Root formation in plants involves the continuous interpretation of positional cues. Physiological studies have linked root formation to auxins. An auxin response element displays a maximum in the Arabidopsis root and we investigate its developmental significance. Auxin response mutants reduce the maximum or its perception, and interfere with distal root patterning. Polar auxin transport mutants affect its localization and distal pattern. Polar auxin transport inhibitors cause dramatic relocalization of the maximum, and associated changes in pattern and polarity. Auxin application and laser ablations correlate root pattern with a maximum adjacent to the vascular bundle. Our data indicate that an auxin maximum at a vascular boundary establishes a distal organizer in the root.

Stem cells self-renew and produce daughter cells that differentiate. How stem cells are specified and maintained is a central question in developmental biology. Plant stem cells occupy a small region or niche in larger zones of mitotic activity called meristems. Here we provide molecular evidence that in the Arabidopsis root meristem, the stem cell population depends on a central group of cells, the quiescent center (QC), which positions the stem cell niche. We show that the putative transcription factor SCARECROW (SCR), first identified by its role in radial patterning, is required cell-autonomously for distal specification of the QC, which in turn regulates stem cell fate of immediately surrounding cells. Received October 21, 2002; revised version accepted November 29, 2002. Stem cell identity in various organisms is maintained in microenvironments called niches (Spradling et al. 2000). Their maintenance depends on local signaling events emanating from nearby cells that can be considered part of the niche (Spradling et al. 2000). Cap cells (Xie and Spradling 1998, 2000) and somatic hub cells (Kiger et al. 2001;Tulina and Matunis 2001) in Drosophila, distal tip cells in Caenorhabditis elegans (Kimble and White 1981), and cells expressing the homeodomain transcription factor WUSCHEL in the Arabidopsis shoot apical meristem (Mayer et al. 1998;Schoof et al. 2000) are examples of cells locally required for the maintenance of stem cells. In these cases, it remains to be established how the signaling cells themselves are specified to define stem cell location.In the Arabidopsis root meristem, the "initial cells" are the stem cells that give rise to all cell types of the root; they surround a small group of mitotically less active cells, the quiescent center (QC), and can be unequivocally identified (Fig. 1a;Dolan et al. 1993). Laser ablation experiments suggested that the QC is a source of cell nonautonomous signals, which prevent differentiation and hence maintain the surrounding stem cells (van den Berg et al. 1997). Hence, the QC and the surrounding cells that contact it can be considered a stem cell niche. The SCARECROW (SCR) gene encodes a putative transcription factor (Di Laurenzio et al. 1996) that is first expressed in QC precursor cells during embryogenesis, after which it extends to the initial cells for the ground tissue (cortex and endodermis) and the endodermis ; this expression pattern persists in the postembryonic root ( Fig. 1a; Di Laurenzio et al. 1996). In scr-1 mutants, the asymmetric cell division of the daughter of the cortex/endodermis initial does not occur, resulting in a single cell layer with mixed identity ( Fig. 1d; Di Laurenzio et al. 1996). Importantly, cells in the src-1 QC region are aberrant in shape and roots ultimately cease growth (Scheres et al. 1995;Di Laurenzio et al. 1996). Here, we provide evidence that these effects are not caused by the cortex/endodermis defect but rather reflect a direct requirement for SCR activity in QC cells for their specification and maintenance ...

Plant postembryonic development takes place in the meristems, where stem cells self-renew and produce daughter cells that differentiate and give rise to different organ structures. For the maintenance of meristems, the rate of differentiation of daughter cells must equal the generation of new cells: How this is achieved is a central question in plant development. In the Arabidopsis root meristem, stem cells surround a small group of organizing cells, the quiescent center. Together they form a stem cell niche [1, 2], whose position and activity depends on the combinatorial action of two sets of genes - PLETHORA1 (PLT1) and PLETHORA2 (PLT2)[3, 4] and SCARECROW (SCR) and SHORTROOT (SHR)[2] - as well as on polar auxin transport. In contrast, the mechanisms controlling meristematic cell differentiation remain unclear. Here, we report that cytokinins control the rate of meristematic cell differentiation and thus determine root-meristem size via a two-component receptor histidine kinase-transcription factor signaling pathway. Analysis of the root meristems of cytokinin mutants, spatial cytokinin depletion, and exogenous cytokinin application indicates that cytokinins act in a restricted region of the root meristem, where they antagonize a non-cell-autonomous cell-division signal, and we provide evidence that this signal is auxin.

Plant growth and development are sustained by meristems. Meristem activity is controlled by auxin and cytokinin, two hormones whose interactions in determining a specific developmental output are still poorly understood. By means of a comprehensive genetic and molecular analysis in Arabidopsis, we show that a primary cytokinin-response transcription factor, ARR1, activates the gene SHY2/IAA3 (SHY2), a repressor of auxin signaling that negatively regulates the PIN auxin transport facilitator genes: thereby, cytokinin causes auxin redistribution, prompting cell differentiation. Conversely, auxin mediates degradation of the SHY2 protein, sustaining PIN activities and cell division. Thus, the cell differentiation and division balance necessary for controlling root meristem size and root growth is the result of the interaction between cytokinin and auxin through a simple regulatory circuit converging on the SHY2 gene.