More than 80,000 angiosperm species produce flowers with petals fused into a corolla tube, a structure that facilitates many specialized plant-pollinator interactions. Very little is known about the genetic control and developmental mechanism of corolla tube formation. Ding et al. show that the tasiRNA-ARF pathway is required for corolla tube formation in the monkeyflower species Mimulus lewisii, by modulating auxin homeostasis during the early stages of flower development. They propose a new conceptual model highlighting the central role of auxin directed synchronized growth of the petal primordia bases and the inter-primordial regions in corolla tube formation. The cover shows a side view of the M. lewisii flower, with a bumblebee about to enter the corolla tube and pollinate the flower.