Fig. 5. Synergistic effects of coexpression of WUS and STM. (A-D) Scanning electron micrographs of seedlings 14 days after germination. (A) ANT::WUS-expressing seedling. An enlarged SAM has formed in place of leaves. The cotyledon petioles (cp) are unaffected and separated from the meristematic cells by a sharp boundary (arrow). h, hypocotyl. (B) ANT::STM-expressing seedling. Cotyledon petioles (cp) are broadened, but do not show meristem-like cells. (C,D) ANT::WUS; ANT::STM coexpressing seedlings. No cotyledon petioles have been formed and fields of small, meristematic cells (arrows) extend into the lamina of cotyledons (c). (E,F) Histological sections of plastic embedded material stained with Toluidine Blue. (E) Longitudinal section through the apex of an ANT::WUS-expressing seedling 8 days after germination. Note the massively overproliferated shoot meristem with small, cytoplasmically dense cells (arrow). (F) Longitudinal section through the apex of an ANT::WUS; ANT::STM-expressing seedling 8 days after germination. The regions of small meristematic cells are expanded into the cotyledons (arrows). The spots of darker stained cells are an artefact of processing. (G,H) In situ hybridization using a CLV3 antisense riboprobe. (G) In ANT::WUS-expressing seedlings, CLV3 mRNA is detected in the outermost cell layers of the enlarged shoot meristem (black arrow), but not in cells of the cotyledon petioles (white arrow). (H) By contrast, ANT::WUS; ANT::STM coexpressing seedlings show CLV3 expression both in the enlarged shoot meristem (black arrow) and in the meristematic regions on the cotyledons (white arrow). Scale bars are 500 µm in A-C, 200 µm in D and 100 µm in E-H.