Most cooks know this much about baking: Pastry flour is best for tender treats like cake, but only bread flour can make a loaf that sticks to the ribs. Geneticists have suspected that two wheat genes help determine these variations in flour. Now they've slipped those genes into rice plants and produced hard evidence that these genes influence key grain traits. The discovery may offer grain breeders new tools to expand the usefulness of their crops.
A crucial feature of any wheat strain is the texture of its endosperm, the starchy flesh that constitutes the bulk of the grain. It takes more strength to mill hard-grained wheats, and their flours are loaded with larger particles and more broken starch granules than flour from soft-grained wheat. For nearly 3 decades scientists have been searching for the genetic basis of this variation. A clue to the mystery was discovered 3 years ago when a team led by geneticist Michael Giroux of Montana State University in Bozeman found that wheat hardness was influenced by mutations in the genes that code for two proteins called puroindolines.
To cinch the case, Giroux and postdoctoral researcher Konduru Krishnamurthy decided to insert the genes into rice, a crop free of puroindolines. After hitching the wheat genes to a marker gene, the researchers shot the mixture into developing cells of a medium-grain rice. They grew the resulting transgenic plants and then harvested and milled the mature grain. It took up to 30% less pressure to fracture rice grains that had activated one or both of the puroindoline genes, the researchers report in the February issue of Nature Biotechnology. And--like flour milled from soft wheat--flour from the transgenic rice had less damage to the starch and finer particles.
The experiment has won praise from rice and wheat geneticists alike. "This is very strong evidence" that puroindoline genes influence the softness trait, says Allan Fritz, a wheat geneticist and breeder at Kansas State University in Manhattan. A softer grain could expand the uses of rice flour, envisions Wally Yokoyama, a cereal chemist with the U.S. Department of Agriculture in Albany, California. "There's an increased use of rice in processed foods now," he says. "A rice grain with different properties might be interesting."