The csu3 cop1-6 mutant shows a three-cotyledon phenotype, and csu3 partially suppresses cop1-6 in the light. (A and B) The cotyledon phenotypes (A) and ratios of three cotyledons (B) of Col, cop1-6, and csu3 cop1-6 mutant seedlings grown under white light conditions for 5 d. (C) The adult phenotypes of Col, cop1-6, and csu3 cop1-6 mutant seedlings grown in long-day (16-h light/8-h dark) conditions for 52 d. (D–K) Hypocotyl phenotypes and lengths of 5-d-old Col, cop1-6, and pid-15 cop1-6 mutant seedlings grown under white (D and E), blue (F and G), red (H and I), and far-red (J and K) light conditions. Error bars represent SE (n ≥ 20). csu3 cop1-6 (2) and csu3 cop1-6 (3) indicate csu3 cop1-6 seedlings with two cotyledons and three cotyledons, respectively. In E, G, I, and K, letters above the bars indicate significant differences (P < 0.05) as determined by one-way ANOVA with Tukey’s post hoc analysis.