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.
Because plant cells do not migrate, cell division planes are crucial determinants of plant cellular architecture. In Arabidopsis roots, stringent control of cell divisions leads to a virtually invariant division pattern, including those that create new tissue layers. However, the mechanisms that control oriented cell divisions are hitherto poorly understood. Here, we reveal one such mechanism in which FEZ and SOMBRERO (SMB), two plant-specific NAC-domain transcription factors, control the delicately tuned reorientation and timing of cell division in a subset of stem cells. FEZ is expressed in root cap stem cells, where it promotes periclinal, root cap-forming cell divisions. In contrast, SMB negatively regulates FEZ activity, repressing stem cell-like divisions in the root cap daughter cells. FEZ becomes expressed in predivision stem cells, induces oriented cell division, and activates expression of its negative regulator, SMB, thus generating a feedback loop for controlled switches in cell division plane.
In plant meristems, dividing cells interpret positional information and translate it into patterned cell differentiation. Here we report the molecular identification of the Arabidopsis HOBBIT gene that is required for cell division and cell differentiation in meristems. We show that it encodes a homolog of the CDC27 subunit of the anaphase-promoting complex (APC). HOBBIT partially complements a yeast nuc2/cdc27 mutant. Unlike other CDC27 homologs in Arabidopsis, its transcription is cell cycle regulated. Furthermore, hobbit mutants show a reduction in DR5 :: GUS auxin reporter gene expression and accumulate the AXR3/IAA17 repressor of auxin responses. HOBBIT activity may thus couple cell division to cell differentiation by regulating cell cycle progression in the meristem or by restricting the response to differentiation cues, such as auxin, to dividing cells.
The embryonic origin of the Arabidopsis root and hypocotyl region has been investigated using histological techniques and clonal analysis. Our data reveal the pattern of cell division in the embryo giving rise to the various initials within the root promeristem. A small region of the root at its connection with the hypocotyl appears not to be derived from the promeristem initials. This region contains two cortical cell layer and [3H]thymidine incorporation data suggest that it lacks postembryonic cell divisions. Sectors marked by transposon excision from the beta-glucuronidase marker gene are used to investigate cell lineages giving rise to root and hypocotyl. The position of end points from sectors with embryonic origin show little variation and hence reveal preferred positions in the seedling for cells derived from different regions of the embryo. The radial extent of complete root sectors is consistent with the radial arrangement of root meristem initials at the heart stage of embryogenesis inferred from histological analysis. Using the clonal data, a fate map is constructed depicting the destiny of heart stage embryonic cell tiers, in the seedling root and hypocotyl. The variability in the sector end points indicates that distinct cell lineages are not restricted for root or hypocotyl fate. In contrast, derivatives of the hypophyseal cell do appear to be restricted to the columella and central cell region of the root.
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