To stimulate expansin production within the meristem that makes new leaves, a group led by Andrew Fleming of the Swiss Federal Institute of Technology in Zurich, Switzerland, created transgenic tobacco plants in which the expansin gene was coupled to a tetracycline-dependent promoter. Induction of expansin expression caused a leaf to sprout at the site. As far as they could determine, the resulting leaves were normal internally and externally, says Fleming—unlike the results from a prior experiment in which dabbing expansin on nontransgenic plants produced only spindly growths.
The authors do not yet know how expansin relaxes the cell wall or how it triggers leaf formation. It may seem surprising that what seems like a small change could unleash a complex process like leaf development. However, says Fleming, the study lends credence to a much-debated hypothesis that cells are not only attuned to their chemical environment, but also to the forces that impinge upon them. “It's possible that the cell responds to the biophysical forces around it and can change its gene expression accordingly,” he says. ▪