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chrystina Kiwan

on 29 April 2013

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Chrystina Kiwan, Rasha Shimlati & Carla Stephan TABLE OF CONTENTS BRS DEFICIENCY BIOSYNTHESIS OCCURRENCE & LOCALIZATION HISTORY STRUCTURE INTRODUCTION TRANSPORT METABOLISM Brassinosteroids act near their site of synthesis --> each organ synthesizes in response to its own BRs.
Therefore, they do not undergo root to shoot translocation. The level of active BRs is regulated by metabolic processes that inactivate BL.
It is also controlled by negative & positive feedback.
Several types of reactions result in BL inactivation:
-->Conjugation to glucose/lipids Brassinosteroids:

group of steroid hormones
over 70 types
most abundant is Brassinolide (BL)
major role in plant development
found in over 27 families of seed plants, as well as green algae & bryophytes Brassinosteroids get their name from the rape plant Brassica napus.
J. Mitchell discovered it 40 years ago from the pollen of rape plant.
Promotes their growth Brassinosteroids are steroidal lactones. Found in low amounts in almost all plant organs including:
(especially abundant in young tissues) Introduction
Occurrence & Localization
Brassinosteroid Deficiency
Effects on Growth & Development
Agricultural Uses Brassinosteroids promote:

1. Cell expansion & division
2. Root Growth
3. Xylem Differentiation
4. Pollen Tubes
5. Seed Germination EFFECTS ON GROWTH & DEVELOPMENT BR & cytokinins stimulate cell proliferation.
Young vegetative tissue is especially responsive.
Auxins also promote elongation but have different kinetics.
BR have lag times of at least 45 min with elongation rates continuing to increase for several hours.
--> slower pathway involving gene transcription CELL DIVISION & ELONGATION Cell expansion involves cell wall relaxation followed by osmotic transport of water into the cell to maintain turgor pressure & cell wall synthesis to maintain wall thickness.
BR increase the uptake of water through aquaporins, enhance cell loosing & induce expression of wall modifying enzymes. (XTH, expansin) CELL EXPANSION ROOT GROWTH Required for normal root elongation.
If applied exogenously, BR promote root growth at low [ ] & inhibit it at high [ ].
BR effect on root growth is independent of auxin & gibberellin action.
At low [ ], BR can induce formation of lateral roots. Yet, in these conditions, BR & auxin act together.

Affects overall root morphology, influencing elongation rate & branching habit. XYLEM DIFFERENTIATION BR play an important role in vascular development by:
Promoting differentiation of the xylem.
Suppressing that of the phloem. Pollen --> a rich source of BR, hence BRs are IMPORTANT for MALE FERTILITY.
BR shown to promote growth of the pollen tube from the stigma through the style to the embryo sac.
Eg: In BR-deficient Arabidopsis mutant cpd, pollen tubes failed to elongate after germination on the stigma --> pollen tube elongation was shown to be partially dependent on BR application. POLLEN TUBE GROWTH BR-insensitive mutant with a defective receptor gene, self-pollinated --> pollen tube fails to develop --> STERILE SEEDS.
Mutant is hand pollinated with wild-type pollen --> FERTILE SEEDS.

Conclusion: For normal pollen tube growth, both BR & BR signaling pathway are needed. POLLEN TUBE GROWTH Reduced male fertility is also attributed to a discrepancy in the heights of stamens vs pistil.
Stamens of the mutant flower are also shorter than those of the wild type.
Because Arabidopsis is self-pollinating, the shorter filaments of dwf4 stamens result in less pollen being deposited on the stigmatic surface. POLLEN TUBE GROWTH SEED GERMINATION Seeds have very high levels of BR.
BR promote seed germination:
Interact with other plant hormones
Gibberellic acid (GA) & Abscissic acid (ABA) play positive & negative roles, respectively, in stimulating seed germination. BR can enhance germination of tobacco seeds, independent of GA signaling.
Needed to overcome inhibitory effect of ABA.
(BR mutants are more sensitive to the inhibition by ABA than the wild type)
Facilitate germination by stimulating embryo growth. Brassinosteroids are also known to:
promote apical dominance
promote leaf senescence
enhance gravitropism
increase ethylene production
inhibit stomatal formation
prevent premature fruit abscission
increase resistance to freezing
increase the yield of crops OTHER DEVELOPMENTS For the past 20 years, tests were done for BRs activity to increase crops yields.
BL: increases bean crop yield (based on weight of seeds/plant) by about 45 % & enhances leaf weight of various lettuce varieties by 25%
Similar increases in yields of rice, barley, wheat & lentils
BL promotes potato tuber growth & increases its resistance to infection.
Works best under stress conditions. AGRICULTURAL USES AGRICULTURAL USES BR: useful aid for plant propagation.
Reduced BR function can contribute to agriculture too.
Eg: Decreased BR synthesis/signaling in rice results in dwarfed plants with an upright leaf habit
-->which allows higher planting densities -->increases biomass & final seed yield. THANK YOU!
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