Figure 1

Model for gene action in the shoot apical meristem and its derivatives, the leaves. Scanning electron micrograph of a vegetative shoot apical meristem. Leaf primordia are shaded in green. Active genes are indicated in red letters; inactive genes are indicated in blue letters. In the meristem STM is on and keeps AS1 off. This allows the KNAT1 genes as well as other genes important for meristem development (and yet to be identified) to be active. In the leaf primordium STM is off in response to some as yet unknown signal. This allows AS1 to be active and to repress KNOX gene expression in the leaf.