HD mutants exhibit loss-of-function phenotype in leaves, seeds, and roots. A, Schematic of wild-type GL2 and mutant gl2^ΔHD and gl2^ΔSTART proteins containing HD, ZLZ, START, and STAD. Amino acid lengths of proteins are indicated (right). Alignment of GL2 and PDF2 HD domains generated by Geneious 7.0.6 (Biomatters). Identical and similar amino acid residues are indicated by black and gray shading, respectively. See Supplemental Figure S1A. The various HD deletions are shown below the alignment. Lysine targeted for missense mutations (K-E or K-T) is indicated by box and asterisk. B and B′, Wild-type GL2 rosettes display trichomes covering leaf surfaces. In contrast, mutants (gl2^ΔST, gl2^ΔHD46, gl2^ΔHD20, gl2^ΔHD11, gl2^K146E, gl2^ΔHD20;ΔST, gl2^K146T;ΔST, and gl2^K146E;ΔST) exhibit trichome differentiation defects. gl2^K146T displays an intermediate phenotype. Bar = 1 mm. C and (C′, Left: Ruthenium red staining of seeds to assay mucilage production. Wild-type GL2 seeds display a normal mucilage layer, while the various gl2 mutants exhibit defective mucilage production. gl2^K146T displays a partial phenotype. Bar = 100 μm. Right: Root hair phenotypes of 4- to 5-day-old seedlings. In contrast to wild-type GL2 and the partial phenotype of gl2^K146T, all other mutants exhibit excessive root hair formation. Bar = 1 mm. D, Total number and branching pattern of trichomes on the first leaves. Gray bars indicate trichomes with two to five branches in plants expressing wild-type EYFP-tagged GL2. Mutant plants of the genotypes gl2-5, and the other gl2 mutants exhibit unbranched trichomes, indicated by red bars. gl2^K146T plants display partial trichome differentiation defects. Values are mean ± sd for n = 20 plants. Letters denote significant differences from ordinary one-way ANOVA (P < 0.05) using Tukey’s multiple comparisons test. E, Numbers of root hairs on primary roots of 4- to 5-day-old seedlings. Genotypes correspond to those in (D). All the gl2 mutants display increases in the number of root hairs. The gl2^K146T seedlings exhibit an intermediate phenotype. Values are mean ± sd for n = 6–10 seedlings. Letters denote significant differences from one-way ANOVA (P < 0.05) using Tukey’s multiple comparisons test.