The molecule responsible for the pattern not only helps attract pollinators but also protects plants from drought
Sunflowers are loved for their size and colourful, irresistible flowers, and it is widely recognized that the bright yellow petals attract pollinators. But within the densely packed arrangement of the petals is an ultraviolet bull’s-eye pattern that, while invisible to humans, is highly visible to some insects and especially bees, drawing them in for their pollination services.
Now, scientists at the University of British Columbia have discovered that the same molecules that create the UV patterns also help the plant respond to the stresses of drought and extreme temperatures. Their discovery came as a surprise as they sought to understand plant adaptation.
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“One of the main goals of our group is to understand how plants adapt to different environments,” said Marco Todesco, research associate at the Biodiversity Research Centre in the university’s department of botany.
“We look at which characteristics differentiate populations of a same species that live in different environments, how these characteristics help them survive, and what are the genes that control these differences.”
He said that sunflowers are a perfect model for this study. As a native North American plant that includes about 50 species, sunflowers grow across the continent in extreme environments including deserts, salt marshes and sand dunes.
In addition, wild sunflowers are closely related to cultivated sunflowers, a major oil crop, so the more that is learned about wild sunflower survival in extreme conditions, the more valuable that knowledge will be in cultivating farmed sunflower varieties to become more resilient.
“A few years ago (from 2016 to 2019), we grew a large population (more than 1,500 plants) from seeds collected across North America, to describe the differences between these plants and to identify the genes responsible for them,” said Todesco. “One of the many characteristics we looked at was ultraviolet floral patterns. These had been described before in sunflowers and other plants and have been known to be important in attracting pollinators. What we didn’t expect was to find such an extreme amount of diversity in the size of these UV patterns from petals being completely UV-absorbing to petals being almost completely UV reflecting. And this set in motion the rest of the study.”
Todesco said that the sunflower “flower” is actually an inflorescence made up of dozens to hundreds of individual flowers, each of which makes one seed. The flowers in the outer whorl have modified enlarged petals technically called ligules.
Their study focused on the wild sunflower Helianthus annuus, which is more widespread and the one most closely related to the cultivated sunflower. They found that genetics plays an important role and is the main determinant of the size of the UV bull’s-eye patterns.
“That said, there is some indication that environmental factors could play a role as well but, at the moment, we are not sure how large a role this is, if any,” he said. “There is a lot of variation for UV bull’s-eye size within different populations of H. annuus, but there is also a lot of variation in UV size between different species of wildflowers.”
During the study, they noticed that sunflowers growing in drier climates had flowers with larger UV bull’s-eyes and they found that these flowers were able to retain water more efficiently, suggesting that the larger UV bull’s-eyes helped plants adapt to the drier environments.
The research team found that a single gene, HaMYB111, was responsible for most of the diversity in floral UV patterns. The gene controls the production of UV-absorbing flavonol compounds, which are known to help plants survive under environmental stresses.
“Flavonols are part of a larger family of chemical compounds produced by plants, the flavonoids,” he said. “These include a lot of the pigments in flowers and fruits and are produced by the plant for a host of reasons (such as) to protect themselves from cold, heat, radiation and also from herbivores and pathogens. The flavonoids that are colourful are also important for the plant to interact with animals, for example to advertise their flowers and fruits.”
The researchers found larger floral UV patterns that have more of these compounds could help reduce the amount of evaporation from a sunflower in environments with lower humidity, preventing excessive water loss. In humid, hot environments, smaller UV patterns would promote evaporation, keeping plants cool and preventing overheating.
He said that petals with larger UV patterns (with more flavonols) lose water at a slower rate than petals with small UV patterns (less flavonols).
“It is possible that as climates get drier, sunflower with larger UV patterns will have an advantage. There is a study from another group that looked at UV patterns in herbarium specimens for a number of different species, and found that, on average, the size of UV patterns has increased over the past 80 years. They suggest this could be due to increased UV radiation due to reductions in the ozone layer, but, given our results, it could also be linked to the fact that climates got drier. These are of course all speculations, and more research would be needed to confirm such a link.”
He said that while larger UV patterns and other traits might help plants cope with drier environments, there is a limit to what adaptation can do in the face of rapid climate change.
The research has shown that, while invisible UV patterns have not been taken into consideration when breeding sunflower lines in the past, they clearly have an impact on important traits.
Todesco said there are three priorities for further research. First, they plan to characterize in more detail the effects of different UV bull’s-eyes on plant fitness, especially in cultivated sunflowers. He wants to evaluate how much of an effect different sizes of UV bull’s-eyes have in terms of how much a plant produces and how well it can withstand drought or other stresses. Secondly, they plan to better understand how the molecular and genetic mechanisms of HaMYB111 regulate the UV patterns and how flavonols affect water retention. Thirdly, they want to identify what genes are responsible for variation in UV bull’s-eyes in other sunflower species. Is it the same gene already discovered or are other genes involved? They will also explore whether UV patterns replicate the dual role in other sunflower species, or whether they have developed different roles.
The study was published recently in the journal eLife.
