The role that microRNAs - short, single-stranded RNAs that bind to mRNA -play in gene regulation during plant and animal development has only recently begun to emerge. Now, Wu and Poethig report that the microRNA miR156promotes the vegetative phase change in Arabidopsis by temporally regulating the transcription factor SPL3, and probably also SPL4 and SPL5 (see p. 3539). In plants,the shoot apex progresses through juvenile and adult phases of vegetative development before switching to reproductive development. Wu and Poethig show that all three SPL genes promote the vegetative phase change and the switch to flowering, and that their effect on development is strongly repressed by miR156. The juvenile-to-adult transition, they report,is accompanied by a decrease in the level of miR156 and an increase in SPL3 mRNA abundance; other experiments indicate that the decrease in miR156 is responsible for this increased SPL3 expression. Thus, conclude the researchers, temporal variation in microRNA expression plays a regulatory role in developmental timing in plants.