Brassinosteroids are a class of plant hormones that were first isolated in 1979. They occur at low levels throughout the plant kingdom, with young tissues and pollen containing the highest amounts. Over 50 brassinosteroids have been identified that are involved in numerous growth and developmental processes in plants. Brassinosteroids signal through a receptor kinase pathway involving BRI1, BAK1, BSU1, BIN2, BZR1/2, and 14-3-3 proteins to regulate gene expression and promote cell expansion, elongation, and stress responses.

1. Brassinosteroids and plant
growth

2. Brassinosteroids:
• polyhydroxy steroids that have been recognized as a sixth class of
plant hormones.
• first explored by Mitchell et al. nearly 40 years ago.
• Brassinolide was the first isolated brassinosteroid in 1979.
• A pollen from Brassica napus was shown to promote stem elongation
and cell divisions.
• The yield of brassinosteroids from 230 kg of Brassica napus pollen
was only 10 mg.

3. Cont…
• Till date about 50 naturally occurring BR have been identified.
• These can be classed as either C27, C28 or C29 steroids.
• BRs are perceived by the plasma membrane receptor BRI1.
• Essential component to regulate the processes driven by BR.

4. Structure of Brassinosteroid:

5. Occurrence in plants:
• Found at low concentrations throughout the plant kingdom.
• They have found in plant organs such as pollens, anthers, seeds,
leaves, stems, roots, flowers, and grains.
• Young growing tissues contain higher levels of BRs than mature
tissues.
• Pollen and immature seeds are the richest sources of BRs with a
range of 1–100 μg kg−1 fresh weight.
• shoots and leaves usually have a lower amounts of 0.01– 0.1 μg kg−1
fresh weight.

6. Biosynthesis:
Campesterol
DET2
Campestanol 6-oxocampestanol
DWF4 DWF4
6-Deoxocathasterone Cathasterone
Late C-6 Early C-6 oxidation pathway
oxidation CPD CPD
Pathway
6-Deoxoteasterone Teasterone
6-Deoxotyphasterol Typhasterole
6-Deoxocastasterone Castasterone
Brassinolide

7. Role in plant growth:
• play key roles in regulation of plant growth and development.
• vegetative and reproductive development, germination, senescence.
Involved in numerous physiological processes.
• Promotion of vascular differentiation.
• Promotion of cell expansion and cell elongation.
• Necessary for pollen elongation for pollen tube formation.

8. Cont..
• Provide some protection to plants during chilling and drought stress.
• Help in leaf development.
• Affect the activity of numerous metabolic pathways.
• Interact with ethylene, abscisic acid, gibberellic acid in terms of
growth regulatory processes.

9. Mode of action of Brassinosteroid:
• Signaling pathway is also essential for BRs to regulate biological
processes.
• Components involved in signaling pathway:
1. BAK1- Associated receptor kinase 1
2. BSU1- Suppressor 1
3. BZR1- Brassinazole resistant 1
4. BIN2- kinase
5.14-3-3 proteins

10. Mechanism:
1. BR binds to the extracellular domain of the BRI1 receptor kinase.
2. Induce dimerization and Causes a conformational change in domain.
3. Activate the kinase catalytic activity.
4. Cause phosphorylation of the BAK1 receptor kinase.
5. Signal is transduced through an mechanism to inhibit BIN2 kinase or
activate BSU1 phosphatase.
6. Leading to dephosphorylation of BZR1/2.

11. BR signaling pathway:

12. Cont..
• Results proteasomal degradation of BZR1/2.
• promoting its binding to the 14-3-3 proteins.
• BR-induced dephosphorylation increases DNA binding activity of
BZR1 and BZR2.
• Leading to BR-responsive gene expression and growth responses.