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Between the Pistil and the Pollen

A visit with Bruce McClure, Professor, Biochemistry

By Noelle Buhidar
Published: - Topics: biology biochemistry plants plant mating plant genetics

Solving intellectual puzzles is a rewarding activity for Bruce McClure, MU Professor of Biochemistry, as he seeks to unravel the mystery of plant mating. Similarly to all sexually reproducing organisms, plants steer clear of mating with close relatives, but how plants decide who is either too closely related or too distantly related proves a tricky question in science—a question whose eventual answer will affect millions. If McClure and his team of researchers can crack the code and understand how breeding barriers work within plants, they will be one step closer to his goal of “making the world better through agriculture.”

Fixing environmental problems through the study of biochemistry has always been of interest to McClure. Once an environmental biochemistry major, McClure vividly recalls his first post-college job with a plant geneticist on a project aimed at modifying the nutritional value of maize. It was in this position that he learned to apply genetic analysis and realized that he could make an impact on the world through plants.

The biggest challenge in working with plants turns out to be keeping them healthy and happy. “We have to maintain them for indefinite periods of time, “ explains McClure. “If they get diseased, or if the person doing your watering doesn’t show up, they die.” Often, his plants are irreplaceable, and their loss is a constant worry. “I have a sleeping bag in my office,” he points out, “because I spend the night on the floor if there’s a snowstorm or something.”

The primary question for McClure is determining how plants recognize their mates. If they are too closely related, inbreeding results; but if they are too distantly related, sterility problems occur. “There’s this window of genetic relationship for a good mating between individuals,” he explains. “We try to figure out how plants are able to ensure that a particular mating is somewhere in that window.” Some of these essential mating decisions are made through the molecular conversation between the pollen (the male part of a flower) and the pistil (the female part of a flower). If you think communication is difficult in human relationships, just think about how challenging it would be if you couldn’t use words. The molecular-level conversation a plant has with itself to avoid self-mating is complicated, and failed communication leads to the decreased health of a plant population, or “inbreeding depression.”

Whereas animals can explore their environment and use their sensory systems to locate an appropriate mate, one major restriction for plants is their inability to move. Instead, they rely on other forces—such as birds, bees, and wind—to move pollen from one plant to another. This dependence is a critical component in plant mating, because it limits the control individuals can exercise over mate selection. “They can have some control by choosing when the plant flowers,” explains McClure, “or by making a particular flower attractive to one kind of animal versus another animal, for example bees versus hummingbirds.” Many of the plants McClure studies are moth-pollinated, obtaining pollen only from other plants that are moth-pollinated, so investigators can control for this variable.

McClure’s work starts at that point in the mating process after the pollen has been transferred and is ready to be recognized by the plant. “All that cross-talk has to come at the biochemical level,” he says. “There’s pressure on the pollen to announce its species identity and genetic identity. There’s pressure on the female side to be able to interpret that information, and then basically decide whether or not to devote resources to that particular pollen.” If this pollen-pistil interaction is successful, then a pollen tube will be able to penetrate through the female structures to the ovules where fertilization occurs.

Choosing which plants to study requires much consideration. For a long time, the focus of McClure’s lab was on plants in the Nicotiana genus, relatives of tobacco. “The primary reason is that they have relatively large pistils,” he notes. “So for the first part of my career, we worked mostly on the female side. If you’re a biochemist, it’s very helpful to have a fairly large structure to work with.” At the present time, McClure is working with tomatoes to study inter-specific pollen recognition and rejection. The wild relatives of cultivated tomatoes are genetically similar to the cultivated ones, so McClure can make inter-specific hybrids that are fertile. His current favorite plant is the potato. “ It’s my new religion,” he jokes. “What have I been doing all my life? I should’ve been working with the potato.”

McClure’s research on potatoes could bring dramatic agricultural benefits to small farmers, especially since the potato is the third most important source of calories worldwide for human beings. “Plant breeding is responsible for huge improvements in production,” the biochemist explains. “But it can only improve within the range of species where you can already make these crosses.“ In other words, investigators must understand when they can make breeding crosses and when they can’t. For example, there are useful disease resistances in wild relatives of potatoes that McClure wants to move into the cultivated varieties. Drawing upon what he has learned about species recognition in potatoes, he can remove the breeding barrier to allow for the transfer of disease-resistant genes from wild to cultivated types. The transfer of these valuable traits could increase crop productivity and prevent a catastrophe—such as the Irish potato famine—from ever happening again.

As a basic research problem, McClure doesn’t see his study as having an end-point. “Our goal at the basic level is understanding, and I don’t think understanding is something that can ever be complete.” This doesn’t mean that certain milestones can’t be achieved. A distinct, well-defined question, such as what particular factor contributes to the ability of the two potato species to cross, will yield a specific answer at a specific time. But, he adds, “there will always be something you don’t understand,” and for that reason his search for the missing pieces of the puzzle will also continue.