Asymmetric cell divisions play an important role in the establishment and propagation of the cellular pattern of plant tissues. The SHORT-ROOT (SHR) gene is required for the asymmetric cell division responsible for formation of ground tissue (endodermis and cortex) as well as specification of endodermis in the Arabidopsis root. We show that SHR encodes a putative transcription factor with homology to SCARECROW (SCR). From analyses of gene expression and cell identity in genetically stable and unstable alleles of shr, we conclude that SHR functions upstream of SCR and participates in a radial signaling pathway. Consistent with a regulatory role in radial patterning, ectopic expression of SHR results in supernumerary cell divisions and abnormal cell specification in the root meristem.

Positional information is pivotal for establishing developmental patterning in plants, but little is known about the underlying signalling mechanisms. The Arabidopsis root radial pattern is generated through stereotyped division of initial cells and the subsequent acquisition of cell fate. short-root (shr) mutants do not undergo the longitudinal cell division of the cortex/endodermis initial daughter cell, resulting in a single cell layer with only cortex attributes. Thus, SHR is necessary for both cell division and endodermis specification. SHR messenger RNA is found exclusively in the stele cells internal to the endodermis and cortex, indicating that it has a non-cell-autonomous mode of action. Here we show that the SHR protein, a putative transcription factor, moves from the stele to a single layer of adjacent cells, where it enters the nucleus. Ectopic expression of SHR driven by the promoter of the downstream gene SCARECROW (SCR) results in autocatalytic reinforcement of SHR signalling, producing altered cell fates and multiplication of cell layers. These results support a model in which SHR protein acts both as a signal from the stele and as an activator of endodermal cell fate and SCR-mediated cell division.

Plants rely on the maintenance of stem cell niches at their apices for the continuous growth of roots and shoots. However, while the developmental plasticity of plant cells has been demonstrated1, it is not known whether the stem cell niche is required for organogenesis. Here we explore the capacity of a broad range of differentiating cells to regenerate an organ without the activity of a stem cell niche. Using a root-tip regeneration system in Arabidopsis to track the molecular and functional recovery of cell fates, we show that re-specification of lost cell identities begins within hours of excision and that the function of specialized cells is restored within one day. Critically, regeneration proceeds in plants with mutations that fail to maintain the stem cell niche. These results show that stem cell-like properties that mediate complete organ regeneration are dispersed in plant meristems and are not restricted to niches, which nonetheless appear necessary for indeterminate growth. This regenerative reprogramming of an entire organ without transition to a stereotypical stem cell environment has intriguing parallels to recent reports of induced transdifferentiation of specific cell types in the adult organs of animals2,3.

Many lines of evidence indicate that cell fate determination in plants is based primarily on position, even though well-defined cell lineages exist (Kidner et al., 2000;van den Berg et al., 1995). Position-dependent fate determination must rely on highly coordinated intercellular interactions, which in plants are complicated by the presence of cell walls. Membrane-lined pores called plasmodesmata (PD) pierce the cell walls, creating a cytoplasmic continuum (symplast) that becomes a possible route of communication between cells in plants. Intercellular communication can thus occur either via secreted signals diffusing through the cell wall continuum (apoplast), or directly through the symplast. Positional information using either mode of cell-cell communication can be regulated by limiting the extent of travel of the signal (Lenhard and Laux, 2003), by the distribution of the competence to respond to the signal, or both.Our previous work provided evidence that the SHORT ROOT (SHR) protein, a member of the GRAS family of putative transcription factors, acts as a positional signal essential to radial pattern formation. Genetic analyses in Arabidopsis first revealed a role for SHR in root radial patterning (Benfey et al., 1993). The radial organization of the root encompasses concentric rings of epidermis, cortex,

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