How petunias know when to start smelling sweet

"Plants emit these scents when they want to attract their pollinators," says Takato Imaizumi. "It makes sense that they should time this with when the pollinators will be around." (Credit: Andrea_44/Flickr)

Researchers have identified a key mechanism plants use to decide when to release their floral scents to attract pollinators.

The findings connect the production and release of these fragrant chemicals to the innate circadian rhythms that pulse through all life on Earth.

The researchers turned to the common garden petunia, a white-flowered hybrid that releases an aromatic, sweet-smelling fragrance in the evening to attract insect pollinators, such as hawk moths.

“Plants emit these scents when they want to attract their pollinators,” says Takato Imaizumi, associate professor of biology at University of Washington. “It makes sense that they should time this with when the pollinators will be around.”

The researchers discovered a major gene that controls when the petunia releases its fragrance. The gene—known by its acronym LHY—is found in many plant species and is a key component of the plant “circadian clock.”

Biologists have long recognized that creatures like plants, humans, and even tiny bacteria all have circadian clocks—genes that keep cells synchronized to the 24-hour cycle of life on Earth and regulate cellular activities based on the time of day.

Opposite timing

Researchers had previously shown that LHY is a component of the circadian clock in other flowering plants, but the new paper, published in the Proceedings of the National Academy of Sciences, marks the first time biologists have connected LHY activity to flower scent.

“Now we’re finding out what the bridge is between the circadian clock and scent production and release,” says Myles Fenske, a doctoral student in biology.

Since no one had ever studied the LHY gene in petunias, the researchers gathered basic information about LHY to show that it has the same circadian functions as it does in other plant species.

Many circadian clock genes are only active at specific times of the day, when they influence the activity of other genes that control what cells are doing. The petunia LHY gene is most active in the morning, at the opposite time of day when the flower releases its fragrant evening scent.

A burst of morning activity

The scientists hypothesized that LHY’s morning activity might repress the production of scented chemicals. When they prolonged LHY’s activity into the evening, the petunias didn’t release their fragrant chemicals at all.

“That was perfect,” Imaizumi says. “It is exactly what I would hope to see.”

If LHY’s activity truly did have a negative effect on scent production, then petunia plants that lacked the LHY gene’s burst of morning activity might produce and release their scents earlier in the day.

When the researchers created petunia plants with reduced LHY activity, those plants produced and released fragrant chemicals four to eight hours earlier in the day.

The team even discovered how LHY represses floral scent production. It interferes with the activity of ODO1, another petunia gene that promotes the production and release of floral scents.

‘Change the cues’

By repressing ODO1 activity early in the day, LHY stops the floral scent assembly line in its tracks. When the LHY gene becomes less active later in the day, ODO1 is able to ramp up production of the fragrant chemicals just in time for the evening aromatic release.

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Since genes like LHY and ODO1 are present in most—if not all—flowering  plants, Imaizumi and his team believe that the interactions between these two genes may be a common mechanism for a plant’s circadian clock to influence or control the production of fragrant floral scents.

If so, then changes to the strength or timing of the LHY-ODO1 bridge may explain how flowers change the timing of scent production as they evolve.

The researchers are now testing if pollinators have a preference between normal garden petunias or petunias with altered LHY activity. In time, these experiments may pave the way for scientists to improve the pollination efficiency of other plants, including important crop species.

“We think you can really change a plant’s success by changing these cues,” Imaizumi says.

The National Institutes of Health and the National Science Foundation funded the work.

Source: University of Washington