The source of BR
Pollen and immature seeds. Young growing tissues contain higher BR levels, and BR in pollen extract promotes stem elongation and cell division.
BR Biosynthesis and Metabolism
BR biosynthesis follows the mevalonic acid/isoprenoid pathway. Key enzymes in the pathway include cytochrome P450s (e.g., CYP85A3, responsible for converting castasterone to brassinolide).
Localized Synthesis: Evidence suggests BRs are synthesized within specific plant tissues where they act. Grafting experiments reveal that localized BR production is critical for normal growth.
BR Physiological Effects
Cell Elongation: BRs promote elongation by modifying cell wall extensibility and microtubule organization.
Root Growth: Their effects depend on concentration; low levels promote elongation, while high levels inhibit it.
Reproductive Functions: Necessary for filament elongation, pollen viability, and tube formation.
Fruit Ripening: Endogenous BR levels rise during ripening, and their application can accelerate the process.
BR Mutants and deficiencies
BR-deficient mutants, like those with defective CYP85A1 or CYP85A3 genes, exhibit dwarfism, male sterility, and vascular irregularities.
Inhibitors like brassinazole disrupt normal BR biosynthesis, mimicking mutant phenotypes and altering tissue differentiation patterns.
BR in plants vs animal steroids
While animal steroids act via intracellular receptors, plant BRs function through membrane-bound receptors, emphasizing the evolutionary divergence in hormone signaling mechanisms between plants and animals.
Key Roles of BR
Growth and Development: BRs regulate cell division, elongation, and differentiation. They enhance stem elongation and hypocotyl growth.
Reproductive Development: BRs are critical for stamen and pollen development, affecting male fertility and pollen tube growth.
Stress Tolerance: They enhance resistance to biotic (pathogens) and abiotic (drought, salinity) stresses.
Defense Mechanisms: BRs contribute to plant immunity and repair.
Vascular Differentiation: They promote the formation of xylem and phloem, essential for nutrient and water transport.
BR signaling
- BR is perceived by a plasma-membrane localized
receptor, BRI1 - In the absence of BR, BRI1 is complexed with BKI1
- BR-binding causes BRI1 to autophosphorylate and
bind BAK1, enhancing BRI1’s activity - The BRI1/BAK1 complex phosphorylates and
activates BSU1, a protein phosphatase, which
dephosphorylates BIN2, inactivating it - Dephosphorylated BES1and BZR1 move into the
nucleus and activate or repress BR-regulated genes
