Without eyes or ears, plants must rely on the interaction of molecules to determine appropriate mating partners and avoid inbreeding. In a new study, University of Missouri researchers have identified pollen proteins that may contribute to the signaling processes that determine if a plant accepts or rejects individual pollen grains for reproduction.
Like humans, the mating game isn’t always easy for plants. Plants rely on external factors such as wind and animals to bring them potential mates in the form of pollen grains. When pollen grains arrive, an introduction occurs through a “conversation” between the pollen (the male part of the flower) and the pistil (the female part of the flower). In this conversation, molecules take the place of words and allow the pollen to identify itself to the pistil. Listening in on this molecular conversation may provide ways to control the spread of transgenes from genetically-modified crops to wild relatives, offer better ways to control fertilization between cross species, and lead to a more efficient way of growing fruit trees.
“Unlike an animal’s visual cues about mate selection, a plant’s mate recognition takes place on a molecular level,” said Bruce McClure, associate director of the Christopher S. Bond Life Sciences Center and researcher in the MU Interdisciplinary Plant Group and Division of Biochemistry. “The pollen must, in some way, announce to the pistil its identity, and the pistil must interpret this identity. To do this, proteins from the pollen and proteins from the pistil interact; this determines the acceptance or rejection of individual pollen grains.”
In the study, researchers used two specific pistil proteins, NaTTS and 120K, as “bait” to see what pollen proteins would bind to them. These two pistil proteins were used because they directly influence the growth of pollen down the pistil to the ovary where fertilization takes place.
Three proteins, S-RNase-binding protein (SBP1), the protein NaPCCP and an enzyme, bound to the pistil proteins. This action suggests that these proteins likely contribute to the signaling processes that affect the success of pollen growth.
“Our experiment was like putting one side of a Velcro strip on two pistil proteins and then screening a collection of pollen proteins to see which of the pollen proteins have the complementary Velcro strip for binding,” McClure said. “If it sticks, it’s a good indication that the pollen proteins work with the pistil proteins to determine the success of reproduction.”
In previous studies, McClure showed that S-RNase, a protein on the pistil side, caused rejection of pollen from close relatives by acting as a cytotoxin, or a toxic substance, in the pollen tube.
For their study, the MU team used Nicotiana alata, a relative of tobacco commonly grown in home gardens as “flowering tobacco.” The study, “Pollen Proteins Bind to the C-Terminal Domain of Nicotiana Alata Pistil Arabinogalactan Proteins,” was published in the Journal of Biological Chemistry and was co-authored by McClure; Kirby N. Swatek, biochemistry graduate student; and Christopher B. Lee, post-doctoral researcher at the Bond Life Sciences Center.
Faculty from six of MU’s colleges and schools perform interdisciplinary research in the Christopher S. Bond Life Sciences Center with a vision to become a recognized world-wide center of scientific excellence and leadership in life sciences research, innovation and education. The Center integrates the strengths of multiple, often disparate, disciplines to promote discovery that boosts the production and quality of food, improves human and animal health and enhances environmental quality. The Center enriches the state of Missouri and its people by generating new businesses and jobs, fueling the economy through the creation and dissemination of new knowledge, and training young people to solve complex interdisciplinary problems.
Sagebrush exhibits communication only when air contact is allowed, says Rick Karban, shown here bagging sagebrush. When air contact is blocked with plastic bags there is no indication that communication has occurred.
-"To thine own self be true" may take on a new meaning-not with people or animal behavior but with plant behavior.
Plants engage in self-recognition and can communicate danger to their "clones" or genetically identical cuttings planted nearby, says professor Richard Karban of the Department of Entomology, University of California, Davis, in groundbreaking research reported in the current edition of Ecology Letters.
Karban and fellow scientist Kaori Shiojiri of the Center for Ecological Research, Kyoto University, Japan, observed that sagebrush responded to cues of self and non-self without physical contact.
The sagebrush communicated and cooperated with other branches of themselves to avoid being eaten by grasshoppers, Karban said. Eventhough the research is in its early stages, the researchers suspect that the plants warn their own kind of impending danger by emitting volatile cues. This may involve secreting chemicals that deter herbivores or make the plant less profitable for herbivores to eat, he said.
What this research means is that plants are "capable of more sophisticated behavior than we imagined," said Karban, who researches the interactions between herbivores (plant-eating organisms) and their host plants.
"Plants are capable of responding to complex cues that involve multiple stimuli," Karban said. "Plants not only respond to reliable cues in their environments but also produce cues that communicate with other plants and with other organisms, such as pollinators, seed disperses, herbivores and enemies of those herbivores".
In their UC Davis study, Karban and Shiojiri examined the relationships between the volatile profiles of clipped plants and herbivore damage They observed that plants within 60 centimeters of an experimentally clipped neighbor in the field experienced less leaf damage over the season, compared with plants near an unclipped neighbor. Plants with root contact between neighbors, but not air contact, failed to show this response.
"We explored self-recognition in the context of plant resistance to herbivory ," he said. "Previously we observed that sagebrush (Artemisa tridentata) became more resistant to herbivores after exposure to volatile cues from experimentally damaged neighbors".
The ecologists wrote that "naturally occurring herbivores caused similar responses as experimental clipping with scissors and active cues were released for up to three days following clipping. Choice and no-choice experiments indicated that herbivores responded to changes in plant characteristics and were not being repelled directly by airborne cues released by clipped individuals".
In earlier research, Karban observed that "volatile cues are mandatory for communication among branches within an individual sagebrush plant. This observation suggests that communication between individuals appears to be a by-product of a volatile communication system that allows plants to integrate their own systemic physiological processes".
The researchers made cuttings from 30 sagebrush plants at the UC Sagehen Creek Natural Reserve and then grew the cutting in plastic pots. They grew the cuttings at UC Davis and then placed the pots near the parent plant or near another different assay plant (control group) in the field.
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