On a beautiful fall day in 2019, Miranda Sinnott-Armstrong was walking down Pearl Street in Boulder, Colorado when she noticed something: a small, particularly shiny blue fruit, on a shrub known as Lantana strigocamara. While the small clusters of pink, yellow and orange flowers and blueberries usually adorn the pedestrian zone in the spring, city workers were tearing out these common Lantanas to prepare for the winter season.
Sinnott-Armstrong, a postdoctoral researcher in ecology and evolutionary biology at CU Boulder, was quick to ask if she could take a specimen back to the lab. She wanted to know: what made these berries so blue?
Sinnott-Armstrong’s results are now published in the journal New phytologist† The study confirms that Lantana strigocamara is the second documented case of a plant that creates blue-colored fruits with layered fat molecules. She and her co-authors published the very first documented case, in Viburnum tinusin the year 2020.
The two plants are among only six in the world known to create the hues of their fruits using a trick of the light known as structural color. But Sinnott-Armstrong has the idea that there are more.
“We literally find these things in our backyards and on our streets. People just aren’t looking for structurally colored plants,” said Miranda Sinnott-Armstrong, lead author of the new study. “And yet, if you just walk down Pearl Street, you’re like, ‘Oh, there’s one!'”
Structural color is common in animals. It’s what gives peacocks’ otherwise brown feathers their brilliant green, and many butterflies their bright blue. But according to Sinnott-Armstrong, this optical illusion is much rarer in plants.
To create its unique color, these blue fruits use microscopic structures in their skin to manipulate light and reflect the wavelengths that our eyes perceive as blue, giving it a distinctive metallic finish. Pigmented color does the opposite, absorbing selected visible wavelengths of light. This means that structurally colored berries have no color of their own; if you squashed them, they wouldn’t turn blue.
In fact, if you peel the skin of a Lantana fruit and hold it up to the light, it will look completely translucent. But if you place it against a dark background, it will look blue again, because of the nanostructures on the surface that are responsible for reflecting the color.
The evolution of color
What is especially unique about Lantana strigocamara – besides the fact that the color blue is quite scarce in nature, especially in fruits – is that it creates this structural color in its skin using layers of lipid molecules or fats.
Viburnum tinus is the only other plant known to do the same, and Lantana and Viburnum last shared a common ancestor more than 100 million years ago. This means that the two plants developed this shared trait completely independently of each other.
“It puts us on the hunt for other groups where this is happening because we know it can be done in multiple ways,” he said Stacey Smithco-author of the publication and associate professor of ecology and evolutionary biology.
The researchers also often talk about why such a thing would evolve. Does structural color offer an evolutionary advantage?
Some theorize that structural color could aid in seed dispersal. Although few structurally colored plants are known, they are widespread worldwide. Lantana itself is invasive in many parts of the world, especially in tropical regions. It’s possible that the fruit’s metallic, glossy nature provides a stark contrast to the surrounding foliage, attracting animals to eat them and disperse their seeds, the researchers said.
“But being blue and glittering might be enough for an animal to think it’s decorative,” Smith said.
The researchers noted that many birds, especially in Australia, like to use structurally colored fruits to decorate their arbor and attract mates. Interestingly, humans may also contribute to the spread of Lantana for the same reason.
“The fact that they have made their way into horticulture suggests that we are susceptible to the same things that other animals find attractive,” Smith says. “We’re like, oh, look at that sparkly, cute thing. I should put that in my garden.”
Another possibility is that the thick, fatty layer that creates this unique color is a protective mechanism for the plant, protecting against pathogens or improving the fruit’s structural integrity, Sinnott-Armstrong said.
The color blue itself may also be a clue.
Pigmented and structural color aren’t mutually exclusive in plants, but perhaps plants came across structural color as a way to create blue because it’s not as easy to create in other ways, she said.
Some researchers in Silvia Vignolini’s lab at the University of Cambridge — where Sinnott-Armstrong is currently based — are now trying to make colored paints, fabrics and more from structural color, by better understanding the assembly of cellulose nanocrystals in colored fruits.
Researchers hope to learn more about the possible evolutionary clues to this mechanism as more structurally colored fruits are discovered.
“They’re out there,” Sinnott-Armstrong said. “We just haven’t seen them all yet.”
Co-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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