This fruit attracts birds with an unusual way of making itself metallic blue
VIDEO: A 3D SCHEMATIC OF THE RECONSTRUCTED GLOBULAR LIPID LAYERS OF THE FRUIT'S CELLS. view more
CREDIT: MIRANDA SINNOTT-ARMSTRONG
There's a reason why blue fruits are so rare: the pigment compounds that make fruits blue are relatively uncommon in nature. But the metallic blue fruits of Viburnum tinus, a popular landscaping plant in Europe, get their color a different way. Instead of relying solely on pigments, the fruits use structural color to reflect blue light, something that's rarely seen in plants. Researchers reporting August 6 in the journal Current Biology show that the fruits use nanostructures made of lipids in their cell walls, a previously unknown mechanism of structural color, to get their striking blue--which may also double as a signal to birds that the fruits are full of nutritious fats.
"Structural color is very common in animals, especially birds, beetles, and butterflies, but only a handful of plant species have ever been found to have structural color in their fruits," says co-first author Miranda Sinnott-Armstrong, a postdoctoral researcher at the University of Colorado-Boulder. "This means that V. tinus, in addition to showing a completely novel mechanism of structural color, is also one of the few known structurally colored fruits."
This image shows the drupe fruits of the Viburnum tinus plant.
Senior author Silvia Vignolini (@VignoliniLab), a physical chemist at the University of Cambridge, has been interested in the plants for nearly 10 years. "I actually found this Viburnum in a garden in Italy and observed that they looked weird, so we measured them at the time but didn't have conclusive results. It was kind of always on the back of my mind," she says. As her team grew, they become more interested in V. tinus and eventually had the capability to examine the structure of the fruits using electron microscopy. "Before we got the images, we were just seeing all these blobs," she says. "When we found out that those blobs were lipids, we got very excited."
While most plants have cell walls made of cellulose, used to make cotton and paper, V. tinus fruit cells have much thicker walls with thousands of globular lipids arranged in layers that reflect blue light. The structure formed by this so-called lipid multilayer allows the fruits to create their vibrant blue color while containing no blue pigment. "This is very strange because globular lipids like these are not usually found in this arrangement in the cell wall, as they are normally stored inside the cell and used for transport," says co-first author Rox Middleton, a physicist who studied the optical response of the fruits during her PhD and is now a postdoctoral researcher at the University of Bristol. "We also believe that this lipid may contribute to the fruit's nutrition. That means that the fruit can demonstrate how nutritious it is by being a beautiful, shiny blue."
This extra nutrition would be important for V. tinus's main consumers: birds that disperse the plant's seeds. Although the researchers can't say for sure whether the lipids are used as fat by the birds that consume them, there is reason to believe they might be. If so, the researchers suggest that the metallic blue color made by the lipid multilayer could indicate to the birds that if they see this striking blue, the fruit in question will have enough nutrients to make it a worthwhile meal. "While birds have been shown to be attracted to blue fruits," says Vignolini, "other blue fruits that we have studied essentially don't have any nutritional value."
Microscopy view of Viburnum tinus
Going forward, the researchers want to see how widespread blue structural color is in fruits to understand its ecological significance. They had never seen this type of lipid multilayer in a biomaterial before, but since their discovery, they've begun to take notice of other species. "We actually realize now that there are some older electron microscopy pictures from other plants where you can see the blobs. The researchers didn't know that they were lipids at the time, or that lipids could even form this type of structure, but our research suggests that they very well could be, meaning this structure may not be limited to Viburnum," Vignolini says.
Additionally, learning how V. tinus can use such a unique mechanism to make color may have implications for how we color our own foods. "There are lots of problems connected to food coloration," says Vignolini. She adds that once this mechanism is better understood, it could potentially be used to create a healthier, more sustainable food colorant.
But right now, Vignolini is just excited her initial hunch paid off: "I've been working on this type of photonic structure for quite a while, and I was beginning to think there were no new ways to make it--at some point you've seen so many that you think, 'This is more or less the end, it's going to be difficult to find something new,'" she says. "Instead, we discovered much more than what we expected."
Closeup of viburnum tinus
This work was supported by the EPSRC NanoDTC, BBSRC David Phillips fellowship, ERC SeSaME, a microMORPH Cross-Training Grant, a Yale Institute for Biospheric Studies grant, and the National Science Foundation.
