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Shedding light on abscission

Study indicates that knowing how plants drop leaves, flower petals and fruit can provide better products.

By Jeff Sossamon W

hen parts of a plant, such as dead leaves, flowers or ripe fruit detach, the process is

called abscission. A new study from the University of Missouri sheds light on the process that governs how and when plants shed their parts.

Knowing how the process works will help increase understanding of both plant development and responses to environmental signals—such as drought and pest infection—while allowing scientists to control the process for flower, fruit and vegetable industries. The earliest steps of abscission involve changes in a special layer of cells, called the abscission zone, located at the base of the flower. As a flower matures, cells in this layer begin to separate from one another along the entire zone, creating a clean rift between the base of the flower and the petals. As the rift enlarges, the petals fall off and are sent tumbling to the ground.

“Scientists have long wondered how a plant regulates this cell separation process, in particular the molecular mechanism that both triggers and powers the process,” said O. Rahul Patharkar, a post-doctoral fellow in the Division of Biological Sciences and lead author of the study.

“We know that when a plant is close to dropping its petals, many genes are activated. A lot of this gene activity, or transcription, is exponentially increased in a relatively short time, ultimately leading to abscission.”

One gene that gets a boost in its activity is called HAESA, a gene required for floral abscission to occur. Previous studies have shown that activity of this gene increases by a

18 British Columbia Berry Grower • Summer 2015

magnitude of 27-fold from the time the flower bud opens to when it drops its petals, a period of about two days. Patharkar’s new research identified two important connections in the mechanisms that explain this rapid increase in HAESA gene expression. The research team found that plants that overexpress a certain regulator protein do not activate HAESA and do not drop their flower petals. The findings suggest that the protein found is a negative regulator of HAESA, meaning it prevents expression of the gene. Additionally, the protein also acts as a molecular “switch” responsible for turning the process on and off and it is this “positive feedback loop” that is important in the abscission process.

“A good analogy for the positive

Study team leader O. Rahul Patharkar.

feedback loop is that it’s the turbocharger for the process of abscission,” Patharkar said. “Basically, it amplifies the power of abscission causing the plant to drop its leaves or flowers.”

“These findings are a ‘tour de force’ in abscission research,” said John C. Walker, Professor of Biological Sciences in the College of Arts and Science and corresponding author of the publication. “The study puts together a number of different genes and proteins into a new model that helps explain how plants precisely control floral organ abscission. Eventually, the findings will provide researchers with new methods of controlling the process that could help the fruit and flower industries that want their products to stay in place until ready for harvest.”

Findings are a ‘tour de force,’ says study’s corresponding author, John Walker.

The study was published in the Proceedings of the National Academy of Sciences, with support from the National Science Foundation.

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