The etiolation response in TRS120 phosphovariants. (A) TRS120-WT (P_TRS120::TRS120:GFP in trs120-4/trs120-4) (Rybak et al., 2014). (B) The non-phosphorylatable TRS120 mutant TRS120-SαβγA in trs120-4/trs120-4. (C) The phosphomimetic TRS120 mutant TRS120-SαβγD in trs120-4/trs120-4. Seedlings were grown on ½ MS media under light and dark conditions. All mutants showed a significant adaptation to light versus dark conditions, i.e., increased hypocotyl growth and reduced root growth leading to a higher hypocotyl/root ratio under dark conditions. For each mutant, four biological replicates were analyzed, and the representative line based on the RQ and P values is depicted here. Values are means ± SD of measured seedlings per condition, number (n) of seedlings are given in gray below the graphs. Stars indicate statistical significance with a two-tailed Student’s t test (*****: P < 0.00001). (D–F) Response quotients (RQ) of (D) the hypocotyl, (E) the root, and (F) the hypocotyl/root ratio of the non-phosphorylatable TRS120-SαβγA and the phosphomimetic TRS120-SαβγD mutants for light-to-dark adaptation. RQ response quotients are normalized to the TRS120-WT control, P_TRS120::TRS120:GFP in trs120-4/trs120-4. A value of 1 (vertical red line) corresponds to an identical adaptation to dark conditions as the control. TRS120-SαβγA and TRS120-SαβγD phosphovariants did not measurably differ from the control (TRS120-WT) line, with RQ values close to 1 (TRS120-SαβγA: mean RQ_ratio = 0.86; TRS120-SαβγD: mean RQ_ratio = 0.83). Note that due to the opposite adaptations of the hypocotyl and root from light-to-dark conditions (increase in hypocotyl, decrease in root length and therefore increased hypocotyl/root ratio), the respective thresholds for attenuated and enhanced responses are opposite. Dots represent biological replicates, mean RQ values are given on the right. Related to Fig. 7.