Fig. 5. A genetic model for adaxial-abaxial pattern formation and outer integument development during ovule development in Arabidopsis thaliana. (A). Initiation of INO expression. ANT and NZZ act antagonistically and are needed for the correct timing of the onset of INO expression. BEL1 is a pre-requisite for INO expression. (B) Feedback regulation of INO expression. After initiation, INO positively regulates ANT expression, which in turn leads to positive regulation of INO and NZZ expression. NZZ in turn forms a negative feedback on ANT thereby maintaining the levels of ANT and INO expression. (C) Spatial regulation of INO expression. The dashed lines with a question mark indicate that the interaction between NZZ, ATS and SUP is unclear. Lines with arrows indicate activation of transcription. Lines with barred ends represent an inhibitory input.