Fig. 1. Ectopic STM expression suppresses cell differentiation. (A) Light micrograph of a non-transgenic stm5 mutant seedling 8 days after germination. Cotyledon petioles are fused and no leaves have been formed. (B) Light micrograph of an stm5 mutant seedling expressing CLV1::STM 8 days after germination. The first pair of leaves formed by the SAM is visible (arrow). The bases of the cotyledon petioles are fused as in the seedling shown in A. We used the CLV1 promoter, which is active in the centre of the embryonic shoot meristem primordium from heart-stage onward, and whose initial activation does not require STM function (Long and Barton, 1998), since no STM promoter has been described that mimics the endogenous mRNA expression pattern. (C,D) Micrographs of DAPI-stained seedlings. (C) stm5 mutant seedling 5 days after germination. No meristematic cells are visible inside the fused cotyledon petioles (arrow). (D) CLV1::STM-expressing stm5 mutant seedling 5 days after germination. A meristematic region is evident from the bright signal from cytoplasmically dense cells inside the fused petioles (arrow). (E,F) Scanning electron micrographs. (E) Wild-type seedling 10 days after germination. c, cotyledon; l, leaf. (F) ANT::STM-expressing seedling with a strong phenotype 21 days after germination. The petioles of the cotyledons (cp) are broader than in wild type (compare with E). Leaves (l, arrow) are not expanded and are rolled up at their margins. h, hypocotyl. (G) Light micrograph of a mature second rosette leaf of a wild-type plant. (H) Light micrograph of a mature second rosette leaf of an ANT::STM-expressing plant with a weak phenotype. The petiole (asterisk) is broader than wild type and lateral outgrowths have developed into leaf-like structures (arrow). (I-L) Cross-sections of plastic-embedded leaf material from seedling 12 days after germination, stained with Toluidine Blue. (I) Petiole of the first rosette leaf of a wild-type plant. A vascular bundle (arrow) with differentiated cells lacking cytoplasm is surrounded by large, vacuolated cells. (J) Basal part of the first rosette leaf of an ANT::STM-expressing seedling. The cells in place of the vascular strand (arrow) are cytoplasmically dense and the cells throughout the petiole are less expanded than in G. (K) The lamina of the first rosette leaf of a wild-type plant. Note the high degree of vacuolation and the large intercellular spaces (asterisk). (L) The lamina of the first rosette leaf of an ANT::STM-expressing seedling. Cells throughout the leaf are smaller than in I and contain more cytoplasm, indicating that differentiation is suppressed. Scale bars are 500 µm in C-H, 100 µm in I-L.