Embryo sac development is delayed and arrests early in 35S:NTAP:AtCTF7∆B ovules. (A-E) and (J-M) Wild type ovules. (F-I) and (N-P) 35S:NTAP:AtCTF7∆B ovules. (A) FG0 ovule. No FM is identified. (B) Early FG1 ovule showing the FM and DM. The nucellus is not surrounded by the integument. (C) FG1 ovule. The nucellus is surrounded by the outer integument but not the inner integument. (D) FG3 ovule containing a two-nucleate embryo sac. The nucellus is enclosed by the inner integument. (E) FG4 ovule containing a four-nucleate embryo sac. (F-I) Embryo sac development in 35S:NTAP:AtCTF7∆B ovules observed by CLSM. (F) Ovule containing megaspore(s). (G) Ovule containing a FG1 embryo sac. Chalazal end megaspore becomes functional megaspore like (FML) and the other megaspores are degrading. (H) FG1 embryo sac. FML locates at a more chalazal position. (I) FG3 embryo sac containing two nuclei with a vacuole between them. (J-M) Embryo sac development in WT ovules visualized by DIC (also see Additional file 1: Figure S3). (J) Meiotic ovule containing a dyad (stage 2-IV). (K) FG1 ovule. FM (arrow) is uni-nucleate. (L) FG3 ovule, containing an embryo sac with two nuclei and a vacuole. (M) FG4 ovule. (N-P) Embryo sac development in 35S:NTAP:AtCTF7∆B ovules observed by DIC. (N) FG1 embryo sac. The nucellus is surrounded by the outer integument and the inner integument. Extra cells are present between the FM (arrow) and L1 cells. (O-O’) Ovule containing a two-nucleate embryo sac. Non-degenerated L1 cells are present. (P) Ovule with a FG3 embryo sac. FMLs are identified as having distinctly bright nuclear autofluorescence and DMs contain a diffuse signal throughout the cells, but no clearly defined nucleus, defined according to Barrell and Grossniklaus . Size bar = 10 μm. Developmental stages in CLSM and DIC are defined according to Christensen et al. [34,35] and Schneitz et al. , respectively.