Banner image: Detail shot of lantana strigocamara in the Ramaley greenhouse. (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 particularly bright little blue fruit, on a shrub known as lantana strigocamara. While its tiny clusters of pink, yellow, and orange flowers and blue berries commonly adorn the pedestrian mall in spring, city workers were plucking these common lantanas to prepare for the winter season.
Sinnott-Armstrong, a postdoctoral researcher in ecology and evolutionary biology at CU Boulder, immediately asked if she could take a sample back to the lab. She wanted to know: what made these berries so blue?
The Sinnott-Armstrong findings are now published in the journal New phytologist. The study confirms Lantana strigocamara as the second documented case of a plant that creates 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 render their fruit colors using a trick of light known as a structural color. But Sinnott-Armstrong has the feeling that there are more of them.
“We’re literally finding these things in our backyards and on our streets, people just haven’t been looking for structurally colored plants,” said Miranda Sinnott-Armstrong, lead author of the new study. “Still, just walking down Pearl Street, you’re like, ‘Oh, there’s one!'”
Structural color is very common in animals. It is what gives the otherwise brown feathers of peacocks their bright greens and many butterflies their bright blue. But this sort of optical illusion is much rarer in plants, according to Sinnott-Armstrong.
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 by absorbing certain visible wavelengths of light. This means that structurally colored berries have no color inside them; if you were to crush them, they would not be dyed blue.
In fact, if you peel off the peel of a Lantana fruit and hold it up to the light, it appears completely translucent. But if you put it on a dark background, it looks blue again, due to the nanostructures on the surface responsible for color reflection.
The evolution of color
What makes it particularly unique lantana strigocamara– besides the fact that the color blue is quite poor 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 thing, and Lantana And Viburnum last shared a common ancestor more than 100 million years ago. This means that the two plants have developed this shared trait that is completely independent of each other.
“It puts us looking for other groups where this happens, because we know it can be done in a number of ways,” said Stacey Smith, co-author of the publication and associate professor of ecology and evolutionary biology.
Researchers also often talk about why such a thing would have evolved. Does structural color provide an evolutionary advantage?
Some theorize that structural color could help with seed dispersal. Although there are very few structurally colored plants known, they are widespread globally. Lantana itself is invasive in many parts of the world, especially in tropical regions. It’s possible that the fruit’s metallic and shiny nature offers a strong contrast to the surrounding foliage, luring animals to eat them and disperse their seeds, according to the researchers.
“But being blue and sparkling might be enough to make an animal think it’s decorative,” Smith said.
Researchers have noted that many birds, especially in Australia, like to use structurally colored fruits to adorn their pergolas and attract mates. Humans, curiously, could 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 in them,” Smith said. “We’re like, oh, look at that shiny, cute thing. I should put it in my garden. ”
Another possibility is that the thick, fatty layer 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 have come across structural color as a way to create blue because it’s not that easy to create in other ways, he said.
Researchers from Silvia Vignolini’s laboratory at Cambridge University, where Sinnott-Armstrong is currently based, are now trying to make colored paints, textiles and more with structural color, by better understanding the assembly of cellulose nanocrystals in colored fruits.
The researchers hope to learn more about the possible evolutionary stimuli for this mechanism, as more structurally colored fruits are discovered.
“They are 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.