Brassinosteroids

Jia Li, University of Oklahoma, Norman, Oklahoma, USA
Xiaoping Gou, University of Oklahoma, Norman, Oklahoma, USA

Published online: April 2007

DOI: 10.1002/9780470015902.a0020092

Brassinosteroids (BRs) are specific phytosteroids necessary for ordinary plant growth and development. BRs share similar chemical structures with animal steroidal hormones, but show a distinctive signal perception mechanism. The importance of BRs is illustrated by the typical phenotypes of mutants with lesions in key biosynthetic or signalling proteins. These mutants exhibit severe dwarfism, curled and round leaves, considerably delayed senescence, reduced male fertility, altered light-regulated developmental programmes, changed responses to abiotic and biotic stresses and modified gene expression profiles. Extensive studies within the last three and a half decades have resulted in a comprehensive picture of BRs, from their nearly completed biosynthesis and physiological functions, to their mechanisms of action.

Keywords: brassinosteroids; phytohormones; brassinolide; arabidopsis; BRI1; BAK1

Figure 1. BL chemical structure and representative BR signalling and biosynthetic mutant phenotypes. Brassinolide, the most active and widespread BRs, is a C28 steroidal lactone. The carbons are numbered; and rings are labelled by letters. There are over 60 BRs identified in plant kingdom. They can be grouped as C28, C29 and C30 BRs. The variations are basically found in rings A and B, as well as in the side-chain. BR mutants, either signal transduction mutants (such as bri1-5) or biosynthetic mutants (such as det2-28) show similar defective phenotypes.
Figure 2. A simplified BR biosynthetic pathway. BRs are biosynthesized from primary metabolic products, acetyl-CoA, to form mevalonate. C6 mevalonate is then converted into C5 isopentenyl pyrophosphate (IPP), which is the building unit for various terpenoids including BRs. Steps from campesterol to BL are considered BR-specific biosynthesis pathway. There are two coexisting pathways converting campestanol to CS; one is called later C-6 oxidation pathway; and the other is called early C-6 oxidation pathway. Recently, it was found that conversion of campesterol to campestanol is not a single step as predicted. Several intermediates were isolated and these steps were named early C-22 oxidation pathway. The enzymes indicated are all from Arabidopsis. Several corresponding enzymes were also identified in other plant species and they are not marked in this figure. The dashed arrow represents the unconfirmed step. Modified from Fujioka and Yokota (2003).
Figure 3. Representatives of BR inactivation reactions. From feeding experiments, it was predicted that many reactions can turn an active BR to an inactive form. Up-to-date, only three inactivation reactions have been experimentally confirmed and their catalytic enzymes have been identified, which are shown in this figure.
Figure 4. The structures of BRI1 and BAK1. BRs are perceived by a cell surface LRR-RLK, BRI1. The second, LRR-RLK, BAK1, is also critical in regulating BR signal perception via its dimerization with BRI1. BL interacts with the extracellular portion of BRI1, including the 70-aa ‘island’ and the 22nd LRR. The direct interaction of BL with BAK1 extracellular domain has not been demonstrated.
Figure 5. A proposed BR signal transduction pathway based on updated information as discussed in the text. In this model, the inactive form of BRI1 is a dimer. BKI1 interacts with the kinase domain of BRI1 and blocking its association with BAK1. There may be an inhibitory protein or peptide that can block the binding of BL to its receptor. BRS1 may be involved in degrading this unidentified inhibitory protein. Binding BL with BRI1 extracellular domain can trigger BRI1 autophosphorylation, and subsequently induce its heterodimerization with BAK1. The activated BAK1 is responsible for an indirect modification of BIN2 via an undefined protein (kinase) and inactivate BIN2. If BAK1 is not activated, the active BIN2 can enter the nucleus and inactivate two major transcription factors BZR1 and BES1.
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Related Sub-Topics:

Intracellular and Extracellular Signalling | Plant Secondary Metabolism
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Article Title: Brassinosteroids
 
How to Cite close
Li, Jia, and Gou, Xiaoping(Apr 2007) Brassinosteroids. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020092]