Current Biology, Middleton et al.: "Viburnum tinus fruits use lipids to produce metallic blue structural colour" https://www.cell.com/current-biology/fulltext/S0960-9822(20)30995-7
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.
Metallic blue fruits use fat to produce color and signal a treat for birds
IMAGE: VIBURNUM TINUS OWES THE DAZZLING BLUE COLOUR OF ITS FRUIT TO FAT IN ITS CELLULAR STRUCTURE, THE FIRST TIME THIS TYPE OF COLOUR PRODUCTION HAS BEEN OBSERVED IN NATURE. view more
CREDIT: ROX MIDDLETON
Researchers have found that a common plant owes the dazzling blue colour of its fruit to fat in its cellular structure, the first time this type of colour production has been observed in nature.
The plant, Viburnum tinus, is an evergreen shrub widespread across the UK and the rest of Europe, which produces metallic blue fruits that are rich in fat. The combination of bright blue colour and high nutritional content make these fruits an irresistible treat for birds, likely increasing the spread of their seeds and contributing to the plant's success.
The researchers, led by the University of Cambridge, used electron microscopy to study the structure of these blue fruits. While there are other types of structural colour in nature - such as in peacock feathers and butterfly wings - this is the first time that such a structure has been found to incorporate fats, or lipids. The results are reported in the journal Current Biology.
"Viburnum tinus plants can be found in gardens and along the streets all over the UK and throughout much of Europe -- most of us have seen them, even if we don't realise how unusual the colour of the fruits is," said co-first author Rox Middleton, who completed the research as part of her PhD at Cambridge's Department of Chemistry.
Most colours in nature are due to pigments. However, some of the brightest and most colourful materials in nature - such as peacock feathers, butterfly wings and opals - get their colour not from pigments, but from their internal structure alone, a phenomenon known as structural colour. Depending on how these structures are arranged and how ordered they are, they can reflect certain colours, creating colour by the interaction between light and matter.
"I first noticed these bright blue fruits when I was visiting family in Florence," said Dr Silvia Vignolini from Cambridge's Department of Chemistry, who led the research. "I thought the colour was really interesting, but it was unclear what was causing it."
"The metallic sheen of the Viburnum fruits is highly unusual, so we used electron microscopy to study the structure of the cell wall," said co-first author Miranda Sinnott-Armstrong from Yale University. "We found a structure unlike anything we'd ever seen before: layer after layer of small lipid droplets."
The lipid structures are incorporated into the cell wall of the outer skin, or epicarp, of the fruits. In addition, a layer of dark red anthocyanin pigments lies underneath the complex structure, and any light that is not reflected by the lipid structure is absorbed by the dark red pigment beneath. This prevents any backscattering of light, making the fruits appear even more blue.
The researchers also used computer simulations to show that this type of structure can produce exactly the type of blue colour seen in the fruit of Viburnum. Structural colour is common in certain animals, especially birds, beetles, and butterflies, but only a handful of plant species have been found to have structurally coloured fruits.
While most fruits have low fat content, some - such as avocadoes, coconuts and olives - do contain lipids, providing an important, energy-dense food source for animals. This is not a direct benefit to the plant, but it can increase seed dispersal by attracting birds.
The colour of the Viburnum tinus fruits may also serve as a signal of its nutritional content: a bird could look at a fruit and know whether it is rich in fat or in carbohydrates based on whether or not it is blue. In other words, the blue colour may serve as an 'honest signal' because the lipids produce both the signal (the colour) and the reward (the nutrition).
"Honest signals are rare in fruits as far as we know," said Sinnott-Armstrong. "If the structural colour of Viburnum tinus fruits are in fact honest signals, it would be a really neat example where colour and nutrition come at least in part from the same source: lipids embedded in the cell wall. We've never seen anything like that before, and it will be interesting to see whether other structurally coloured fruits have similar nanostructures and similar nutritional content."
One potential application for structural colour is that it removes the need for unusual or damaging chemical pigments - colour can instead be formed out of any material. "It's exciting to see that principle in action - in this case the plant uses a potentially nutritious lipid to make a beautiful blue shimmer. It might inspire engineers to make double-use colours of our own," said Vignolini.
The research was supported in part by the European Research Council, the EPSRC, the BBSRC and the NSF.
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