Appropriate Pollens | Illumination

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Appropriate Pollens

Appropriate Pollens

Because genetic diversity gives species a competitive edge, most living things seek to avoid inbreeding. For plants, this means distinguishing between incestuous pollens and those of more eligible breeding partners. It sounds simple enough. But, in fact, the question of how plants actually make such distinctions has long vexed biologists.

Now, thanks in part to the work of Bruce McClure, associate director of the Christopher S. Bond Life Sciences Center and an associate professor of biochemistry at MU, scientists are coming closer to an answer. McClure describes his contribution as a form of "eavesdropping" on plants' most intimate, molecular-level conversations. The strategy is yielding results. In the Feb. 16 issue of the British journal Nature, and as reported on the National Science Foundation web site in March, McClure demonstrated that the complex system of "molecular recognition" plants use to evaluate and finally weed out inappropriate pollen is more intimate than anyone realized.

For a plant to successfully reproduce, McClure explains, the "male" pollen must germinate and grow within the "female" pistil before it reaches the plant's ovule. Previous research has shown that many plants literally poison inbred pollen en route to the ovule using a toxin known as S-RNase. But until now the specifics of this self-incompatibility system were a mystery. "We've known there is a molecular conversation going on between S-RNase, a protein on the pistil side, and SLF, a protein on the pollen side, and that the result of this conversation is a decision about whether or not the pollen will be allowed to fertilize the ovule. In other words, who to mate with and who to reject," McClure says.

McClure and his colleagues, working at an advanced microscopy facility in the Molecular Cytology Core of the Life Sciences Center, used Nicotiana alata, a garden plant sometimes called "flowering tobacco," to illuminate a crucial, previously undescribed step in the process. After the pistil injects S-RNase into the pollen, McClure and his team found the toxin was whisked away into a sort of holding compartment where a complement of proteins made the "self" or "non-self" decision. If the pollen was deemed acceptable, the toxin was held safely in the compartment and the fertilization process moved forward.

"What's really new here is the finding that pollen protects itself from the toxin in a different way than we previously thought, and we're starting to understand how these other proteins work together with S-RNase," McClure says. "The combination of a great team and great facilities made this possible. We had team members from Argentina, Japan, India, Mexico and the United States. It was an incredible collaborative effort."

His research group is now determining the molecular information responsible for sequestering and releasing S-RNase. "This latest development in the pollen-pistil story is not only significant in its own right as we strive to understand the intricacies of plant breeding, it's also a superb example of how stepwise advances in fundamental knowledge lead to our greater understanding of a complex system," says Susan Lolle, the NSF program manager for McClure's research, in a statement from the agency.

For his part, McClure says he's not content to simply share this "greater understanding" among his scientific peers. He also believes it is an excellent way to more fully engage undergraduates. "It's a great system for helping students see how we connect genetics and biochemistry and use them to build an understanding of how living things work," he says.

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Published by the Office of Research.