Header image: detail photo of Lantana strigocam in the greenhouse at Ramaley. (Credit: Patrick Campbell/CU Boulder)
On a beautiful fall day in 2019, Miranda Sinnott-Armstrong was walking down Pearl Street in Boulder, Colorado, when something caught her eye: a small, particularly bright blue fruit on a bush known as Lantana strigocam. While their tiny clusters of pink, yellow, and orange flowers and blue berries commonly grace the pedestrian mall in spring, these common Lantanas were plucked by city workers to prepare for the winter season.
Sinnott-Armstrong, a postdoctoral researcher in ecology and evolutionary biology at CU Boulder, quickly asked if she could bring a sample to the lab. She wanted to know: What made these berries so blue?
Sinnott-Armstrong results are now published in the journal new phytologist. The study confirms Lantana strigocamara as the second documented case of a plant creating blue-colored fruits with layered fat molecules. She and her co-authors published the first documented case, in Viburnum tinusin 2020.
The two plants are among only six in the world known to tone their fruit using a light trick known as structural color. But Sinnott-Armstrong has a feeling there is more.
“We’re literally finding these things in our backyards and on our streets, people just haven’t been looking for plants with structural colors,” said Miranda Sinnott-Armstrong, lead author of the new study. “And yet, just walking down Pearl Street, you’re like, ‘Oh, there’s one!'”
Structural color is very common in animals. It’s what gives the brown feathers of peacocks their bright greens, and many butterflies their bright blues. But this kind of optical illusion is much rarer in plants, according to Sinnott-Armstrong.
To create their unique color, these blue fruits use microscopic structures in their skin to manipulate light and reflect the wavelengths our eyes perceive as blue, giving it a distinctive metallic finish. Pigmented color does the opposite, absorbing selected wavelengths of visible light. This means that structural color berries themselves have no color; if you crushed them, they wouldn’t stain blue.
In fact, if you peel the skin off a Lantana fruit and hold it up to a light, it looks completely translucent. But if you put it on a dark background, it turns blue again, due to the nanostructures on the surface responsible for reflecting the color.
the evolution of color
What is especially unique about Lantana strigocamIn addition to the fact that the color blue is quite rare in nature, especially in fruits, it creates this structural color in your skin using layers of lipid molecules, or fats.
Viburnum tinus is the only other plant known to do the same, and lantana Y viburnum they 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.
“It puts us on the hunt for other groups where this happens, because we know it can be done in a variety of ways,” said Stacey Smith, co-author of the paper and associate professor of ecology and evolutionary biology.
Researchers also often discuss why such a thing would evolve. Does structural color provide an evolutionary advantage?
Some theorize that structural color could help with seed dispersal. Although very few structurally colored plants are known, they are widespread throughout the world. Lantana itself is invasive in many parts of the world, especially tropical regions. It’s possible that the fruit’s shiny, metallic nature provides a stark contrast to the surrounding foliage, attracting animals to eat them and disperse their seeds, according to the researchers.
“But being blue and shiny 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 fruit to adorn their perches and attract mates. Interestingly, humans may also be contributing to the spread of Lantana for the same reason.
“The fact that they have been introduced into horticulture suggests that we are susceptible to the same things that other animals find attractive in them,” Smith said. “We’re like, oh look at that cute, shiny thing. I should put that in my garden.”
Another possibility is that the thick layer of fat that creates this unique color is a protective mechanism for the plant, providing defense against pathogens or improving the structural integrity of the fruit, Sinnott-Armstrong said.
The blue color itself could also be a clue.
Pigmented and structural color aren’t mutually exclusive in plants, but perhaps plants stumbled upon structural color as a way to make blue because it’s not as easy to create in other ways, he said.
Some researchers in Silvia Vignolini’s lab at the University of Cambridge, where Sinnott-Armstrong is currently based, are now trying to make color paints, fabrics and more out of structural color, by better understanding the assembly of cellulose nanocrystals in colored fruits.
The researchers hope to learn more about possible evolutionary indications of this mechanism, as more structurally colored fruits are discovered.
“They’re out there,” Sinnott-Armstrong said. “We just haven’t seen all of them 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.