Faculty Focus: Bruce McClure
Researcher's discovery sheds light on how plants control mating
Photo by Adam Masloski, Illumination Magazine
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 MU 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," says 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 a study published in 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," McClure says. "It means the conversation is a lot more intricate and two-sided than we had thought."
Pollen tubes (white) growing in a pistil. S-RNase (blue) and 120K (green) are secreted by pistil cells and taken up by the pollen tube. S-RNase and 120K end up in a pollen tube vacuole and the image shows how 120K surrounds the S-RNase. The pollinated pistil was fixed and stained with antibodies. The image was obtained with a confocal microscope and represents a single 300nm thick optical section and is about 70 um square. Photo courtesy of Ariel Goldraij, Mayandi Sivaguru, and Bruce McClure.
McClure, who received funding for the research from both the National Science Foundation and the MU-Monsanto Plant Biology Program, says 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.
McClure engages undergraduate biochemistry students 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," McClure says.