Optimal brassinosteroid homeostasis is required to maximize Arabidopsis hypocotyl growth in the dark

Plants produce a group of steroid hormones named brassinosteroids (BRs), with brassinolide (BL) and castasterone (CS) exhibiting highest biological activity. The maintenance of appropriate BR homeostasis is crucial for plant growth and adaptation to environmental conditions. BRs negatively regulate Arabidopsis thaliana seedling photomorphogenesis by promoting hypocotyl elongation in the light. However, both BL and the BR biosynthesis inhibitor brassinazole (BRZ) suppress Arabidopsis hypocotyl growth in the dark, indicating a seemingly contradictory role of BRs in seedling skotomorphogenesis. Using wild-type Col-0 and two independent BR-deficient mutants previously generated by ectopically overexpressing the grapevine BR-inactivating enzyme CYP734A15 in Arabidopsis, we quantified dark-grown hypocotyl length changes in response to BL or BRZ treatments at a wide range of concentrations. The results revealed that BR homeostasis in wild-type Arabidopsis seedlings is nearly optimal yet slightly excessive for achieving maximal hypocotyl growth in the dark. Gentle BRZ treatments slightly reduce seedling BR levels to maximize hypocotyl growth. Treatments with BL and higher concentrations of BRZ suppress hypocotyl growth via distinct mechanisms of overdosing BRs to toxic levels and further reducing BRs to insufficient levels, respectively. For dark-grown BR-deficient mutants, mild BL treatments boosted their BR levels to promote hypocotyl growth, whereas further increases of BL supply were overdosed for hypocotyl growth. BRZ treatments on BR-deficient mutants further reduced the already insufficient BR levels, making their hypocotyls even shorter. In conclusion, optimal BR homeostasis is required to maximize Arabidopsis hypocotyl growth in the dark.