Sep. 12, 2001 ITHACA, N.Y. -- Familiarity breeds contempt. Nonfamiliarity produces seed.
Just as humans have a natural aversion toward marrying kin, some food crop plants have genes that allow them to avoid being fertilized by "self-related" pollen. Now Cornell University's biologists have solved one more piece of the puzzle of how plants' self-incompatibility works on the molecular level.
The discovery, as reported in the today's journal Science , could enable genetic engineers to short-circuit the reproduction process and more easily hybridize improved varieties of plants.
Many commercial crops are genetic hybrids. Obtaining seed to plant commercial quantities of these crops, such as tomatoes, for example, requires the labor-intensive work of manual crossing. Without the process of manual crossing, the plants would not have the desired qualities of hybrids. But nature has come up with an efficient system for making hybrid seed, which, when understood at the molecular level, can have applications on a commercial scale. This process, termed self-incompatibility, "prevents inbreeding and promotes out-crossing and variability in plants," says June Nasrallah, Cornell professor of plant biology, and the lead author on the Science paper.
In addition to Nasrallah, co-authors of "Allele-Specific Receptor-Ligand Interactions in Brassica Self-Incompatibility" include Mikhail Nasrallah, Cornell professor of plant biology; Aardra Kachroo, Cornell postdoctoral researcher in plant biology; and Christel R. Schopfer, a former Cornell postdoctoral researcher who now conducts research in Germany.
Funding for the research was provided by a four-year grant from the National Institutes of Health for the purpose of understanding cellular communication systems. The Nasrallah group examined the reproductive processes of Brassica plants. Like humans and animal species, plants use eggs and sperm in order to make seed and multiply. On the plant's pistil is the stigma, which is the site for capturing pollen. Pollen, which carries the male sperm, is released by stamens and is carried by wind or insects, and it is drawn to a plant's stigma.
If genetically unrelated (nonmatching) pollen lands on the stigma, the pollen germinates and produces a pollen tube that then runs through the plant's pistil and into the plant's ovaries. Fertilized eggs then develop into seed ready to be grown in a garden or a producer's field.
However, if "self-related" pollen lands on the stigma, the stigma's outer (epidermal) layer genetically recognizes the type of pollen and precipitates a self-incompatible reaction that inhibits the pollen tubes from growing. The Cornell group found that pollen recognition is based on highly specific lock-and-key interactions between receptors (the lock) on the stigma surface and ligands (the key) on the pollen surface. "If the pollen is matching kin, the receptor on the stigma is activated to prevent pollen tube growth," says Kachroo. "If the pollen is nonmatching, the receptor is not activated and pollen tubes can grow."
With this revelation, scientists are one step closer to understanding the reproductive barriers of flowering plants and their evolution. "The potential is to finally grasp -- at the molecular level -- which genes are needed for pollen rejection," says Mikhail Nasrallah. "The ability to silence, mutate and transfer the genes that control the self-incompatibility barrier could be a boon to breeders. Even self-fertilizing crops like tomatoes and rice can benefit from increased genetic variability."
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