Asymmetric flowers
Left-right asymmetry breaks with the expected symmetry. Think of flowers with their style and anthers located respectively left and right of the middle. Now, a group of international researchers show how a plant breaks with symmetry.
Some flowers are asymmetrical, their style and anthers are located at opposite sides of the middle. Plants do this to prevent self-pollination. But also, pollinators are dusted asymmetrical with pollen. To make sure that the flowers will get pollinated, is in a part of the population the asymmetry reversed. So, while in one part of the population the style is at the left, in the other part of the population the style will be at the right of the middle.
There are two ways a plant can regulate this. It can be genetically determined, for example with dominant and recessive versions of a gene. But it can also be that environmental factors are influencing the asymmetry. The researchers decided to find this out for the South African Cyanella alba. A long-lived plant that flowers each year from a new growth centre with multiple yellow flowers. Mostly with the same left-right asymmetry.
Cyanella alba has is no genetic basis for its left-right asymmetry of its flowers
Because of the flower phenotype the researchers expected that the asymmetry was genetically organised. To confirm this, and to find out which gene was responsible the researchers read the genomes of 25 Cyanella alba plants. In 15 the style was located left and in 10 at the right side of the middle. To the surprise of the researchers, it turned out that there was no genetic basis for the observed asymmetry.
Subsequently the researchers studied 188 plants for multiple years. During this they noticed that in about half of the plants for which the style was located at the right, that in the subsequent year the style was at the left. Closer observation showed that not only the style switched orientation but the rosette from which the flowers grow as well. When the leaves of the rosette grew clockwise then the style of the flowers would be left of the middle. But when the leaves in the rosette grew counterclockwise the style of the flowers would be right of the middle.
Applying auxin can reverse the left-right asymmetry
The next question that the researchers studied was how the flower actually got asymmetric. By closely observing the researchers noticed that that the carpels at the top of the flower did not stretch evenly. This created a mechanical asymmetry at the base of the style and the anthers. The farther stretching carpel forced the style away into the opposite direction and pulled the anthers towards it.
Because the asymmetric distribution of the plant hormone auxin plays a role asymmetry in plants the researchers checked if this was here also the case. By applying auxin at the base of the developing carpels, the researchers could switch the left-right asymmetry.
Although this study shines light on how Cyanella alba gets it asymmetric flowers, we are left with the question how the rosette gets its direction. The clockwise or counterclockwise twisting of the leaves of the rosette must be a reaction to something in its environment. But what it precisely is that the plant reacts to remains elusive.
Literature
Robertson, C., Xue, H., Saltini, M. et al. Spiral phyllotaxis predicts left-right asymmetric growth and style deflection in mirror-image flowers of Cyanella alba. Nat Commun 16, 3695 (2025). https://doi.org/10.1038/s41467-025-58803-5
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