The researchers found that the TOPLESS gene encodes a protein (TOPLESS) that is a transcriptional co-repressor. In plants and animals, co-repressors regulate gene expression by inhibiting the activity of transcription factors, which act as switches to activate target genes. Transcription factors control gene activity by binding to DNA sequences adjacent to a gene, but they are deactivated when they recruit and interact with co-repressors.
Long explains that the normal function of TOPLESS protein is to silence genes required for root development in the top or shoot half of a plant embryo. "In plants with a mutant TOPLESS, genes that should be kept off in order to produce a shoot aren't, so a root is produced," he explains.
Since transcriptional repression plays such a key role in animal embryonic development, the finding that a co-repressor controls polarity in plants was surprising, says Long. "We thought that because plants and animals are believed to have largely developed independently from each other, they would have used a different set of processes to maintain polarity," he says. "But they have a very similar toolbox of genes to set up some aspects of their body plan."
However, a critical difference between plants and animals in terms of polarity development is how much more tolerant plants are of manipulation. Long says, "In animals, early cell divisions are very important to development of polarity and if something goes awry with co-repressors, the animal quickly dies. But in plants, polarity can be changed much later in embryogenesis."
"Plants are so plastic. Our topless mutants can survive very well," he continues. "This suggests that the actual polarity in the embryo isn't fixed until late in development, and that offers us an opportunity to change the fate of the