Fig. 7. Surgical ablation of the L₁ layer leads to aberrant cell division and differentiation in L₂ and L₃. (A-C) Consecutive video images of a single meristem from which the left half of the L₁ layer was removed. Cut primordia are coloured in dark green, and the youngest primordia are highlighted in yellow: (A) t₀, (B) 2 days and (C) 4 days after ablation. Organ formation continues from the unperturbed half, and the meristem centre is shifted to the right. (D-F) Consecutive video images of a meristem from which most of the L₁ layer has been removed: (D) t₀, (E) 2 days, (F) 4 days. Meristem activity ceases. Note that a final primordium is formed at the normal position (I₁). (G-I) Longitudinal sections through meristems after a surgical ablation as in D. (G) Note the continuity of the L₂ layer at the site of the ablation (between arrows). (H) Two days after ablation as in (D). A last primordium was formed (I₁), while the cells at the ablated site (arrowhead) became vacuolated and started to divide periclinally. (I) Five days after ablation as in (D). Note stacks of cells resulting from repeated periclinal division, and increasing vacuolisation (arrowhead). (J) Close up of (I). Note different cell division patterns and cell shape in the area where a primordium had been removed at the beginning of the experiment (arrow), compared to the site at which the L₁ was ablated (arrowhead). (K-M) In situ hybridisations with a ³⁵S-labelled antisense probe against LeT6. Tomato meristems were treated as in (D), and fixed for analysis either immediately (K), or after 2 days (L), and 5 days (M). LeT6 signal can be observed at the ablated site until 2 days after the ablation (L). After 5 days, the vacuolated cells exhibited low levels of LeT6 mRNA, whereas high levels of LeT6 remained in the lower L₃ cells that exhibit less vacuolisation. P₄, P₃, P₂ and P₁ indicate the bases of preexisting leaf primordia that were removed at the beginning of the experiment; I₁ and I₂ indicate primordia formed after the ablation. Scale bars: 100 µm (A-I, K-M); 50 µm (J).

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