Research reveals the science behind the blue berries of this plant CU Boulder Today

Banner image: Photo detail Lantana strigocamara At the Ramaley Greenhouse. (Credit: Patrick Campbell / CU Boulder)

On a beautiful fall day in 2019, as Miranda was walking down Pearl Street in Boulder, Colorado, Miranda was caught by something: a small, bright blue fruit in a bush known as her. Lantana strigocamara. While its small bouquet of pink, yellow and orange flowers and blue berries usually adorn the pedestrian center in the spring, city workers ripped out these plain Lantana to prepare for the winter season.

Sinnott-Armstrong, a postdoctoral researcher in ecology and evolutionary biology at CU Boulder, was quick to ask if he could take a specimen to the lab. He wanted to know: What made these berries so blue?

The Sinnott-Armstrong results are now published in the journal New phytologist. The study confirms that Lantana strigocamara is the second case of a plant that produces blue fruits with a layered fat molecule. He and his authors published the first documented case in the year Viburnum tinus2020.

The two plants are among some of the six known in the world, using the nuances of their fruit as a trick of light known as the structural color. But Sinnott-Armstrong believes there are more.

“We’re literally finding these things in our backyards and on our streets, people aren’t looking for structural colored plants,” said Miranda Sinnott-Armstrong, lead author of the new study. “And yet, as he walked down Pearl Street, ‘Oh, there’s one!'”

Structural color is very common in animals. This is what gives the palms a bright green feather that is otherwise brown, and many butterflies a bright blue. But this kind of optical illusion is much rarer in plants, according to Sinnott-Armstrong.

To create its unique color, these blue fruits use microscopic surface structures to manipulate light and reflect the wavelengths that our eyes perceive as blue, giving them a distinctive metallic finish. Pigmented colors do the opposite, absorbing visible wavelengths of light. This means that the structure-colored berries have no color in them; if you squashed them, they wouldn’t be stained blue.

In fact, if you peel and lighten the skin of a Lantana fruit, it looks completely translucent. But if you put it against a dark background, it looks blue again, due to the surface nanostructures that are responsible for reflecting color.

Stacey Smith, co-author of the publication and associate professor of ecology and evolutionary biology, removes the skin of a Lantana fruit.
Stacey Smith, co-author of the publication and associate professor of ecology and evolutionary biology, removes the skin of a Lantana fruit.

Top: Lantana strigocamara At the Ramaley Greenhouse. Below: Stacey Smith, co-author of the publication and associate professor of ecology and evolutionary biology, peels a Lantana fruit. (Patrick Campbell / CU Boulder)

Evolution of color

What is especially special Lantana strigocamaraThe blue color is relatively rare in nature, especially in fruit, which creates this structural color on its skin using lipid molecules or layers of fat.

Viburnum tinus is the only other plant that does the same thing, and Lantana and Biburnoa he last shared a common ancestor more than 100 million years ago. That is, the two plants developed this shared trait completely independently of each other.

“This puts us in search of other groups where this is happening, because we know it can be done in a variety of ways,” said Stacey Smith, author of the publication and associate professor of ecology and evolutionary biology.

Researchers also often talk about why such a thing would evolve. Does structural color provide an evolutionary advantage?

Some theorize that structural colors can help disperse seeds. Although very few colored plants are known for their structure, they are widespread throughout the world. Lantana itself is invasive in many parts of the world, especially in tropical regions. The metallic and shiny nature of the fruit may be in stark contrast to the surrounding leaves, attracting animals to eat and scattering the seeds, according to the researchers.

“But being blue and bright may be enough to make you think that animals are decorative,” Smith said.

Researchers point out that many birds, especially Australian birds, like to use structurally colored fruit to decorate their boards and attract mates. Interestingly, humans can also contribute to the spread of Lanthanate for the same reason.

“Being on the path in gardening suggests that we suffer the same things that other animals find attractive,” Smith said. “Oh, look at that bright, pretty thing. I should put that in my garden. ‘

Another option is to have a thick, greasy layer that creates this unique color as a protective mechanism for the plant, to protect against pathogens, or to improve the structural integrity of the fruit, Sinnott-Armstrong said.

The same color blue could also be a clue.

Pigmented and structural colors are not mutually exclusive in plants, but perhaps the plants stumbled upon structural color as a way to make blue, as it is not easy to create in other ways, he said.

Silvia Vignolini, a researcher at the University of Cambridge’s laboratory, now home to Sinnott-Armstrong, is trying to make paints, fabrics and more in a color-structured color that better understands the assembly of cellulose nanocrystals in colored fruits.

Researchers hope to learn more about the possible evolution of this mechanism as more structural colored fruits are found.

“They are there,” Sinnott-Armstrong said. “We haven’t seen them all yet.”

The authors of this publication include: Yu Ogawa, Université de Grenoble Alps; Gea Theodora van de Kerkhof, University of Cambridge; and Silvia Vignolini, University of Cambridge.

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