Pennsylvania Weed Reveals Plant-Pollination Secrets

Steve Travers, a postdoctoral scholar in the research group headed by Andy Stephenson, professor of biology, uses a modified electric toothbrush to harvest pollen from flowers growing in the Buckhout Greenhouses on the University Park campus for the group's newest research project.

Photos: Greg Grieco, Penn State

The research group of Andy Stephenson, professor of biology, recently has begun studying the horse-nettle plant, a major weed in agricultural fields in Pennsylvania and other states, which is in the same "Solanaceae" family as tomato, potato, eggplant, tobacco, and petunia plants. "Horse nettle is a particularly damaging weed because any pesticides that kill the weed also kill the economically important plants in the Solanaceae family," Stephenson says.

Until the Stephenson group began its research, the horse nettle was thought to be unable to fertilize itself--an unusual trait for a weed. "We have found evidence that after 20 to 40 flowers on a horse-nettle plant have failed to set fruit because there is no pollen around from other plants of its species, the plant does become partially able to pollinate itself," Stephenson says. "The plant's first several clusters of flowers appear to strongly reject that plant's own pollen; however, if those flowers do not produce any fruits the plant will begin to make just a few fruits by self-pollination that have a relatively small number of seeds. If there continues to be no 'cross' pollen around from other horse-nettle plants, more and more fruits will form with more and more seeds as a result of self-pollination."

The group uses an electric toothbrush--modified by replacing the brush with a flower-support loop--to remove pollen from the flowers and collect it in miniature test tubes made from half a pharmaceutical gel cap. "The electric toothbrush vibrates at the same frequency as the wings of a bee that is shaking pollen out of these flowers," Stephenson explains. The researchers are selectively pollinating their 30 sets of horse-nettle plants--each group genetically identical to one of the 30 wild parent plants they collected from fields near Penn State--by dipping the female parts of the flowers into the miniature test tubes to coat them with a particular plant's pollen.

A plant that cannot fertilize itself recognizes its own pollen cells as "incompatible" and uses biochemical means to prevent them from growing--thereby preventing the plant from making seeds containing the pollen's genes. "We are studying how plants naturally control inbreeding by looking at the molecular, genetic, population, and environmental factors that affect the growth of the pollen tube during the fertilization process," Stephenson says. "The horse nettle is a particularly useful species for these studies."

Another aspect of the plant's allure for Stephenson is that the lab of Penn State's Teh-Hui Kao, professor of biochemistry and molecular biology, is one of the worldwide leaders in fertilization molecular biology in the entire family of plants that includes the horse nettle. "By making use of the molecular biology that Teh-Hui Kao already has worked out, we now have more time and resources to focus on the ecological and evolutionary questions we really want to answer concerning mating and breeding systems in these plants," Stephenson explains.

"We hope to answer such questions as what are the consequences of inbreeding, what effects does the environment have on the breakdown of self-incompatibility, what can we do to overcome those environmental factors, and what are the genes that modify the strength of incompatibility," Stephenson says. "Mostly I'm interested in how plants regulate which pollen grains actually achieve fertilization and what impact does that selection process have for the genetics and the evolution of a plant species."

"One important reason to study this plant is to learn how to control it, especially since it is becoming an increasingly prevalent pest in Pennsylvania." Stephenson says he also hopes his team's research with the horse nettle will help to reveal how to control other invasive species of native and non-native plants, and will develop new knowledge about the basic biology of the interactions between plant cells during fertilization that could benefit breeders of hybrid crops.