BRASSINOSTEROID

Brassinosteroids (BRs) are a class of plant steroid hormones that regulate growth and development. They are polyhydroxylated sterols, with brassinolide and castasterone among the most active forms. BRs are found throughout plant tissues and act at very low concentrations to influence processes such as cell elongation, vascular differentiation, reproductive development, and responses to light and stress.

Biosynthesis and metabolism occur from sterol precursors through a branched pathway involving several cytochrome P450 enzymes, including DWF4 and CPD. The pathway culminates in the production of brassinolide and castasterone. BR biosynthesis is tightly regulated by feedback mechanisms, and inhibition of BR production—by chemicals such as brassinazole, for example—leads to dwarf, dark-green phenotypes in many plants.

Perception begins at the cell surface with the BR receptor BRI1, which forms a complex with the co-receptor BAK1 upon BR binding. This activates a phosphorylation cascade that suppresses the activity of the GSK3-like kinase BIN2. Inactivation of BIN2 allows dephosphorylation and activation of the transcription factors BZR1 and BES1, which accumulate in the nucleus and regulate BR-responsive gene expression.

Physiological roles of BRs include promotion of cell expansion and shoot growth, regulation of leaf and stem development, and involvement in vascular differentiation and reproductive development. BR signaling interacts with other hormones such as auxin and gibberellins, contributing to photomorphogenesis, stress responses, and environmental adaptation. The BR pathway is essential for normal growth in many species, and its disruption yields characteristic dwarf phenotypes.

In agriculture, BRs and components of BR signaling are studied for crop improvement, while BR-deficient or BR-insensitive mutants provide valuable models for understanding plant growth regulation.