Research uncovers the science behind this plant’s blue berries

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 shiny blue fruit, on a shrub known as Lantana strigocamara . While its tiny clusters of pink, yellow and orange flowers and blue berries usually adorn the pedestrian mall in the spring, city workers were tearing up these common Lantanas 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 specimen back to the lab. She wanted to know: What made those berries so blue?

Sinnott-Armstrong results are now published in the journal New Phytologist. The study confirms that Lantana strigocamara is the second documented case of a plant creating blue-colored fruits with layered fat molecules. She and her co-authors published the very first documented case, in Tinus Viburnumin 2020.

The two plants are among only six in the world known to tint their fruit using a trick of light known as structural color. But Sinnott-Armstrong has a hunch there are others.

“We literally find these things in our backyards and on our streets, people just haven’t looked for structurally colored plants,” 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 is what gives the otherwise brown feathers of peacocks their brilliant green and many butterflies their bright blue. But this kind 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 wavelengths that our eyes perceive as blue, giving them a distinctive metallic finish. Pigmented color does the opposite, absorbing certain visible wavelengths of light. This means that structurally colored berries have no color in themselves; if you were to crush them, they wouldn’t stain blue.

In fact, if you peel the skin off a Lantana fruit and hold it up to the 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 particularly unique about Lantana strigocamara, besides the fact that the blue color is quite rare in nature, especially in fruit, is that it creates this structural color in its skin using layers of molecules lipids or fats.

Tinus Viburnum is the only other plant known to do the same, and Lantana and Viburnum last shared a common ancestor over 100 million years ago. This means that the two plants developed this common trait completely independently of each other.

“It puts us on the lookout for other bands where this is happening because we know it can happen in a number of ways,” said Stacey Smithco-author of the publication 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 may aid in seed dispersal. Although there are very few structurally known colored plants, they are widely distributed throughout the world. Lantana itself is invasive in many parts of the world, especially in tropical regions. According to the researchers, it is possible that the metallic and shiny nature of the fruit provides a strong contrast to the surrounding foliage, attracting animals to eat them and disperse their seeds.

“But just being blue and sparkly can be enough for an animal to think it’s decorative,” Smith said.

Researchers have noted that many birds, especially in Australia, like to use structurally colored fruit to adorn their arbors and attract mates. Humans, interestingly, can also help spread Lantana for the same reason.

“The fact that they’ve made their way into horticulture suggests that we’re sensitive to the same things that other animals find attractive about them,” Smith said. “We’re like, oh, look at this shiny, cute thing. I should put this in my garden.

Another possibility is that the thick, oily 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 color and structural color are not mutually exclusive in plants, but perhaps plants have stumbled upon structural color as a means of producing blue, since it’s not as easy to create otherwise, a she declared.

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 nanocrystals. of cellulose in colored fruits.

Researchers hope to learn more about possible evolutionary prompts for this mechanism as more structurally colored fruits are discovered.

“They’re out there,” Sinnott-Armstrong said. “We just haven’t seen them all yet.”

The co-authors of this publication are: Yu Ogawa, University of Grenoble Alpes; Gea Theodora van de Kerkhof, University of Cambridge; and Silvia Vignolini, University of Cambridge.