To the novice, gene transfer in plants seems like a pretty simple notion -- choose a gene that controls a specific trait, introduce it into the plant and watch for the expected results. However, work on flower color in transgenic petunias illustrates that gene expression involves an intricate hierarchy of mechanisms, many of which are not yet fully understood. When copies of a gene that controls blue, purple and red flower color are transferred into a purple petunia, UC Davis geneticist Richard Jorgensen found that nearly half of the flowers on the resulting plants were surprisingly white or variegated white and purple, indicating that the introduced gene had somehow shut down the existing pigment-producing gene in a process known as "co-suppression." Drawing on research begun at DNA Plant Technology Corp., Jorgensen speculates that the diverse pigment patterns produced by co-suppression are the result of a handful of factors involving not only the coding and uncoding of the DNA's genetic information, but also changes taking place in the cell nucleus as the plant goes through various developmental stages. "This takes us back to some of the old questions of genetics that were addressed in the 1950s, suggesting that inheritance doesn't involve only the DNA," says Jorgensen, who presented an overview of co-suppression research related to flower color patterns in the May 5 issue of Science.