Brassinosteroids (BRs) and photomorphogenesis: Hormonal regulation of light-mediated plant development

Abstract

To adapt to varying environmental conditions, plants have developed complex mechanisms to optimize growth and development throughout their lifecycles. Light is the primary energy source that drives photosynthesis and act as an environmental signal, which is involved in regulating various plant physiological functions. Plants detect changes in external light conditions through multiple photoreceptors. Photomorphogenesis is characterized by the opening and expansion of the cotyledon, suppression of hypocotyl elongation, and the development of proplastids into fully mature chloroplasts in response to light. The Brassinosteroids (BRs) are polyhydroxylated steroidal hormones—important for plant growth, development, and productivity, including photomorphogenesis. While promoting hypocotyl elongation, BRs fine-tune light-responsive photomorphogenesis in plants. Since BRs are eco-friendly and non-toxic phytohormones, they promote plant adaptability to changing environmental conditions and various biotic and abiotic stresses, thereby maintaining ecological equilibrium. Light and BR antagonistically regulate the transition from skotomorphogenesis in darkness to photomorphogenesis in light. Photomorphogenic repressors, such as Arabidopsis (Arabidopsis thaliana) G-protein β subunit (AGB1), PHYTOCHROME-INTERACTING FACTORS (PIFs), and CONSTITUTIVELY PHOTOMORPHOGENIC PROTEIN 1 (COP1), have been reported to enhance BR response, whereas factors that promote photomorphogenesis, including NF-YCs, bzr1–D suppressor1 (BZS1), and ELONGATED HYPOCOTYL 5 (HY5), have been reported to inhibit BR signaling. BRASSINAZOLE RESISTANT 1 (BZR1) and BRASSINOSTEROID-INSENSITIVE 2 (BIN2) cross-talk with the transcription factors (TFs), playing a crucial role in the light signaling pathway to orchestrate photomorphogenesis. This review discusses BRs, photomorphogenesis, and the interaction between photoreceptors and BRs. It outlines the physiological roles of BRs in plant development during photomorphogenesis and elucidates the molecular mechanism of BRs in regulating photomorphogenesis.