In a plant root, SHR moves from stele to endodermal cells.


Location, location, location—the mantra of real estate also holds true for plant development, where the fate of a plant cell depends largely on its position rather than its lineage. In the core of a growing root, cells learn their position from their neighbors through the actions of a transcription factor that travels between cell layers, according to a study conducted at New York University, NY. This protein is novel because of its dual function: it both induces cell division and signals cell identity.Philip Benfey and colleagues report that the SHORT-ROOT protein (SHR) is made only in the innermost layer of the Arabidopsis root, but it travels to the adjacent layer. There, it stimulates cell division by turning on a growth-promoting gene called SCARECROW, and it helps establish the identity of endodermal cells, which arise from this layer. So far, only plants have been shown to pass transcription factors like SHR from cell to cell, Benfey says.

In a mysterious twist, SHR's advance stops at this second layer. Benfey says this pattern raises two burning questions he and his coworkers are hoping to answer. First, how does SHR get from one layer to the next? It may travel through the plasmo-desmata, but there is no firm evidence for this yet. And second, what prevents the protein from moving farther? If SHR is passing through the plasmodesmata, then these channels or something associated with them may be acting as border guards to limit SHR's movements. ▪


Nakajima, K., et al. 2001. Nature. 413:307–311.