Source: National Science Foundation

New data suggest that molecular communication between the plant sexes--specifically the pollen of males and pistils of females--is more complicated than originally thought. Plants, like animals, avoid inbreeding to maximize genetic diversity and the associated chances for survival. For decades, scientists have sought to fully understand the plant's molecular system for recognizing and rejecting "self" so that inbreeding does not occur.

Now, Bruce McClure at the University of Missouri-Columbia (UMC), together with his colleagues, report in the Feb. 16 issue of the journal Nature that plant "self" recognition systems involve multiple players and lots of male-female "conversation," at least at the molecular level.

For successful reproductions to occur in plants, the pollen must make its way to the plant's female parts. That is, it germinates and grows within the pistil in order to reach the ovule. In one system plants use to prevent inbreeding, the pistil literally poisons the pollen en route to the ovule using a toxin known as S-RNase. Until now, the specifics of this self-incompatibility system perplexed scientists.

McClure and his colleagues showed that after the pistil injects S-RNase into the pollen, the toxin is whisked away to a holding compartment where it can do no harm until the "self" or "non-self" decision is made. McClure's work also suggests that at least three other proteins may be involved in this decision-making process.

McClure said, "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's group is now determining the molecular information responsible for the sequestering and release of S-RNase.

McClure also engages UMC freshman biochemistry laboratories in studying this plant self-recognition system to learn and perform advanced molecular biology. "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," said McClure.

McClure first showed that RNases were involved in controlling plant mating some 17 years ago. In 1994, he and Teh-hui Kao at Pennsylvania State University, both supported by the National Science Foundation (NSF), independently determined the toxin's function. Recently, Kao's lab made another key advance by showing that a pollen protein called SLF helps pollen recognize S-RNase.

Susan Lolle, the NSF program manager for McClure's current research said, "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 great example of how stepwise advances in fundamental knowledge lead to our greater understanding of a complex system."

Missouri University investigators' discovery sheds light on how plants control mating

Source: University of Missouri-Columbia
By: Melody Kroll

Like animals, most plants avoid mating with close relatives. But, how plants decide who is a relative has been a mystery to science. New research from Missouri University investigators suggests that this decision-making process is more intricate than previously considered.

In plants, some of the important mating choices are made through an intimate "conversation" between the pollen (the male) and the pistil (the female part of a flower). The conversation is carried on with molecules instead of words. One molecular-level conversation scientists have been eavesdropping on for a long time is the one a plant may have with itself to avoid self-mating, or inbreeding. How this conversation occurs is now turning out to be quite complex.

"We've known that 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 plant.in other words, who to mate with and who to reject," said Bruce McClure, Associate Director of the Christopher S. Bond Life Sciences Center and the lead investigator for the research. "We used to think these two proteins interacted pretty directly."

In this week's Nature, McClure and fellow MU investigators show that S-RNase is taken up into a compartment inside a growing pollen tube.

"That S-RNase is sequestered in a compartment — away from the SLF protein — really changes how we think about this interaction," continued McClure. "It means the conversation is a lot more intricate and two-sided than we had thought."

McClure, who received funding for the research from both the National Science Foundation and the MU-Monsanto Plant Biology Program, said the finding is important for a basic understanding of plant biology, but may also offer insight into other issues, like the spread of transgenes from genetically modified crops to wild relatives.

In studies in 1989 and 1990 (also in Nature), McClure showed that S-RNase causes rejection of pollen from close relatives by acting as a cytotoxin (toxic substance) inside the cytoplasm of the growing pollen tube. What scientists know now is that pollen keeps S-RNase safely stored in a bag-like compartment where it cannot cause damage unless it is released. The molecular conversation between S-RNase and SLF controls this release. With this new finding, scientists now envision the pollen-pistil conversation as involving a whole new set of interactions.

"It's as much where the molecules are as what they are," McClure said. "We used to think that the important molecular decision made between mating partners was whether or not to degrade S-RNase. We now know the important decision is whether or not to release the S-RNase from this compartment. This takes us in a whole new direction of research."

For their studies, the MU team used Nicotiana alata, a relative of tobacco commonly grown in home gardens as "flowering tobacco." The advanced microscope facilities in the Molecular Cytology Core at the Bond Life Sciences Center were critical for discovering the S-RNase compartment.

"The combination of a great team and great facilities made this possible," said McClure. "We had team members from Argentina, Japan, India, Mexico, and the United States. It's an incredible collaborative effort."

Among the team members are an MU graduate student in the Division of Biological Sciences, Christopher Lee, and three MU investigators, Katsuhiko Kondo, Mayandi Sivaguru, and Thomas E. Phillips. Nathan Hancock, an MU alumnus, also participated in the research, as did Ariel Goldraij, from the National University of Cordoba, Argentina, and Sonia Vasquez-Santana and Felipe Cruz-Garcia, from the National University of Mexico.