Fig. 3. The pattern of GFP accumulation (green) in embryos from plants bearing the GL2::GFP transgene viewed by fluorescence microscopy. The red background in the images is due to autofluorescence. Owing to the progressive increase in the level of GL2::GFP expression during embryogenesis, the GFP signal was amplified in embryos at earlier stages (e.g. B and C) relative to later stages (e.g. I and J), so accurate comparisons of signal intensity between the stages cannot be made with these images. Propidium iodide staining (red) was performed with the mature embryos shown in K and L to visualize cell boundaries. (A) Median longitudinal optical section from a globular stage embryo. Bar, 50 µm. (B,C) Surface view of two different embryos from the late-triangular/early-heart stage. Bar, 50 µm for B-D. (D) Median longitudinal optical section from a late-triangular/early-heart stage embryo. (E) Surface view of a heart stage embryo. Bar, 50 µm for E and F. (F) Median longitudinal optical section from a heart stage embryo. (G) Surface view of a torpedo stage embryo. Bar, 50 µm for G and H. (H) Median longitudinal optical section from a torpedo stage embryo. (I) Surface view of a mature embryo. Bracket indicates the location of the root-hypocotyl junction region. Bar, 40 µm for I and J. (J) Median longitudinal optical section from a mature embryo. (K) Magnified view of the hypocotyl region of a mature embryo. (L) Magnified view of the root region of a mature embryo. The insets in K and L show optical sections of the underlying cortical cell layer, revealing that epidermal cells outside the anticlinal cortical cell walls (marked by arrows) lack GFP expression. Bar, 15 µm for K and L.