1. Chapter 24

2.  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 2

3.  Polyhydroxylated sterols  Sterols are terpenoid  Identified in 27 families  Gymnosperm  Angiosperm  Pteridophyte -- Equisetum  Bryophyte -- Marchantia  Algal -- Hydrodyction 3

4.  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 4

5.  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 auxin 5

6.  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! 6

7.  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 7

8.  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 8

9.  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 9

10.  Promote xylem differentiation  Suppress phloem differentiation  Deficient mutants  Fewer vascular bundles  Irregular spacing 10

11.  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 11

12.  Fiber development in cotton  Increasing crop yield – especially under stress conditions  Propagation of woody plants  Improved tissue culture 12

13.  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 13