Key to scent of flowers
The tight coupling between pollination and decreased scent emission makes sense from an evolutionary point of view.
A RECENT Purdue University study has uncovered the processes responsible for shutting down scent production in certain flowers once they've been pollinated — a finding that may help the horticulture industry enhance floral scent.
Natalia Dudareva, associate professor of horticulture, and her colleagues have recently identified the molecular mechanisms that cause petunias and snapdragons to decrease scent production after they've been visited by pollinators such as bees or moths.
The researchers also proved that fertilization, the reproductive process that follows pollination, triggers a decline in scent production. In addition, their research has identified a new role for the plant hormone ethylene. The study will appear in the journal The Plant Cell and is published in The Plant Cell Preview.
``This study gives us a better understanding of how plants regulate floral scent production and how to improve floral scent in unscented flowers.'' Over years of breeding for characteristics such as longevity, colour and flower size, many commercially produced flowers have lost their scent.
``It makes sense. To increase shelf life, a flower needs to save energy, and maybe the trade-off was that these flowers don't expend energy on producing scent anymore,'' Dudareva said.
To her surprise, she found that while petunias and snapdragons rely on some of the same compounds and processes to produce scent, these flowers regulate their post-pollination scent production in different ways at the molecular level.
In all flowers, a variety of substances known as volatile compounds contribute to floral scent, Dudareva said.
A volatile compound called methylbenzoate is one of the most abundant scent compounds in many flowers, including petunias and snapdragons. Dudareva previously showed that both produce methylbenzoate through a process called methylation.
During methylation, an enzyme adds a small molecular unit, called a methyl group, to a compound called benzoic acid, found in the petals. Two different, but related, enzymes called BAMT in snapdragons and BSMT in petunias are responsible for the methylation reaction that produces methylbenzoate and a bouquet's fragrance.
In the present study, Dudareva and her colleagues found that in petunias the plant hormone ethylene, which is produced after pollination, suppresses activity of the gene that triggers the creation of BSMT.
Without BSMT, the flower cannot produce the methylbenzoate responsible for its scent.
Ethylene plays a role in many plant development processes, including fruit maturation, leaf drop and various stress responses, but has not previously been shown to play a role in regulating scent.
Snapdragons, Dudareva found, are somewhat sensitive to ethylene, but not to the extent of petunias.
While ethylene essentially shuts down scent emission in petunias after pollination, the hormone does not elicit this effect in snapdragons.
Instead, scent emission in snapdragons is regulated by a change in the ratio of two compounds produced in snapdragon flowers, Dudareva said. One of these compounds, called SAM, donates the methyl groups used in the methylbenzoate-producing reaction.
The other compound, called SAH, is produced as a result of the methylation reaction. Taken together, the relative amounts of these two compounds are called the `methylation index'.
``What we found very interesting is that fertilization, not just pollination, gives a signal to down regulate floral scent,'' she said. ``For two days after pollination, scent does not go down in snapdragons.''
This tight coupling between pollination and decreased scent emission makes sense from an evolutionary point of view, she said.
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