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SCHIZORIZA controls an asymmetric cell division and restricts epidermal identity in the Arabidopsis root

Panagiota Mylona^1,*,, Paul Linstead¹, Rob Martienssen² and Liam Dolan¹

¹ Department of Cell and Developmental Biology, John Innes Centre, Colney, Norwich NR4 7UH, UK
² Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA
^* Present address: The Sainsbury Laboratory, Norwich Research Park, Norwich NR4 7UH, UK

View larger version (151K): [in a new window]   Fig. 1. Morphology of 3-day-old scz root. Scanning electron micrograph of the root at the beginning of the differentiation zone. The tubular outgrowths are root hairs. (A) The alternating files of trichoblasts (t) and atrichoblasts (at) can be seen in the wild-type Landsberg erecta. (B) There are no alternating files of trichoblasts and atrichoblasts in scz mutants. Furthermore, root hairs can be seen originating from the underlying cells (arrows). Bar, 50 µm.

View larger version (100K): [in a new window]   Fig. 2. The subepidermal layer of scz mutants develops root hairs and expresses epidermal cell specific markers. (A) Median longitudinal optical section through the scz mutant root showing two cells with root hairs arising from subepidermal cell layers. (B) Transverse sections through the root shown in A at positions indicated by arrows illustrate that the root hair originates from the cortex (asterisk). (C,D) Expression of GFP-enhancer trap line J0481 in the differentiation zone of a scz mutant root (C) and wild-type root (D). Median longitudinal sections stained with propidium iodide are shown on the left, GFP expression (green) is in the middle and superimposed images are shown on the right. The enhancer trap is expressed in the epidermis and ground tissue in scz mutants (C), in contrast to wild type where expression is restricted to the epidermis (D). Bar, 25 µm.

View larger version (115K): [in a new window]   Fig. 3. TTG, GL2 and CPC act in the subepidermal layer of scz mutants. (A) scz; (B) scz gl2; (C) scz ttg and (D) scz cpc. (Upper panels) Light micrographs of the beginning of the differentiation zone. scz gl2 and scz ttg form more root hairs than scz, while scz cpc form fewer root hairs. (Lower panels) Median longitudinal sections through the beginning of differentiation zone, showing that the number of subepidermal root hairs is altered in the double mutants when compared to scz single mutant. Bar, 50 µm.

View larger version (90K): [in a new window]   Fig. 4. Defects in the cellular organisation of scz meristem. (A) Schematic representation of meristem organisation of the Arabidopsis root. The colour code identifies the different cell types. The initials of the epidermis/lateral root cap and cortex/endodermis are shown in white. (B,C) Median longitudinal section through 3-day-old wild-type (B) and scz (C) roots. (B) In wild type, the asterisk indicates a cell that has divided asymmetrically to generate epidermal (inner) and lateral root cap (outer) daughter cells and the arrowhead indicates a cell that has divided asymmetrically to produce the endodermis (inner) and cortex (outer) daughter cells. (C) In scz, the epidermis/lateral root cap daughter cells are not clearly defined (asterisk). Division pattern of the cortex/endodermis daughter cell is abnormal. Often an anticlinal division occurs instead of a periclinal one (arrowhead). More than one periclinal division can occur (arrows) resulting in supernumerary layers (C). Bar, 25 µm.

View larger version (87K): [in a new window]   Fig. 5. An epidermis-specific enhancer trap is expressed in cells of the ground tissue in scz meristems. Expression of GFP-enhancer trap line N9173 in 3-day-old wild-type (A) and scz (B) meristems. (A) In wild type, the enhancer trap is expressed in the lateral root cap and the epidermis. Expression is evident immediately after division of the epidermis and lateral root cap daughter cells (arrow). (B) In scz mutants, expression in the epidermis starts higher in the root (arrow). Moreover, in the ground tissue clusters of cells show a lower level of expression (arrowheads). (C) Close-up of a cluster of cells shown in B illustrating expression of the marker in the ground tissue. Bar, 25 µm in A,B; 56 µm in C.

View larger version (135K): [in a new window]   Fig. 6. The expression of a cortex/endodermis-specific enhancer trap is altered in the scz meristem. Expression of the GFP-enhancer trap line J0571 in 3-day-old wild-type (A) and scz (B) meristems. (A) In wild type the marker is expressed in the quiescent centre, the cortex/endodermis initial, the cortex/endodermis daughter cells and in all the cells of the cortex and the endodermis. (B) The expression of the marker is displaced upwards (asterisk) in scz mutants. The expression is discontinuous (arrow). There are clusters of cells in the extra outer ground tissue layer that show low levels of expression (arrowhead). Bar, 25 µm.

View larger version (57K): [in a new window]   Fig. 7. scz interacts with scr and shr. (A,B) Median longitudinal optical sections through the meristem of 3-day-old scz scr (A) and scz shr (B) double mutant roots. Both scr and shr suppress periclinal divisions in the ground tissue of scz mutants. Occasionally periclinal divisions occur in the ground tissue of scz scr (arrow). (C,D) Median longitudinal section in the differentiation zone of 3-day-old scz scr (C) and scz shr (D) double mutant roots. Cells of the ground tissue in the scz scr double mutant form root hairs (C). Few of the ground tissue cells in the scz shr double mutants form root hairs (D), as shown in an optical cross section (E, asterisk). Bar, 25 µm in A,B,D,E; 50 µm in C.

View larger version (83K): [in a new window]   Fig. 8. Expression of SCR in scz. RT-PRC was performed to compare the amounts of SCR mRNA in scz mutant and wild-type Ler roots (upper panel). The Arabidopsis elongation factor 1-alpha 4 (EF1) was used as a standard (lower panel). Lane G shows the size of the fragment synthesised when genomic DNA is used as a template.

View larger version (111K): [in a new window]   Fig. 9. Cell division defects in scz embryos. Wild-type (left panels) and scz mutant (right panels) embryos stained with Toluidine Blue. (A) In torpedo stage scz embryos, a periclinal division of the presumptive cortex/endodermis initial daughter cell occurs earlier (arrow) than in wild type. (B) In walking stick embryos, an extra periclinal division in scz mutants results in the formation of a new layer in the ground tissue (arrowhead). The periclinal division of the epidermis/lateral root cap initial is indicated with an arrow. Bar, 25 µm.

View larger version (17K): [in a new window]   Fig. 10. (A) Schematic representation of cell divisions and the resulting ground tissue lineage in wild type (wt) and scz. The periclinal divisions are shown with a dotted line. The cortex/endodermis initial is shown in white, the cortex/endodermis initial daughter cell in yellow, the cortex in blue, the endodermis in green and the extra cell layer of scz in red. (B) Model of interaction of SCZ, SCR and SHR. See text for a detailed discussion.