Flowers use adjustable ‘paint-by-numbers’ petal patterns to attract pollinators, researchers say

Flowers like hibiscus use an invisible template established very early in petal formation that dictates the size of their bull’s-eyes, a crucial pre-template that can have a significant impact on their ability to attract pollinating bees.

The study, conducted by researchers at the Sainsbury’s Laboratory at the University of Cambridge, also found that bees preferred larger bullseyes to smaller ones and flew 25% faster between artificial flower disks with larger bullseyes, potentially increasing the efficiency of both the bees and the flowers. The findings are published in Scientific progress.

The patterns on plant flowers guide insects, such as bees, to the center of the flower, where nectar and pollen await them, increasing the plant’s chances of successful pollination. Despite their importance, surprisingly little is known about how these petal patterns form and how they evolved into the wide variety we see today, including spots, stripes, veins, and bull’s-eyes.

Using a small hibiscus plant as a model, the researchers compared closely related plants with the same flower size but three different sized bull’s-eye patterns featuring a dark purple center surrounded by white: H. richardsonii (small bull’s-eye covering 4% of the flower disk), H. trionum (medium bull’s-eye covering 16%), and a transgenic (mutation) line of H. trionum (large bull’s-eye covering 36%).

They found that a preliminary pattern forms on the surface of the petals very early in the flower’s formation, long before the petal shows any visible color. The petal acts like a paint-by-numbers canvas, where different regions are predetermined to develop specific colors and textures long before they begin to differentiate from each other.

Research also shows that plants can precisely control and modify the shape and size of these patterns using multiple mechanisms, which could have implications for plant evolution. By fine-tuning these patterns, plants could gain a competitive advantage in the competition for pollinators or perhaps begin to attract different species of insects.

Dr Edwige Moyroud, who leads a research team studying the mechanisms underlying petal pattern formation, explains: “If a trait can be produced by different methods, it gives evolution more options to modify it and create diversity, like an artist with a large palette or a builder with a large set of tools. By studying how bull’s-eye patterns change, what we are really trying to understand is how nature generates biodiversity.”

Lead author Dr Lucie Riglet investigated the mechanism behind the formation of hibiscus petal patterns by analysing petal development in three hibiscus flowers that had the same overall size but different bull’s-eye patterns.

She found that the pre-pattern begins as a small crescent-shaped region long before the target is visible on tiny petals less than 0.2 mm.

Dr Riglet said: “At the earliest stage of dissection, the petals had about 700 cells and were still greenish in colour, with no visible purple pigment and no difference in cell shape or size. When the petal grows further to 4,000 cells, it still has no visible pigment, but we identified a specific region where the cells were larger than their surrounding neighbours. This is the pre-model.”

These cells are important because they mark the position of the target boundary, the line on the petal where the color changes from purple to white: without a boundary, there is no target.

A computer model developed by Dr Argyris Zardilis provided additional insights and, by combining both computer models and experimental results, the researchers showed that hibiscus can vary target dimensions very early during the pre-structuring phase or modulate growth in either region of the target, adjusting cell expansion or division, later in development.

Dr Riglet then compared the relative success of bull’s-eye patterns in attracting pollinators using artificial flower disks mimicking the three different dimensions of the bull’s-eye.

Dr Riglet explains: “Not only did the bees prefer the medium and larger oxeyes over the small oxeyes, they also visited these larger flower disks 25% faster. Foraging is energy intensive and if a bee can visit four flowers rather than three at the same time, this is probably beneficial for the bee, but also for the plants.”

The researchers believe that these pre-patterning strategies may have deep evolutionary roots, potentially influencing the diversity of floral patterns across different species. The next step for Moyroud’s research team is to identify the signals responsible for generating these early patterns and to explore whether similar pre-patterning mechanisms are used in other plant organs, such as leaves.

This research not only advances our understanding of plant biology, but also highlights the complex connections between plants and their environment, showing how precise natural designs can play a central role in the survival and evolution of species.

For example, H. richardsonii, which has the smallest bull’s-eye of the three hibiscus studied in this study, is a critically endangered plant native to New Zealand. H. trionum is also found in New Zealand, but is not considered native. It is widespread in Australia and Europe and has become a naturalized weed in North America.

Further research is needed to determine whether the larger bull’s-eye size helps H. trionum attract more pollinators and improve its reproductive success.