Function of the START domain in HD-Zip transcription factor GL2 from Arabidopsis . (A) Rosettes exhibiting leaf trichomes in wild-type (WT), and reduction of trichomes in the gl2 null mutant. Scale bar: 2 mm. (B) Roots were germinated on 0.8% agar medium and imaged after 3 to 5 days. gl2 mutant exhibits excessive root hairs in comparison to WT. Scale bar: 200 μm. (C) Mucilage of WT seeds was stained with ruthenium red after imbibition. gl2 mutants lack mucilage layer. Scale bar: 20 μm. (A, B, C) EYFP:GL2 displays complete rescue of the mutant phenotype while EYFP:GL2-StAR-START (pink) and EYFP:GL2-ATML1-START exhibit partial rescue. The phenotypes of EYFP:gl2ΔSTART, EYFP:GL2-EDR2-START and EYFP:GL2-REV-START are indistinguishable from gl2. (D) Schematic diagrams of the EYFP:GL2 and EYFP:gl2ΔSTART proteins depicting enhanced yellow fluorescence protein (EYFP), homeodomain (HD), Zip-Loop-Zipper (ZLZ) and START-adjacent domain (SAD). For specific constructs, the START domain was deleted or replaced with a heterologous START domain. (E) Trichomes were counted on the first leaves of 1-week-old plants grown on soil. Error bars indicate standard deviations for n = 20. (F) Amino acid similarities and identities of Arabidopsis ATML1, REV and EDR2 START domains (green) and mouse StAR START domain (pink) in comparison to the Arabidopsis GL2 START domain. Amino acid sizes of corresponding START domains are indicated in parentheses. aa, amino acid; ATML1, Arabidopsis thaliana Meristem Layer 1; EDR2, Enhanced Disease Resistance 2; EYFP, enhanced yellow fluorescent protein; GL2, Glabra2; HD, homeodomain; REV, Revoluta; SAD, START adjacent domain; StAR, steroidogenic acute regulatory protein; START, StAR-related lipid transfer; WT, wild type.