Age-related responsiveness to vernalization controlled by the miR156/SPL and miR172/AP2-like modules in A. alpina, a perennial relative of Arabidopsis. Top row, Young plants that have not achieved competence to flower are exposed to vernalization. In the shoot apex, miR156 is broadly expressed across the SAM and leaves, and the abundance of miR156 remains high during vernalization. Flowering does not occur during vernalization. After vernalization, miR156 is down-regulated and SPL-encoding genes are expressed, but flowering does not proceed until the plants are vernalized. The AP2 ortholog PEP2 is expressed at all stages throughout the plant and is shown here at the SAM. Bottom row, Older plants that have achieved competence are exposed to vernalization. The age-related down-regulation of miR156 occurred, allowing the expression of SPL-encoding genes at the shoot apex. The level of miR172 is increased markedly at the SAM during vernalization, presumably through the activity of SPLs. The increase of miR172 at the SAM inhibits the accumulation of the floral repressor PEP2 in the center of the meristem. Flowering is induced during vernalization, and floral meristem identity genes such as LFY are expressed. In another perennial Brassicaceae species, C. flexuosa, the miR172 level was shown to be increased during the growth of adult plants even before vernalization. The miR172/AP2 module also plays a role in the floral meristem formed on the flanks of the SAM to determine the identity of developing floral organs. This figure is based on data from Bergonzi et al. (2013) and Zhou et al. (2013).