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First published online 24 January 2007
doi: 10.1242/dev.001008

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Growth from two transient apical initials in the meristem of Selaginella kraussiana

¹ Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK.
² Systems Biology Laboratory, 127 Milton Park, Abingdon, Oxfordshire OX14 4SA, UK.

View larger version (123K): [in this window] [in a new window]   Fig. 1. X-ray irradiation of S. kraussiana var. aurea yields cell-autonomous sectors. (A) Wild-type +/+ (dark green), heterozygous au/+ (pale green) and homozygous au/au (white) plants, and the predicted outcome of X-ray-induced aneuploidy. (B) Shoot morphology 14 and 21 days after irradiation with 0-100 Gy. (C) Growth perturbations in plants irradiated at 75 Gy and above. Note perturbations in leaf shape at high doses (dotted white circles), shoot termination at 100 Gy (dotted lines) and dark-green sectors (arrowed). (D-G) Single-cell +/- (D,E) and au/-(F,G) sectors in the epidermis (E,G) and mesophyll (D,F) layers. Scale bars: D,E,G, 10 µm; F, 20 µm.

View larger version (92K): [in this window] [in a new window]   Fig. 2. S. kraussiana leaves comprise three chloroplast-containing cell layers. (A) Transverse section through leaves arranged around an apex showing the three main cell layers of the leaf, and the downwards orientation of chloroplasts in the mesophyll. The asterisk indicates the position of the meristem; red arrowheads indicate the abaxial surfaces of the dorsal (d) and ventral (v) leaves; black arrows indicate large chloroplasts in the mesophyll; the blue arrow indicates a guard cell. Scale bar: 40 µm. (B) Scan through all three layers of the leaf showing a white au/- sector in the upper two layers of the leaf. The abaxial epidermis is au/+. Scale bar: 10 µm.

View larger version (43K): [in this window] [in a new window]   Fig. 3. Leaves are initiated from a pair of epidermal cells. (A-D) Schematic of full-length leaf sectors that traversed all three Ad-Ab leaf layers and support a two-cell (A), four-cell (B), six-cell (C) and eight-cell (D) mediolaterally extended strip of cells that are recruited from the meristem to make a leaf. (E) Schematic of sectors occupying two of the three leaf layers (i.e. those that are formed when the Ad-Ab axis comprises two cell layers). Sectors are a maximum of a one-sixth leaf width indicating that the Ad-Ab axis is not established until at least six cells have been recruited in the M-L axis. Notably, some sectors traverse the full length of the leaf indicating that Ad-Ab axis formation precedes P-D axis formation. (F) Schematic of sectors occupying just one cell layer (i.e. those that are formed when the Ad-Ab axis comprises three cell layers). No sectors traverse the full length of the leaf showing that the P-D axis is established before divisions in the Ad-Ab axis are completed. (G) Summary of earliest cell division patterns in the leaf as inferred from 1362 sectors. At the six-cell stage, the next cell division can either be in the M-L axis to form an eight-cell strip, or in the Ad-Ab axis to produce 12 founder cells.

View larger version (78K): [in this window] [in a new window]   Fig. 4. Shoot growth occurs from two apical initials. (A) Dorsal view of the flattened S. kraussiana shoot showing major and minor branch structures. White square indicates meristem position. (B) Scanning electron micrograph of a meristem, and schematic showing the four axes that are established during shoot growth. The oval indicates four cells at the tip. (C-F) Transverse sections through the shoot apex at 3 µm intervals showing: two central initials flanked by two lateral merophytes on the surface (C); the division of the two central initials to produce dorsal and ventral merophytes, and radial division of the two lateral merophytes (D); further radial divisions in the epidermal cell layer (E); and finally, at five cell layers below the surface, approximately 30 large epidermal cells, and a ventral region of five to six small epidermal cells in the predicted position of leaf initiation (p) (F). (G-J) Schematics of sections C-F showing epidermal cell division patterns deduced from both histology and sector analysis. Black lines indicate the most recent division, and insets indicate stages that are not apparent in the sections shown but that must have occurred between the layer shown and the next layer down. m, merophyte. (K-M) Schematic representation of shoot sectors occupying one-half (K), one-quarter (L) and one-tenth (M) of the shoot circumference. The sector in L is a twin spot, most likely induced by somatic recombination rather than by independent aneuploidy events in two neighbouring cells. Scale bars: A, 5 mm; B, 22 µm; C-F, 18 µm.

View larger version (32K): [in this window] [in a new window]   Fig. 5. Apical initials are not permanent. (A) Schematic of a shoot sector that first occupies segments of the front and back of the shoot, then occupies half of the shoot, and finally takes over the whole shoot. (B-E) Hypothetical patterns of inheritance of the mutant sector if two apical initials are functional throughout shoot growth (B,C), or if the number of initials decreases from two to one (D,E). Initials are indicated with an asterisk and the direction of cell division with arrows; the jagged arrow indicates an X-ray-induced aneuploidy event in one initial; light grey depicts au/+ tissue and dark grey depicts +/-tissue. Cell patterns in the surface (B,D) and subtending (C,E) layers of the apex are depicted.

View larger version (80K): [in this window] [in a new window]   Fig. 6. Apical initial number varies during shoot growth and branching. (A-F) Frontal longitudinal sections through recently bifurcated meristems (A,E,F), the meristems of major branches that have initiated at least two (B) or three (C) leaf pairs, and an apex that has visibly begun to split into two (D). Arrowheads point to large epidermal cells that are deduced from sector analysis and KNOX and ARP in situ hybridisation patterns (Harrison et al., 2005) to be apical initials (red) or merophytes (black). Scale bars: A, 32 µm; B, 20 µm; C, 27 µm; D, 24 µm; E, 32 µm; F, 27.5 µm. (G-J) Schematic representation of sectors that span branch points. (K-N) Deduced cell division patterns giving rise to the sectors in G-J. The boxes represent the presumed number of initials and merophytes present in the surface layer. Light grey represents au/+ tissue and dark grey represents +/-tissue. M, merophyte; I, initial. The ventral branch-point leaf is represented at the six-cell stage.