Flowers use adjustable 'paint by numbers' petal designs to attract pollinators, researchers discover

Venice Mallow, also called Flower-of-an-hour, (Hibiscus trionum) was selected by Edwige Moyroud as a new model plant for studying petal pattern development. Native to Australia, H. trionum also now occurs in gardens and has become naturalized in some parts of the world. Credit: Lucie Riglet and Edwige Moyroud

Flowers like hibiscus use an invisible blueprint established very early in petal formation that dictates the size of their bullseyes—a crucial pre-pattern that can significantly impact their ability to attract pollinating bees.

The study, by researchers at the University of Cambridge's Sainsbury Laboratory, also found that bees prefer larger bullseyes over smaller ones and fly 25% faster between artificial flower disks with larger bullseyes—potentially boosting efficiency for both bees and blossoms. The findings are published in Science Advances.

Patterns on the flowers of plants guide insects, like bees, to the center of the flower, where nectar and pollen await, enhancing the plant's chances of successful pollination. Despite their importance, surprisingly little is known about how these petal patterns form and how they have evolved into the vast diversity we see today, including spots, stripes, veins, and bullseyes. 

Researchers compared the relative success of the bullseye patterns in attracting pollinators using artificial flower discs that mimicked the three different bullseye dimensions. The bees not only preferred the medium and larger bullseyes over the small bullseye, they were also 25% quicker visiting these larger flower discs. Credit: Lucie Riglet

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

They found that a pre-pattern is set up on the petal surface 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 start looking different from one another.

Color map of the cell area across the adaxial epidermis (top surface) of Hibiscus trionum petals during early developmental stages. The petal pre-pattern becomes visible early in development before any sign of color and when the petal is half a millimeter across with ~4000 cells. Credit: Lucie Riglet. Published in Science Advances.

The research also shows plants can precisely control and modify the shape and size of these patterns using multiple mechanisms, with possible implications for plant evolution. By fine-tuning these designs, plants may gain a competitive advantage in the contest to attract pollinators or maybe start attracting different species of insects.

Dr. Edwige Moyroud, who leads a research team studying the mechanisms underlying pattern formation in petals, explained, "If a trait can be produced by different methods, it gives evolution more options to modify it and create diversity, similar to an artist with a large palette or a builder with an extensive set of tools. By studying how bullseye patterns change, what we are really trying to understand is how nature generates biodiversity."

Lead author Dr. Lucie Riglet investigated the mechanism behind hibiscus petal patterning by analyzing petal development in the three hibiscus flowers that had the same total size but different bullseye patterns.

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

Dr. Riglet said, "At the earliest stage we could dissect, the petals have around 700 cells and are still greenish in color, with no visible purple pigment and no difference in cell shape or size. When the petal further develops to 4,000 cells, it still does not have any visible pigment, but we identified a specific region where the cells were larger than their surrounding neighbors. This is the pre-pattern."

Researchers compared the relative success of the bullseye patterns in attracting pollinators using artificial flower discs that mimicked the three different hibiscus bullseye dimensions - H. richardsonii (small bullseye covering 4% of the flower disc), H. trionum wild type (medium bullseye covering 16%) and a transgenic line (mutation) of H. trionum (large bullseye covering 36%). Credit: Lucie Riglet

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

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

Dr. Riglet then compared the relative success of the bullseye patterns in attracting pollinators using artificial flower disks that mimicked the three different bullseye dimensions.

Dr. Riglet explained, "The bees not only preferred the medium and larger bullseyes over the small bullseye, they were also 25% quicker visiting these larger flower disks. Foraging requires a lot of energy and, so if a bee can visit four flowers rather than three flowers in the same time, then this is probably beneficial for the bee, and also the plants."

The researchers think that these pre-patterning strategies could have deep evolutionary roots, potentially influencing the diversity of flower patterns across different species. The next step for Edwige 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.

Bumblebees can distinguish between two artificial bullseyes based on size and show a clear preference for the larger bullseye size of Hibiscus trionum compared to the smaller pattern of Hibiscus richardsonii. The researchers also found that bumblebees fly 25% faster between artificial flower discs with larger bullseyes. Credit: Lucie Riglet

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

For example, H. richardsonii, which has the smallest bullseye of the three hibiscus plants studied in this research, is a critically endangered plant native to New Zealand. H. trionum is also found in New Zealand, but not considered to be native, and is widely distributed across Australia and Europe and has become a weedy naturalized plant in North America.

Additional research is needed to determine whether the larger bullseye size helps H. trionum attract more pollinators and enhance its reproductive success.

 

 

by University of Cambridge

Source: Flowers use adjustable 'paint by numbers' petal designs to attract pollinators, researchers discover (phys.org)