of multiple fibres, and then when the total is thirteen, it results in a non-twisted cable. But are there less than thirteen, then the cable twist clockwise, and by more than thirteen the cable turns counter clockwise.
In this way an analysis about tropism can end in a study about root twisting and microtubulin.
Literature
Sheng, H., Bouwmeester, H.J. and Munnik, T. (2024), Phosphate promotes Arabidopsis root skewing and circumnutation through reorganisation of the microtubule cytoskeleton. New Phytol. https://doi.org/10.1111/nph.20152
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