Working with a decorative plant, the petunia, scientists at Cornell University have identified a gene that restores pollen production to sterile plants. The finding, reported the week of July 8 in Proceedings of the National Academy of Sciences , points the way to probable locations of similar restorer genes in approximately 150 other plant species with the so-called cytoplasmic male-sterility (CMS) defect and could facilitate crop-plant hybridization for increased yields.
The fertility restorer gene is located in the plant cell nuclei of certain petunia varieties and somehow prevents an abnormal gene in the cells' mitochondria from disrupting pollen production, says Maureen R. Hanson, the leader of a six-year effort to identify the gene and the Liberty Hyde Bailey Professor of Plant Molecular Biology at Cornell. Oilseed rape, cauliflower, sunflower and rice are among the food plants known to have similar naturally occurring restorer genes, and knowing the general location in one plant genome should help pinpoint it in others, Hanson says.
Identification of a crop plant's own restorer gene will help plant breeders transfer the gene more quickly to advanced breeding lines, either by traditional sexual crosses or by using genetic engineering techniques. Once the restorer gene is incorporated into a breeding line, the plants can be used in hybrid seed production.
No other plant gene that can turn off the expression of a defective mitochondrial gene has previously been discovered. In addition to Hanson, the finding was reported in PNAS by the research team of Stephane Bentolila and Antonio A. Alfonso, postdoctoral research associate and graduate student, respectively, in Cornell's Department of Molecular Biology and Genetics.
Cytoplasmic male sterility is a valuable trait that is used in a special strategy by seed producers, lead author Bentolila explains. First, a male-sterile plant variety, lacking any pollen, is grown in fields near a plant variety that contains fertility restorer genes and makes pollen. All the seeds collected from the male-sterile plants must be hybrid, resulting from cross pollination. Plants arising from hybrid seed are more vigorous and productive than inbred genetically uniform seed that arises when plants self-pollinate.
For example, hybrid rices have been bred in China using natural fertility restorer and male-sterile lines, and the result has been as much as a 30 percent higher yield compared with inbreds in some areas. Hybrid rice breeding efforts are under way at many rice research centers, including the Philippine Rice Research Institute (PhilRice), where team member Alfonso will return after completing his Ph.D. at Cornell.
To map the petunia gene, DNA from nearly 1,000 petunia plants was extracted and analyzed. Then using special enzymes, petunia DNA was sliced into many pieces, which were put into 86,000 different bacterial colonies, each containing a different piece of petunia DNA. DNA isolated from one of the bacterial colonies was found to carry a marker near the fertility restorer gene. When that DNA was transferred into cells from a male-sterile plant, using tissue culture, all plants arising from the transformed cells were able to produce pollen – proving that the Cornell researchers had identified the fertility restorer gene.
The study was funded by the U.S. Department of Agriculture's National Research Initiative and by a Rockefeller Foundation fellowship to Alfonso.
But identifying and locating the fertility restorer gene doesn't explain everything, Hanson adds. "We still don't understand the molecular mechanism of action of the petunia restorer gene and how it turns off expression of the abnormal mitochondrial gene. And we also need to find out why a form of the gene present in nonrestoring lines does not allow normal pollen development in the presence of the male-sterility cytoplasm."