The wanderer is SHORTROOT (SHR), which is made in the root vascular tissue and then diffuses outward. In the neighboring cell layer, it partners with another transcription factor, called SCARECROW (SCR), to help create the plant endodermal layer. The Arabidopsis endodermis is a single layer surrounding the root vasculature. Benfey and colleagues wondered why SHR's diffusion stops here.
Two cell lines with reduced SCR activity provided the answer. The lower the SCR activity, the more SHR leaked past the first cell layer and began to specify extra endodermis. The effects suggest that SCR physically tethers its partner in the nucleus of the cells where the job must be done. SHR and SCR orthologues also interact in rice and are expressed in similar patterns, suggesting the strategy may be common among plants.
The endodermis's job is to absorb nutrients into the vasculature. Benfey speculates that the diffusive stop helps to ensure that nutrients only have to pass through a single endodermal layer along their way. It may seem baroque, he says, but no more so than diffusion gradients in animals.