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Plants engineer their petals to be visually attractive to bees

Flowering plants produce brightly colored petals in order to attract pollinators. It is essential that plants attract these insects so that sexual reproduction can take place and genetic variation of plant offspring can be ensured. However, there is a lot of competition for the attention of pollinators and a new study from the University of Cambridge suggests that the bright pigments and patterns are not the only means by which flowers may lure the insects closer. 

It seems that there are microscopic, three-dimensional folds, or ridges, that develop on the surface of some petals, and that these parallel striations reflect particular wavelengths of light to produce an iridescent optical effect that is not always visible to human eyes, but is visible to bees. The striations involve the buckling of the cuticle, which is a thin, protective layer on the surface of young, growing petals. 

Research led by Professor Beverley Glover’s team at Cambridge’s Department of Plant Sciences suggested that these semi-ordered ridges act as diffraction gratings that reflect different wavelengths of light to create a weak, iridescent blue-halo effect in the blue–UV spectrum that bumblebees can see. However, it was not known how the striations form, or why they form only on certain flowers, or on certain areas of the petals. More research was needed in order to answer these questions

Edwige Moyroud, who started this research in Professor Glover’s lab and is now leading her own research group at the Sainsbury Laboratory, has developed the Australian native hibiscus, known as Venice mallow (Hibiscus trionum), as a new model species to try to understand how and when these nanostructures develop.

The researchers noticed that the striation patterns first appear when cuticle cells elongate, which suggested that growth was an important factor in the formation of these structures. The experts then measured physical parameters related to growth, such as cell expansion and cuticle thickness, to see whether these variables could adequately predict the patterns observed. They could not. Some other factor was clearly involved, and this required further investigation.

The researchers then developed a new method from the chemistry field to analyze the molecular composition of the cuticle at specific points across petals. They found that petals with different textures – striated or smooth – also had cuticles with different chemical compositions. In particular, striated cuticle had high levels of dihydroxy-palmitic acid and waxes, and low levels of phenolic compounds, when compare to smooth cuticle. 

“Our initial model predicted that how much cells grow and how much cuticle those cells make were key factors controlling the formation of striations, but when we started to test the model using experimental work in Venice mallow we found out that their formation is also highly dependent on cuticle chemistry, which affects how the cuticle responds to the forces that cause buckling,” said Dr. Moyroud.

To validate their findings, the researchers used transgenic techniques in Hibiscus to alter cuticle chemistry directly in the plants, using genes similar to those known to control the production of cuticle molecules in a different model plant, Arabidopsis. The results, published in the journal Current Biology, showed that cuticle texture can be modified, without changing cell growth, simply by modifying cuticle composition.

Thus, it seems, plants can regulate and change the chemistry of their petal surface to create patches of striations that produce iridescent signals visible to bees. The researchers think that changes in cuticle chemistry affect the mechanical properties of the cuticle, allowing the development of microscopic ridges in places.

“The next question we want to explore is how different chemistries can change the mechanical properties of the cuticle, as a nanostructure-building material,” said Dr. Moyroud. “It may be that different chemical compositions result in a cuticle with differing architecture or with different stiffness and hence different ways of reacting to the forces experienced by cells as the petal grows.” 

This project revealed that there is a combination of processes that work together and allow plants to shape their surfaces. “Plants are formidable chemists and these results illustrate how they can precisely tune the chemistry of their cuticle to produce different textures across their petals,” said Dr. Moyroud. “Patterns formed at the microscopic scale can fulfil a range of functions, from communication with pollinators to defense against herbivores or pathogens. They are striking examples of evolutionary diversification and by combining experiments and computational modelling we are starting to understand a little bit better how plants can fabricate them.”

“These insights are also useful for biodiversity and conservation work because they help to explain how plants interact with their environment,” said Professor Glover, who is also director of the Cambridge University Botanic Garden, in which the researchers first noticed the iridescent flowers of Venice mallow. 

“For example, species that are closely related but that grow in different geographic regions can have very different petal patterns. Understanding why petal pattering varies and how this might affect the relationship between the plants and their pollinators could help to better inform policies in future management of environmental systems and conservation of biodiversity.” 

By Alison Bosman, Staff Writer

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