Men have long used flowers to help them win a mate--but what do flowers use? Scientists still don't know, but University of Chicago researchers have found flowers that have lost part of it.
Male pollen granules, once they have settled upon the female pistil, grow pollen tubes toward the eggs deep within in a race to deliver their sperm for fertilization. The pollen tubes, which grow with near-perfect efficiency, avoiding eggs that have already interacted with other pollen tubes, probably respond to both attractive and repellent signals from the female parts of the flower.
Now researchers at the University of Chicago Medical Center have found a clue to a breakdown in communication between the sexes. They have identified mutations of a small flowering plant that leave the pollen tubes wandering aimlessly about the pistil, unable to find and fertilize an egg. The genetic change results in mutant plants that are incapable of fertilizing themselves, but which can still pollinate or be pollinated by normal plants -- a genetic event similar to what plant biologists think occurred over and over again in evolution, as plant species tended toward self-sterility to force outcrossing and maintain a wide genetic diversity.
The finding is reported in the November 29 issue of the journal Science.
"The property of these lost, wandering pollen tubes is very novel -- nobody has seen that before in plants where eggs are present," says Daphne Preuss, Ph.D., assistant professor of molecular genetics and cell biology, who led the research.
Self-sterility, or the inability of an individual plant to pollinate itself, has arisen frequently among the flowering plants by such means as the physical structure of the flower or the maturation of male and female parts of the same plant at different times. Biochemical self-incompatibility has also been identified and is a heritable trait.
"Most sterile mutants are either exclusively male or female, but ours is clearly a defect in male-female communication -- both parties are at fault," Preuss said.
The finding is of broad interest to biologists because the phenomena of cell adhesion and guidance are similar to the processes that occur in the outgrowth of axons from neurons during the development of the nervous system in animals. For plant biologists, the finding of these mutant strains represents a major step toward identifying elusive signaling molecules required for reproduction, and the self-sterile nature of the plants serves as a model for the evolution of self-incompatibility.
"This is similar to what occurs in nature," said Preuss. "Self-sterility is an evolutionary advance, since it broadens the gene pool."
Preuss and her co-author on the Science report, graduate student Laura Wilhelmi, found a mutant with alterations in two distinct genes by screening 80,000 specimens of the plant Arabidopsis thaliana, a small, inconspicuous weed that has become the "fruit-fly" of plant genetics for its ease of laboratory growth and genetic manipulation.
Understanding the genetics and molecular basis of fertilization would be useful for crop engineering, where breeders may desire sterility in one generation and fertility in the next. Arabidopsis, a member of the mustard family, is related to such important crop plants as cabbage, broccoli, cauliflower, rape seed, and bok choy.
The research reported in Science was funded by the National Science Foundation and the Department of Energy.