Chapter 24

Considered plant hormones because of their ability to cause dramatic changes in growth and differentiation at low concentrations

Brassica napus 40 kg pollen → 4 mg Effects only demonstrable

with exogenous application

until Arabidopsis mutants discovered 10-7 M concentrations produces effects

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Polyhydroxylated sterols Sterols are terpenoid

Identified in 27 families Gymnosperm Angiosperm Pteridophyte -- Equisetum Bryophyte -- Marchantia Algal -- Hydrodyction

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Brassinosteroids -- wide range of functions Stimulation of stem elongation Pollen tube elongation Stimulation of cell division (with auxin and

cytokinins) Seed germination Leaf morphogenesis Apical dominance Inhibition of root elongation

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Brassinosteroids -- wide range of functions Vascular differentiation Accelerated senescence Programmed cell death Responses to biotic and abiotic stress

Salt Drought Temperature Pathogens

Most responses also controlled by auxin5

Promotes both cell expansion and cell division in shoots Dwarf mutants show smaller and fewer cells Brassinosteroid mutants have few microtubules

Brassinosteroids cause grass leaves to become horizontal – just like bending of stems under effect of auxin!

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Cell expansion kinetics between brassinosteroids and auxins different Brassinosteroid – elongation begins after 45

min lag – max rate after several hours Auxins – elongation begins after 15 minute lag –

max rate within 45 minutes Brassinosteroid may involve gene transcription! Auxin-related gene expression may be greater

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Operate synergistically Implication each hormone requires the other

Brassinosteroids enhance auxin transport, stimulation of lateral root growth, differential responses to gravitropism, etc.

Brassinosteroids increase uptake of water via aquaporins aiding in cell wall loosening in cell expansion

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Promotes and inhibits root growth Concentration dependent Low – promote root growth (incl. lateral roots)

Synergy with auxins in lateral root formation Promotes gravitropic response

Effects independent of both auxin and gibberellins

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Promote xylem differentiation Suppress phloem differentiation Deficient mutants

Fewer vascular bundles Irregular spacing

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Required for pollen tube growth Pollen excellent source of brassinosteroids Both the brassinosteroids and the signalling

pathway required Promotes seed germination

Interactions with other hormones Can cause tobacco seed germination independent

of GA Mutants more sensitive to ABA inhibition

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Fiber development in cotton Increasing crop yield – especially under

stress conditions Propagation of woody plants Improved tissue culture

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Auxins – cell elongation, apical dominance & tropisms

Gibberellins – regulators of plant height and seed germination

Cytokinins – regulators of cell division Abscisic Acid – seed maturation and stress

response Ethylene – only gaseous hormone Brassinosteroids – cell expansion and

development

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