A small, shiny blue fruit, on a shrub known as Lantana strigocamara, caught the attention of Miranda Sinnott-Armstrong as she walked down Pearl Street in Boulder, Colorado. While its small clusters of pink, yellow and orange flowers and blue berries adorn the pedestrian mall in the spring, city workers were ripping these common Lantanas out to prepare for the winter
Sinnott-Armstrong asked if she could return the specimen to the lab. She wanted to know what made the berry blue.
The results are in a journal. Lantana strigocamara is the second documented case of a plant making blue-colored fruits with layers of fat molecule. The first documented case was published in 2020.
Only six plants in the world are known to make their fruit's colors using a trick of the light known as structural color. Sinnott-Armstrong thinks there are more.
"We're finding these things in our backyards and on our streets, people just haven't been looking for colored plants," said Miranda Sinnott-Armstrong, lead author on the study. You're like, "Oh, there's one!" when you walk on Pearl Street.
Animals have structural colors. It is what gives peacocks' brown feathers their brilliant greens. Sinnott-Armstrong says that this optical illusion is very rare in plants.
The blue fruits use tiny structures in their skin to manipulate light and reflect the wavelengths our eyes see as blue, creating a metallic finish. The color of the light is absorbed by the color of it. If you were to crush them, they wouldn't stain blue.
If you hold a Lantana fruit up to the light, it will look translucent. If you put it against a dark background, it will look blue again because of the nanostructures on the surface.
The evolution of colors.
Lantana strigocamara is the only plant in the world that creates a structural color in its skin using layers of fat and cholesterol.
The only other plant that does the same thing is Viburnum tinus, which has a common Ancestor with Lantana. This trait is completely independent of one another.
"It puts us on the hunt for other groups where this happens, because we know it can be done multiple ways," said the co-author of the publication.
The researchers talk a lot about why such a thing would change. Structural color may provide an evolutionary advantage.
Structural color may be able to help with seed dispersal. There are few known plants with colored skin. Many parts of the world are invaded by Lantana. According to the researchers, it is possible that the metallic, shiny nature of the fruit attracts animals to eat them and distribute their seeds.
It might be enough for an animal to think it's decorative if it's blue and shiny.
Many birds in Australia like to use colored fruits to decorate their bowers and attract mates. Humans might be contributing to the spread of Lantana for the same reason.
Smith said that the fact that they made their way into horticulture suggests that we are vulnerable to the same things as other animals. We're looking at that cute thing. I should plant that in my garden.
It is possible that this unique color is a protective mechanism for the plant, providing defense against pathogens or improving the structural integrity of the fruit.
It's possible that the blue itself is a clue.
She said that plants may have stumbled across structural color as a way to make blue because it's not as easy to make other colors.
Some researchers at the University of Cambridge are trying to make colored paints, fabrics and more out of structural color by better understanding the assembly of cellulose nanocrystals in colored fruit.
As more colored fruits are found, researchers hope to learn more about the evolutionary prompt for this mechanism.
Sinnott-Armstrong said that they are out there. We haven't seen all of them.
There are three co-authors on this publication.
More information: Miranda A. Sinnott‐Armstrong et al, Convergent evolution of disordered lipidic structural colour in the fruits of Lantana strigocamara (syn. L. camara hybrid cultivar), New Phytologist (2022). DOI: 10.1111/nph.18262 Journal information: New Phytologist
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