Mutations that affect auxin-mediated apical-basal patterning in Arabidopsis and the role of TOPLESS. (a) Schematic diagram of wild-type (WT), topless (tpl), monopteros (mp), bodenlos (bdl), mp tpl, bdl tpl, and bdl mp seedling phenotypes. The temperature-sensitive tpl mutant displays apical patterning defects at permissive temperatures (24°C), whereas restrictive temperatures (29°C) result in complete homeotic transformation of the seedling shoot to root. In the double mutants, tpl can suppress patterning defects in both mp and bdl. (b) Proposed function of the TOPLESS (TPL) corepressor protein in the nucleus. TPL can homodimerize, act in a complex with IAA12/BDL and MP, and hypothetically interact with other EAR-domain-containing proteins. The CTLH domain of TPL and the EAR domain of BDL mediate the interaction between TPL and BDL. Domains III and IV of BDL are found in all AUX/IAA proteins and mediate interaction with similar domains in ARFs, such as MP.