Miranda Sinnott-Armstrong was walking along Pearl Street in Boulder, Colorado on a gorgeous fall day in 2019 when she noticed a little, extremely shiny blue fruit on a bush called Lantana strigocamara. In the spring, when the Lantanas’ delicate clusters of pink, yellow, and orange blooms and blueberries grace the pedestrian mall, city workers were tearing them out to prepare for the winter season.
Sinnott-Armstrong, a postdoctoral researcher in ecology and evolutionary biology at CU Boulder, requested if she could take a sample back to the lab right away. What made these berries so blue, she wondered?
The findings of Sinnott-Armstrong have been published in the journal New Phytologist. Lantana strigocamara is the second-ever known occurrence of a plant producing blue-coloured fruits containing stacked fat molecules, according to the study. In Viburnum tinus, she and her co-authors published the first-ever documented instance in 2020.
The two plants are two of only six in the world that use a light trick called structural colour to colour their fruits. However, Sinnott-Armstrong suspects there are more.
“We’re literally discovering these creatures in our backyards and on our streets,” Miranda Sinnott-Armstrong, the study’s main author, said. “Yet you’re like, ‘Oh, there’s one!’ simply walking down Pearl Street!”
Animals have a lot of structural colours. It’s what gives peacocks their beautiful green feathers and many butterflies their bright blue wings. According to Sinnott-Armstrong, this optical illusion is significantly more common in plants.
These blue fruits employ small structures in their skin to manipulate light and reflect wavelengths that human eyes perceive as blue, giving them a striking metallic finish. Pigmented colour, on the other hand, absorbs certain visible wavelengths. This indicates that architecturally coloured berries have no colour within them and would not stain blue if squished.
When you peel the skin off a Lantana fruit and hold it up to the light, it appears to be completely transparent. However, when placed against a dark background, it appears blue again, thanks to the nanostructures on the surface that reflect the hue.
The History of Color
Lantana strigocamara is remarkable in that it develops its structural colour in its skin using layers of lipid molecules, or fats, rather than the colour blue, which is uncommon in nature, especially in fruits.
The only other plant that does the same way is Viburnum tinus, and Lantana and Viburnum last shared an ancestor more than 100 million years ago. That is, the two plants developed this common property independently of one another.
“It makes us want to look for other populations where this happens, since we know it can be done in a variety of ways,” said Stacey Smith, associate professor of ecology and evolutionary biology and co-author of the study.
The experts also discuss why such a thing would emerge on a regular basis. Is there an evolutionary advantage to structural colour?
Some people believe that structural colour might aid seed dissemination. While architecturally coloured plants are uncommon, they are found all across the world. Lantana is an invasive plant that can be found in many parts of the world, particularly in tropical areas. According to the researchers, the metallic, shining appearance of the fruit may present a stark contrast with surrounding vegetation, luring animals to eat them and disseminate their seeds.
“However, it’s possible that just being blue and glittery is enough for an animal to think it’s decorative,” Smith added.
Many birds, particularly in Australia, use structurally coloured fruits to beautify their bowers and attract mates, according to the experts. Humans, curiously enough, may be contributing to Lantana’s proliferation for the same reason.
“The fact that they’ve made it into horticulture implies that we’re sensitive to the same things those other animals find appealing,” Smith said. “Oh, look at that sparkly, cute thing, we’re thinking. That’s something I should put in my garden.”
Another idea, according to Sinnott-Armstrong, is that the thick, fatty layer that gives the fruit its distinctive hue is a defensive mechanism for the plant, offering defence against infections or increasing the structural integrity of the fruit.
Blue could be a hint in and of itself.
Although the pigmented and structural color is not mutually exclusive in plants, she believes that structural color evolved as a mechanism for plants to manufacture blue because it is difficult to achieve in other ways.
By better understand, the assembly of cellulose nanocrystals in coloured fruits, some researchers in Silvia Vignolini’s group at the University of Cambridge—where Sinnott-Armstrong is currently based—are now attempting to manufacture colored paints, fabrics, and more out of structural colour.
As additional architecturally colored fruits are identified, researchers aim to understand more about the possible evolutionary motivations for this mechanism.
Sinnott-Armstrong stated, “They’re out there.” “We haven’t seen all of them yet.”
Yu Ogawa, Université de Grenoble Alps; Gea Theodora van de Kerkhof, University of Cambridge; and Silvia Vignolini, University of Cambridge, are co-authors on this paper.