Brassinosteroids

Brassinosteroids are a class of polyhydroxylated plant steroids that function as hormones essential for normal plant growth and development. They are synthesized in most vascular plants and include highly active compounds such as brassinolide and castasterone.

Biosynthesis and metabolism occur through a multi-step pathway starting from sterol precursors such as sitosterol and campesterol. Enzymes in the pathway, including cytochrome P450 family members like DWF4 and CPD, introduce hydroxyl groups and other modifications to produce castasterone and brassinolide as end products in many species. The brassinosteroid pathway is tightly regulated by feedback mechanisms that adjust enzyme levels and BR inactivation to maintain hormonal balance.

Perception and signaling begin at the cell surface with the BRI1 receptor kinase, which forms a complex with the co-receptor BAK1. Upon BR binding, a signaling cascade modulates a GSK3-like kinase called BIN2. In the BR-present state, BIN2 activity is suppressed, leading to the dephosphorylation and activation of the transcription factors BZR1 and BES1. These factors accumulate in the nucleus and regulate BR-responsive gene expression, coordinating growth and development. BR signaling also interacts with other hormonal pathways, such as auxin and gibberellins.

Physiological roles of brassinosteroids include promotion of cell elongation and division, vascular differentiation, leaf angle, and fertility. They influence photomorphogenesis and stress responses, contributing to tolerance to temperature stress and drought in some contexts. Exogenous brassinosteroids have been explored as growth promoters and stress mitigators in crops, though responses are species-dependent and subject to regulatory considerations.