Intercellular movement of the putative transcription factor SHR in root patterning

Positional information is pivotal for establishing developmental patterning in plants(1-3), but little is known about the underlying signalling mechan isms. The Arabidopsis root radial pattern is generated through stereotyped division of initial cells and the subsequent acquisition of cell fate(4). s hort-root (shr) mutants do not undergo the longitudinal cell division of th e cortex/endodermis initial daughter cell, resulting in a single cell layer with only cortex attributes(5,6). Thus, SHR is necessary for both cell div ision and endodermis specification(5,6). SHR messenger RNA is found exclusi vely in the stele cells internal to the endodermis and cortex, indicating t hat it has a non-cell-autonomous mode of action(6). Here we show that the S HR protein, a putative transcription factor, moves from the stele to a sing le layer of adjacent cells, where it enters the nucleus. Ectopic expression of SHR driven by the promoter of the downstream gene SCARECROW(SCR) result s in autocatalytic reinforcement of SHR signalling, producing altered cell fates and multiplication of cell layers. These results support a model in w hich SHR protein acts both as a signal from the stele and as an activator o f endodermal cell fate and SCR-mediated cell division.