Fig. 6. L1 layer specification and development of meristem structure. (A) pML1::GFP5-ER reporter (green) was upregulated in a subset of superficial shoot meristem progenitors, marked by the pCUC2::3XVENUS-N7 (red) marker. (B,C) Expression of the pML1::GFP5-ER reporter (green) was L1 specific within the shoot progenitors (B) but not L1 specific in cross-sections of callus (C). (D) 24 hours later, pML1::GFP5-ER expression was upregulated in the shoot progenitors. (E) 72 hours later, pML1::GFP5-ER expression was homogeneously expressed within the L1 of the meristem as primordia (I₁ and P₁) were initiated. (F) 96 hours later, two early primordia (P₁ and P₂) were evident. (G) pPIN1::PIN1-GFP (green) and pSTM::STM-VENUS (red) were upregulated with similar timing in small patches of cells marked by the ubiquitous pRIBO::2XCFP-N7 marker (blue). (H) 24 hours later, pPIN1::PIN1-GFP marked initiating primordia (I₁, I₂). (I) After 48 hours, primordia (P₁ and P₂) labeled by pPIN1::PIN1-GFP have grown outwards and pSTM::STM-VENUS was expressed in the developing meristem. (J) After 72 hours of observation, meristems derived from small numbers of initial cells gave rise to fewer primordia than meristems with more cells. Scale bars: 50 µm (A-J).