Fig. 4. Independent functions of WUS and STM. Light micrographs of live seedlings (A,E-H) and GUS-stained, cleared seedlings (B-D,I-L). (A,B) ANT::WUS; ANT::GUS-expressing wild-type seedlings 12 days (A) and 10 days (B) after germination. An enlarged SAM has developed in place of leaves (A) which strongly expresses the transgenes (B). (C-E) ANT::WUS; ANT::GUS-expressing stm5 mutant seedlings 10 days (C) and 18 days (D,E) after germination. Transgene expression has only been initiated in a few cells (arrow) inside the fused cotyledon petioles in the seedling in C from which a mass of small meristematic cells develops subsequently (D,E arrow). In E, the fused cotyledon petioles have been cut open for clarity. (F) Non-transgenic stm5 mutant seedling 18 days after germination. Several leaves have been formed and have ruptured the fused wall of the cotyledon petioles. (G) ANT::STM-expressing wild-type seedling. Leaves are reduced to finger-like, lobed structures (arrow) and the petioles of the cotyledons (c) are broadened. (H) ANT::STM-expressing wus1 mutant seedling. Leaves (arrow) and cotyledon petioles (c) are affected as in G. (I,J) ANT::STM; ANT::GUS-expressing wild-type (I) and wus1 mutant (J) seedlings. In both cases, strong GUS staining is visible in the vascular strands of the cotyledon petioles (arrowheads) and in young leaf primordia (arrows) at the shoot meristem. (K,L) WUS::NLSGUS- (K) and ANT::STM; WUS::NLSGUS- (L) expressing seedlings. In both cases, GUS staining is restricted to a small central cell group in the shoot apical meristem (arrowheads). The additional smaller region of staining in K is an axillary meristem. (M) Longitudinal section through a GUS-stained WUS::NLSGUS-expressing seedling. GUS activity is detected specifically in a small central cell group of the SAM, reflecting the WUS mRNA expression pattern (Mayer et al., 1998). Scale bars are 1 mm in A-L, 100 µm in M.