1. Regulators of Cell Expansion and Development
BIOL3745 Plant Physiology
Unit 3
Chapter 24
Brassinosteroids:
Regulators of Cell Expansion and Development

2. The brassinosteroids (BRs) are steriod hormones that regulate plant development and processes, including
cell division and cell elongation in stems and roots,
photomorphogenesis,
reproductive development,
leaf senescence,
and stress responses

3. Figure 24.1 Bean second-internode bioassay for brassinosteroids
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4. Figure 24.2 Dwarf rice lamina inclination bioassay for brassinosteroids
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5. Figure 24.3 Dose–response curves for three active BRs in the rice lamina inclination bioassay
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6. Figure 24.4 Structures of brassinosteroids
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7. Bioassays distinguish active BRs from inactive intermediates and permit quantitation
BRs are a group of polyhydroxylated steriod hormones, with brassinolide (BL) being the most widespread and active BR in plants
BRs have been detected in all tissues examined with greatest activity in the apical shoot
BRs are ubiquitous plant hormones that predate the evolution of land plants

8. Figure 24.5 Phenotypes of Arabidopsis BR mutants
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9. Figure 24.6 BL and intermediates of BL biosynthetic pathway restore normal growth to cpd mutant
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10. BR deficient mutants are impaired in photomorphogenesis
det2 and cpd were identified in screening for Arabidopsis seedlings that have a light-grown morphology after growing for several days in total darkness
BR-deficient mutants show abnormal photomorphogenesis, which can be prevented with exogenous application of BL (or intermediates downstream of the steps catalyzed by the mutated enzymes).

11. Figure 24.7 Domain structure of the BR receptor BRI1
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12. Figure 24.8 A model for BR signaling
BAK1: BRI1-associated receptor kinase 1
BKI1: BRI1-kinase inhibitor 1
BSK: BR-signaling kinase
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13. Figure 24.8 A model for BR signaling
BL binds to the BRI1 receptor found in the plasma membrane and endosomal membranes
BL binding activates BRI1, which becomes phosphorylated at multiple sites
The activation of BRI1/BAK1 initiates a signaling cascade that leads to BR-regulated gene transcription
The de-phosphorylated forms of BES1 and BZR1 activate or repress BR target genes
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14. Figure 24.9 BL inhibits the phosphorylation of BES1
Highly phosphorylated form
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De-phosphorylated form

15. Figure 24.10 Simplified pathways for brassinolide (BL) biosynthesis and catabolism
All the enzymes converting campesterol to BL are cytochrome P450 monoxygenases located on the ER.
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16. Figure Structure of brassinazole, a triazole compound that inhibits brassinosteroid biosynthesis
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17. Figure 24.11 Brassinosteroid levels are controlled by both negative and positive feedback
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18. Figure 24.13 Effect of brassinazole on light-grown 14-day-old Arabidopsis seedlings
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19. Figure 24.14 Overexpression of the BR biosynthetic gene DWF4 in Arabidopsis
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20. Figure 24.15 Effects of reciprocal grafting between wild type and BR-deficient mutant of pea
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21. BRs
BR levels are regulated through multiple control mechanisms, including catabolism, conjugation, and negative feedback from the signaling pathway
Plant grown on Brz show BR-deficient phenotype, which can be reversed by the addition of BL to their growth medium
BRs act near their sites of synthesis and do not undergo long-distance transport; each organ synthesizes and responds to its own active BRs

22. Figure 24.16 The kinetics of BR stimulation of soybean epicotyl elongation
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23. Figure 24.17 BRs increase the plastic wall extensibility of soybean epicotyls
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24. Wild-type parenchyma cell
Figure Effect of BR on microtubule organization in Arabidopsis seedlings
Wild-type parenchyma cell
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Green: microtubules
Yellow: chloroplasts
(b) BR deficient mutant parenchyma cell
(c) BR-deficient mutant treated with BR

25. Figure 24.19 BL and IAA act synergistically to promote lateral root development
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26. Figure 24.20 BR is required for a normal vascular development
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27. BRs are essential for this differentiation process
Figure Zinnia leaf mesophyll cell before and after differentiation into a tracheary element
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BRs are essential for this differentiation process

28. Figure 24.22 BR stimulates germination of Arabidopsis seeds
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29. BRs: Effects on Growth and Development
Involved in development of fiber, lateral roots, and vasculature, as well as maintenance of apical dominance, pollen tube growth, seed germination, leaf senescence, and plant defenses
Promote both cell proliferation and cell elongation
Maintain normal microtubule abundance and organization needed for cell wall growth
Promote root growth at low concentrations and inhibit root growth at high concentrations

30. BRs: Effects on Growth and Development
Promote lateral root development by altering polar auxin transport
Promote differentiation of the xylem and suppress that of the phloem
Promote seed germination by interacting with other hormones, such as GA and ABA
BR application to crop plants is most effective under stress conditions
BRs are useful in plant propagation