Banner image: Detailed photo of lantana strigokamar in the greenhouse in Ramali. (Credit: Patrick Campbell / CU Boulder)
On a beautiful fall day in 2019, Miranda Son-Armstrong was walking down Pearl Street in Boulder, Colorado, when something caught her eye: a small, particularly shiny blue fruit, on a bush known as lantana strigokamar. While small clusters of pink, yellow and orange flowers and blueberries usually adorn the pedestrian mall in the spring, city workers plucked these simple lanthans to prepare for the winter season.
The son-Armstrong, a postdoctoral fellow in ecology and evolutionary biology at CU Boulder, quickly asked if he could take a sample back to the lab. She wanted to know: What made these fruits so blue?
Sinnott-Armstrong’s results have already been published in the journal New phytologist. The study confirms that Lantana strigocamara is the second documented case of a plant that produces blue-colored fruit 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 that are known to make their fruits bloom using a trick of light known as structural color. But Son Armstrong has a hunch he has more.
“We literally find these things in our backyards and on our streets, people just haven’t looked for structurally colored plants,” said Miranda Son-Armstrong, lead author of the new study. And yet, just walking around Pearl Street, you say to yourself, “Oh, there’s one!”
Structural color is very common in animals. This gives the otherwise brown peacock feathers brilliant green and many butterflies bright blue. But this kind of optical illusion is much rarer in plants, according to Son-Armstrong.
To create their 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 them a distinctive metallic finish. The pigmented color does the opposite, absorbing selected visible wavelengths of light. This means that structurally colored fruits have no color in them; if you crush them, they will not turn blue.
In fact, if you peel off the peel of the lanthanum and keep it in the light, it looks completely transparent. 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 is especially unique lantana strigokamar– in addition to the fact that the blue color is quite scarce in nature, especially in fruits – is that 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 Lantana and Kalina last shared a common ancestor more than 100 million years ago. This means that the two plants have developed this common feature completely independently of each other.
“This makes us look for other groups where this is happening, because we know it can be done in several ways,” said Stacey Smith, co-author and associate professor of ecology and evolutionary biology.
Researchers also often talk about why something like this would develop. Does structural color provide an evolutionary advantage?
Some theorize that structural color can help disperse seeds. Although there are very few known structurally colored plants, they are widespread worldwide. Lanthanum itself is invasive in many parts of the world, especially in the tropics. It is possible that the metallic, shiny nature of the fruit provides a strong contrast to the surrounding greenery, attracting animals to eat them and scatter their seeds, according to researchers.
“But just being blue and shiny can be enough to make an animal think it’s decorative,” Smith said.
Researchers note that many birds, especially in Australia, like to use structurally colored fruits to decorate their handcuffs and attract mates. Interestingly, people can also contribute to the spread of Lantana for the same reason.
“The fact that they’ve made their way in gardening suggests that we’re susceptible to the same things that other animals find attractive in them,” Smith said. “We’re like, oh, look at this shiny, sweet thing. I have to put it in my garden. ”
Another possibility is the thick fat layer that creates this unique color, is a protective mechanism for the plant, providing protection against pathogens or improving the structural integrity of the fruit, said Son-Armstrong.
The blue color itself can also be a clue.
Pigmented and structural color are not mutually exclusive in plants, but plants may have come across structural color as a way to make blue because it’s not so easy to create in other ways, she said.
Some researchers at Sylvia Vignolini’s laboratory at Cambridge University, where Sinnott-Armstrong is currently based, are now trying to make colored dyes, fabrics and others of structural color, better understanding the assembly of cellulosic nanocrystals in colored fruits.
Researchers hope to learn more about possible evolutionary prompts for this mechanism, as more structurally colored fruits are found.
“They are somewhere,” said Son-Armstrong. “We just haven’t seen them all yet.”
Co-authors of this publication include: Yu Ogawa, Université de Grenoble Alps; Gea Theodora van de Kerkhoff, University of Cambridge; and Sylvia Vignolini of the University of Cambridge.