In plant cell development, who you are is often determined by where you are and what your neighbours are. For example, the differentiation of photosynthetic cells in maize is controlled by their proximity to the veins of a leaf. One mechanism by which this could be achieved is the production of a signal molecule by one cell, which is detected by surrounding cells. Such systems do exist, as in the maintenance of the shoot apical meristem by a feedback loop between wuschel- and clavata -expressing cells. However, working with Arabidopsis roots, Philip Benfey and colleagues at New York University have uncovered a more direct mechanism.

The root of a plant is made up from a number of concentric layers of cells. The innermost of these is the stele, around which is wrapped the endodermis and cortex, both just one cell thick. Pairs of adjacent endodermis and cortex cells are formed from the division of a single cell within the root meristem. This is controlled by a gene called SCARECROW (SCR) — with scr mutant plants having a single combined endodermis–cortex layer.

Benfey and colleagues investigated a gene called SHORT-ROOT (SHR), mutants of which show a similar phenotype to scr plants. They found that SHR is not transcribed in the cortex or endodermis — its messenger RNA being confined to the stele. However, tagging the SHR gene product with green fluorescent protein lit up not only stele cells, but also the nuclei of endodermal cells and the initial cells that divide to produce the endodermis and cortex (green in figure, cell walls are red). In fact, SHR was in the nuclei of all cells directly in contact with the stele.

From its sequence, SHR is predicted to be a transcription factor. So, perhaps SHR that is synthesized in the stele might be exported to adjacent cells, where it is rapidly transported to the nucleus to promote transcription of endodermis-specific genes, including SCR. To test this, Benfey and colleagues made transgenic plants containing an additional copy of SHR under the control of the SCR promoter. The roots of such plants showed a massive expansion of the endodermis, as SHR made in the stele was being exported to — and inducing endodermal character in — adjacent cells. At the same time, SHR was inducing ectopic production of more SHR in the endodermis. This SHR was then exported to cells adjacent to the endodermis, promoting transcription of yet more SHR and an avalanche of endodermis induction.

Some plant proteins are known to move between cells through plasmodesmata — pores that punctuate plant cell walls, actively controlling the flow of material across this barrier. It seems likely that SHR moves by the same route. Equally, other transcription factors, such as LEAFY and DEFICIENS in the floral meristem, are known to move out of the cells in which they are produced, although the functional significance of their emigration is unclear.

The radiation of SHORT-ROOT out from the stele provides, in a single protein, both positional information and a mechanism to determine cell fate. This simple strategy will doubtless be used at the heart of many developing structures, but it is another example of an elegant innovation that is unique to the plant kingdom.