Fig. 6. The loss-of-function phenotypes of wus and stm mutants cannot be rescued by transgenic expression of the respective other gene. Light micrographs of GUS stained cleared embryos or seedlings (A-D,G,J-L,O,P) and of live seedlings (E,F,H,M,N). (A,B) The CLV1::STM transgene is strongly expressed in the SAM primordia (arrows) of wild-type (A) and wus1 mutant (B) embryos as indicated by staining for the activity of a linked CLV1::GUS reporter. Note the flat apex of the wus1 mutant embryo compared to the convex meristem in the wild type, suggesting that the former has terminated. (C) CLV1::STM; CLV1::GUS expression is detected in the SAM of 7-day old wild-type seedlings by GUS staining. (D) CLV1::STM; CLV1::GUS-expressing wus1 mutant seedlings 10 days after germination show strong GUS staining at the shoot apex. (E,F) The meristems in CLV1::STM; CLV1::GUS-expressing wus1 mutant seedlings (F) terminate indistinguishably from the meristems in non-transgenic wus1 mutants (E) (arrows). (G,H) In CLV1::WUS; CLV1::GUS-expressing wild-type seedlings 7 days after germination strong GUS staining is detected at the apex (G) which causes the development of an enlarged meristem (H, arrow). (I) In situ hybridization using a WUS antisense riboprobe on CLV1::WUS-expressing seedlings confirms transgene expression specifically in the centre of the enlarged shoot meristem, yet not on the flanks (arrow) where organs are initiated. (J,K) In CLV1::WUS; CLV1::GUS-expressing stm5 mutant seedlings the first GUS-staining cells are detected 7 days after germination inside the fused cotyledon petioles (arrow in J) which give rise to adventitious meristems (K, compare with M,N). (L-N) While non-transgenic stm5 mutants 12 days after germination show no sign of a SAM inside the fused cotyledon petioles (L), CLV1::WUS-expressing stm5 mutant seedlings (M,N) of the same age contain a conspicuous meristematic structure (arrows) that is surrounded by small leaf primordia (arrowhead in N). (O) In CLV1::WUS; CLV1::GUS-expressing wild-type plants 25 days after germination, the meristem is massively enlarged (arrow). (P) CLV1::WUS; CLV1::GUS-expressing stm5 mutant plants of the same age show only small meristematic regions that express the GUS reporter gene (arrow). In addition, leaves are small and sometimes fused as in non-transgenic stm5 mutant plants. Scale bars are 50 µm in A,B, 1 mm in C-H,J-P, and 100 µm in I.