Figure 2.

Meristem organization in wild-type (WT) and pin1 Arabidopsis. A and B, Longitudinal sections through stage I meristems (10 d after germination). pin1 meristems (B) are comparable to the wild type (A) in size and structure but tend to be flatter. C and D, Stage II meristems (4 weeks old). Both wild-type (C) and pin1 (D) meristems are organized in layers and structurally intact. E and F, Stage III meristems (6 weeks old). Wild-type (E) and pin1 (F) meristems are of similar size and structure. G, Zoom out of D. Even though organs are not initiated in stage II pin1 mutants, meristem growth is not arrested, as evidenced by the small cells (zone of no distinction) between stage I leaves and the meristem boundary (arrowheads). Insets are closeups of the central meristem regions. H to K, Expression of the central zone marker CLV3-GFP is comparable in stage III meristems of the wild type (H) and pin1 (I) as well as in inflorescence wild-type (J) and pin1 (K) meristems. L, Scanning electron microscopy image of a stage III pin1 meristem. Primordia (P) are poorly spaced and clustered on one side of the meristem. The zone of no distinction produced during stage II is clearly visible. The asterisk shows the center of the meristem. Bars = 50 µm in A to K and 100 µm in L.