Bullseye
Flowers go all out for getting noticed by pollinators, like accentuating their centre with a different colour. Now Englisch researchers show in Science Advances that this pattern establishes itself early in hibiscus flower development.
The dark yellow stamen full with pollen form a stark contrast with the blue-purple spot that forms the middle of the hibiscus flower. The transition from the blue-purple to the white of the rest of the petals is abrupt. Researchers wondered how a plant can regulate this that precisely and decided to study it in more detail.
They started with a close-up of the developing petals. Early on in development two regions could be distinguished. One close by the base of the petal, and one at the tip. The cells at the base of the petal started stretching out early, while the cells at the tip of the petal kept dividing for longer. In between, at about one third from the base of the petal, the largest cells were found.
These largest petal cells appeared to be at the location of the future border of the blue-purple centre. To confirm this the researchers studied hibiscus plants that had a smaller centre in the middle of their flowers. Here, as well, the cells at the base of the petals stretched themselves early, while the cells at the tip of the petal were still dividing. Only the largest petal cells were located at one tenth from the base of the petal. Just like the border of the smaller centre.
Pollinators prefer flowers with medium and large sized purple centres
Subsequently the researchers created plants with a large blue-purple centre. They did this through enabling plants to produce more of TCP4.1, a gene regulator. At first sight there appeared to be no differences. The largest petal cells were still located at about one third from the base. But also cells beyond the largest petal cells turned purple.
But zooming in more appeared to be going on. All petal cells divided for a similar length of time. And cells at the base of the petal that stretched themselves got to about the same size, only beyond the one third border the cells stated smaller. The researchers suspect that these changes in cell division and stretching out are the reason why the blue-purple centre stretches itself beyond the one third border.
Now I hear you think, nice. But does it makes a difference for the pollinators. That, the researchers also tested through observing the preference of bees for particular round discs with purple centres and sugar water. They used purple centres of three different sizes. They noticed that the bees clearly prefer the middle sized centres over the small ones. While they did not show a strong preference between the middle and large sized purple centres.
Literature
Lucie Riglet et al., Hibiscus bullseyes reveal mechanisms controlling petal pattern proportions that influence plant-pollinator interactions. Sci. Adv. 10, eadp5574(2024). DOI: 10.1126/sciadv.adp5574
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