Twisting stems and the golden rule
Most plants orient their leaves around their stem according to the principle of the golden rule. This comes down to that every subsequent leaf is oriented at an angle of about 137.5 degrees compared to its predecessor. In this way the leaves are mostly not overlapping and can the plant use the incoming sunlight optimally.
Surprisingly researchers don’t know how plants are able to stick that well to the golden rule. Of course there are theories, like that hormone distribution in the stem plays a role, but they don’t completely explain it. What doesn’t help is that the golden rule is really robust, there are no mutants that consistently diverge from the golden rule.
Therefore, the researchers of the article ‘PLETHORAs shape Arabidopsis phyllotaxis through modulation of patterning robustness and accelerated inflorescence development’ were happy when it appeared a mutant was diverging. This mutant was missing three PLETHORA (3, 5, and 7) genes. But when the researchers studied this plant once more in slightly different growth conditions, this divergence of the golden rule did not appear to be as strong as previously thought. Still the researchers decided to keep studying it.
Thereby the researchers found that the PLETHORA genes, which encode gene-activators, are needed for the activation of genes that are involved in the production and distribution of the hormones auxin and cytokinin. So far so good. Still the researchers where not completely satisfied.
Swaying stems
Therefore, they searched for a plant that was diverging even more from the golden rule. The researchers found this in a plant in which not only the PLETHORA were missing but also did not have a working auxin distribution protein. This so say the researchers suggest that the PLETHORA genes ensure via auxin distribution for a particular robustness in the growth centres of the plant.
In addition, the researchers noticed that plants without the three PLETHORA genes had a longer stem between two leaves. In addition, they noticed that plants that missed the three PLETHORA genes could be divided into two groups when the researchers set off the divergence angle against the stem length between two leaves. In the group in which the divergence angle was smaller, the stem was twisting clockwise, while in the group in which the divergence angle was larger, the stem was twisting counterclockwise.
To be sure that the longer distance between the leaves and the twisting of the stem was enough to explain the divergence of the golden rule, the researchers putt it all in a model. This was able to simulate the phenotype of the PLETHORA mutant. And when the researchers increased the twisting or the length of the distance between two leaves, then the angle of the simulated leaves was diverging even more. The golden rule is therefore possible due to the twisting of plant stems.
Literature
Kerstens, M., van der Klugt, F., Hofhuis, H., Scheres, B. and Willemsen, V. (2026), PLETHORAs shape Arabidopsis phyllotaxis through modulation of patterning robustness and accelerated inflorescence development. New Phytol, 249: 495-511. https://doi.org/10.1111/nph.70620
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