Gene editing technique gives beneficial properties

North Carolina State University researchers have successfully transferred a critical gene from one compartment of a plant cell to another to produce tobacco plants without pollen and viable seeds. Their findings could lead to better ways to produce hybrid seeds to increase crop productivity or to introduce seedlessness into fruit varieties that often lack the desired trait, such as raspberries, blackberries or muscatine grapes.

Anthers of normal tobacco plants (left) produce and shed copious amounts of pollen; Plants with a mitochondrial gene deletion (right) have malformed stamens that produce pollen. Photo courtesy of Ralph Dewey, NC State University.

The researchers began work on mitochondria, the energy-producing part of a cell. In plants, mutations in mitochondrial genes can be associated with an inability to produce pollen, known as cytoplasmic male sterility (CMS), which has been successfully used to produce high-yielding hybrid seeds in many important crops. However, CMS-based systems robust enough to facilitate commercial-scale hybrid seed production will naturally occur.

In their proof-of-concept study, NC State researchers, along with colleagues from Precision Biosciences and Elo Life Systems, employed a unique strategy to test whether the CMS trait could be developed in tobacco, a model species commonly used in plant research. The researchers initially took an essential mitochondrial gene called atp1 and moved it into the nucleus after placing it under the regulatory control of an element — known as a promoter — that they predicted would allow the altered atp1 gene to be expressed in every cell of the plant. Except those responsible for producing pollen. The researchers used gene editing tools to permanently remove the native atp1 gene from the mitochondria.

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Their approach was successful.

„The results exceeded our expectations,” said Philip Morris, NC State professor of crop science and Ralph Dewey, corresponding author of a paper describing the research. „The plants looked completely normal until they began to flower, but then failed to produce pollen because the mutated atp1 gene was not expressed. Importantly, because the original atp1 gene was deleted from the mitochondrial genome, this trait would be maternally inherited, an important consideration for large-scale hybrid seed production.

Pollen is not the only casualty of this technique. When cross-fertilized using pollen from a neighboring normal plant, their tobacco plants unexpectedly produced small, empty seeds, similar to those found in popular „seedless” fruits such as watermelons and grapes.

Normal tobacco seeds (left) are dense and germinated; Seeds of plants with a mitochondrial gene deletion (right) are often hollow and do not germinate. Photo courtesy of Ralph Dewey, NC State University.

„That’s because the promoter we chose fails to express not only during anther development, but also during early seed development,” Dewey said.

Dewey said his team is now working to disentangle these results so that researchers can simultaneously achieve pollen sterility, or the seedless trait.

Dewey also emphasized that the findings should not be limited to tobacco plants. Their next-generation experiments include testing the seedless trait in the tomato, a close relative of tobacco. They will test their novel CMS trait in a cereal such as rice to test the effectiveness of their system in a crop where hybrid seed production is critical for maximum yield.

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„Knowing how the system works, there is no reason to believe that the technology could not be effectively transferred to other plant species,” he said.

This study appears in Frontiers in Plant Science. Devarshi Chelot, H. Carol Griffin, Alison N. Dickey, Derek Jantz, J. Jeff Smith, Anna Mathiadis, Josh Streble, Caitlin Kestel, and William A. Smith co-authored the article. This work was funded in part by a grant from Elo Life Systems and supported by the NC State Plant Breeding Consortium. Dewey and several other researchers have filed a patent for the new technique.


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