In plants, many developmental events, such as flowering and tuberisation(the formation of thickened underground stems), are controlled by day length,a means of determining when they occur in the year. The molecular mechanisms involved are somewhat understood for flowering, but remain largely unknown for tuberisation. Now, on p. 2873, Paula Suárez-López and co-workers demonstrate that in potatoes, the microRNA miR172 regulates both flowering and tuberisation. The authors show that miR172 levels increase when potato plants are grown under tuber-inducing short-day conditions. At the onset of tuberisation, miR172 levels increase specifically in stolons, the stems from which tubers arise. miR172 overexpression promotes not only flowering, as seen in other plants, but also tuberisation,and it increases the mRNA levels of the tuberisation promoter BEL5. The depletion of the photoreceptor PHYB, a repressor of tuberisation,increases stolon levels of both BEL5 mRNA and miR172. Future work should shed light on whether the dual function of miR172 in flowering and tuberisation is conserved in other plant species.