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What are Brassinosteroids?
Brassinosteroids (denoted as BRs) have been identified as the sixth class of plant hormones. They are polyhydroxylated sterol derivatives that are found distributed throughout the plant kingdom. About 70 forms of naturally occurring BRs have been known since their discovery in the 70s. It is a plant hormone that is important for its growth and development. They are quite similar to animal steroid hormones both structurally and functionally.
Discovery
Mitchell et al. first reported elongation in stems and promotion in cell division by treatment with organic extracts of rapeseed pollen (Brassica napus). The first BR, brassinolide, a biologically active molecule, was extracted from the pollen of Brassica napus in 1979. Since then, many chemically related compounds have been discovered and included in the brassinosteroid family.
Biosynthesis of Brassinosteroids
Brassinolide, the most active of the brassinosteroids, has been derived from campesterol. It undergoes a series of reductions, hydroxylations, epimerisations and oxidations to form the compound. The pathway can be branched into two: early and late C-6 oxidation. Early oxidation is when a ketone is added to the C6 carbon before side chain hydroxylation, and late is when the ketone is added after hydroxylation. However, recently, it was concluded that it is just a metabolic grid and not two different pathways.
The site of brassinosteroid synthesis in plants is not known; it is assumed that all parts produce the hormone because they are widely expressed in all plant organs. Genes for the biosynthetic production of brassinosteroids have been cloned in Arabidopsis thaliana.
Functions of Brassinosteroids
Brassinosteroids perform the following functions in a plant:
- It works in conjunction with auxin for cell expansion and elongation.
- It also takes part in cell division and cell wall regeneration.
- It induces signal transduction that promotes vascular differentiation.
- It promotes pollen elongation, which is an important step for pollen tube formation.
- It accelerates senescence in dying tissue cultured cells.
- It provides protection to plants during the time of drought stress and chilling.
- It counteracts abiotic and biotic stresses in plants.
In a scientific study, the application of brassinosteroids to cucumbers showed increased metabolism and removal of pesticides, thus diminishing the amount of pesticides that are digested by humans. Application of the hormone to rice seeds demonstrated an increase in their fresh weights as compared to non-treated plants.
Working of Brassinosteroids
At the time of cell elongation, brassinosteroids alter the mechanical properties of the cell wall by both genomic and non-genomic methods. It allows the expansion of cells by turgor-driven methods. Upon testing phenotypes that were deficient in BR, it was confirmed that they are essential for cell elongation.
Receptors of Brassinosteroids
Brassinosteroids are recognised at the cellular level by a co-receptor complex that is composed of BRI1 (brassinosteroid insensitive-1) and BAK1 (BRI1-associated receptor kinase 1). BRI1 is a kinase that is inhibited by BKI1 (BRI1 kinase inhibitor 1) in the absence of BRs. Upon binding of BR to the co-receptor complex, BKI1 is released, which triggers a phosphorylation cascade and leads to the deactivation of BIN2 (brassinosteroid insensitive 2), another kinase. BIN2 inhibits the transcriptional factors. On the other hand, when BR inhibits BIN2, the transcriptional factors are activated which include several developmental pathways.
Commercial Potential
Application of brassinosteroids to rice, grapes, tomatoes, wheat, corn, potatoes and melon has shown an increase in their yield. It has the capability to improve the quality and quantity of various horticultural crops. It works better when the plant is stressed rather than in optimal conditions.
One of the biggest advantages of using BRs on plants is that it is a natural compound that does not harm the environment. It is referred to as a natural plant-strengthening substance; hence, its use is preferred over pesticides.
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Also Read:
- Plant Hormones: Definition, Types and Function
- Structure, Functions and Uses of Cytokinins
- MCQs on Plant Hormones for NEET 2022
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