Involved in the regulation of tiller growth angle (PubMed:17908158). Promotes horizontal shoot growth (PubMed:17908158). TAC1 and LAZY1 play opposite functions in the regulation of tiller growth angle (PubMed:17908158). Expressed in the leaf sheath pulvinus, tiller base and tiller node. Plants silencing TAC1 exhibit erect tillers with a very small tiller angle. Belongs to the TAC family.