biology 11of organisation and function of cells · "biology", 4th edition ch. 18, plant...
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BIO1PS 2012Plant Science
Lecture 4Hormones Pt. I
Dr. Michael EmmerlingDepartment of BotanyRoom [email protected]
Ladiges et al. (2010), 4th ed., Fig. 16.1
Leaves~4 Lectures
Soil and Roots~3 Lectures
Stems~2 Lectures
Hormones and Coordination~3 Lectures
Ghost gumEucalyptus papuana
Useful website: http://www.plant-hormones.info/
Ladiges et al. (2010), "Biology", 4th EditionCh. 18, Plant hormones and growth responses, pp. 400 - 419
Recommended Reading
Knox et al. (2005), "Biology", 3rd EditionCh. 17, Plant hormones and growth responses, pp. 370 - 392
Plant Hormones• Introduction
• General principles
• Discovery of plant hormones: auxin
• Major groups of plant hormones
Learning Objectives – Part 1•De!ne a plant hormone
•Describe how Went’s curvature test lead to the discovery of auxin
•Describe the major roles of the plant hormone auxin
•Describe the role of conjugation in regulating plant hormone levels
Classes of Plant Hormones•Auxin (IAA)
•Cytokinins
•Ethylene
•Gibberellins (GAs)
•Abscisic acid (ABA)
•Other hormones:
•Brassinosteroids
•Jasmonates
• plants can’t move (!!!!)
• external environment can change dramatically
• plants are very adaptable
Plant Development and Physiology
growth and "owering
Plant hormones in"uence almost every aspect of plant development and physiology
light and shade
dormancy and germination
heat and drought
leaf drop and abscission
cold and frost
Plant DevelopmentIn!uenced by many factors• internal
• genetics – evolution, adaptation
• external• light – amount, quality, direction
• nutrients – soil, CO2
• temperature• water – humidity, rainfall
Plant Hormones• are small molecules
• are effective at very low concentrations
• regulate the development of plants through
• changes in the production
• changes in the sensitivity of the plant/tissue to the hormone
Plant Hormones• are synthesised in a discrete organ or tissue
• are transported to a (speci!c) target tissue
• control a physiological response in a concentration-dependent manner
• regulate gene expression
The Study of Plant Hormones
Traditional approach:
• puri!ed from plants (or other organisms) and applied to an intact plant, or plant segment
Modern approach:
• use of genetics and mutants has clari!ed many aspects of hormone action
Canary grass seedlings
Shoot apex perceives light, and induces bending below the tip, towards the light
Ladiges et al. (2010), 4th ed., Fig. 18.2
Phototropism
Ladiges et al. (2010), 4th ed., Fig. 18.2
Hypothesis/Idea: The shoot apex is producing a diffusible factor that promotes growth
J. R. Eyerman, Time/Life
Went's Avena Curvature TestFrits Warmolt Went, 1956
Went's Avena Curvature Test
http://extension.entm.purdue.edu/pestcrop/2010/issue4/graphic/popups/agron3.jpg and agron5.jpg
Coleoptile
Went's Avena Curvature Test
Ladiges et al. (2010), 4th ed., Fig. 18.3
1926
AuxinThe substance inducing the curving of the coleoptile is auxinorindole-3-acetic acid (IAA)
acetic acidIndole
Ladiges et al. (2010), 4th ed., Fig. 18.3Ladiges et al. (2010), 4th ed., Fig. 18.4
tryptophan
Auxin-Induced GrowthCell elongation• cell wall is acidi!ed• acidi!ed cell wall is more "stretchable"• turgor pressure expands cells
Taiz and Zeiger (2010), 5th ed., Fig. 19.2
IAA Conjugation
• IAA is often bound to sugars or amino acids within the plant
• 50-90% of auxin in maize is conjugated
• the level of active IAA within the plant is dependent on the formation and breakdown of these IAA conjugates
• the level of free IAA is tightly controlled
Auxin - Amino Acid Conjugates
Bakllamaja, V. (2003), J Young Investigators 8 (1)
indole-3 acetic acid (IAA)
IAA-aspartate
IAA-glycine
Want to know more?Ludwig-Müller, J. (2011) Auxin conjugates: their role for plant development and in the evolution of land plants. J. Exp. Bot. 62(2), 1757-1773
Plant Hormones
Auxin GA Cytokinin Abscisic Acid Ethylene
Dormancy
Juvenility
Extension Growth
Root Development
Flowering
Fruit Development
Senescence
ShootTransported in the phloem to the rest of the plant
RootsTransported to the tip in the stele,then laterally in the epidermal/cortical cells
Auxin Transport
GravitropismTendency for
• shoots to grow away from gravity
• roots to grow towards gravity
mediated by auxins
Gravitropism
•Confocal microscopy of live roots•Green Fluorescent
Protein (GFP): auxin-responsive gene expression•Correlates with auxin
concentration
Auxin in the root? Transported via the phloemOttenschläger et al. (2003), Proc Nat
Acad Sci USA 100, 2987
GFPpromoterpromoter
+ auxin
GFP
Redistribution of Auxin
1.5 hours
3 hours
Ottenschläger et al. (2003), Proc Nat Acad Sci USA 100, 2987
high auxin levels in root tissue reduce cell expansion
gravitropism
GFPpromoterpromoter
+ auxin
GFP
ME
Apical Dominance
low dominance
The shoot apical meristem produces an "inhibitory substance" which prevents the outgrowth of other “dormant” buds (axillary meristems)
high dominance
ME
regulates plant architecture
Apical Dominance
MEME
Trimming
Ladiges et al. (2010), 4th ed., Fig. 18.13
Differential Responses to Auxin• Observations:
• auxin in shoot inhibits expansion of lateral buds
• auxin in shoot promotes cell elongation (phototropism)
• auxin in root promotes growth of lateral roots
• auxin in roots reduces cell elongation (gravitropism)
• Different sensitivities to auxin
• Elongation, division, differentiation
Sensitivity to Auxin
Inhibition
shoots
roots
auxinconcentrationinhibits roots
stimulates roots,no effect on shoots
Stimulationstimulates shoots
Sensitivies to Hormones
• different tissues have different responses to hormones
• this is due (in part) to different sensitivities to hormone levels, and
• also due (in part) to interactions with other hormones
IAA and AuxinsInvolved in the regulation of many aspects of plant development:
• cell elongation (and division)• tropisms• differentiation• apical dominance• senescence• abscission• "owering
Synthetic Auxins
2,4-dichlorophenoxy-acetic acid
2,4,5-trichlorophenoxy-acetic acid
• similar in structure to IAA
• more "stable" than IAA• transported less slowly
than IAA
results in excessive cell expansion
Ladiges et al. (2010), 4th ed., Fig. 18.4
• Agent Pink • Agent Green • Agent Purple • Agent Blue• Agent White • Agent Orange
"Rainbow Herbicides"
• Deployed (1960s and 70s) in Korea and Vietnam
Agent Orange
Mix of 2,4-D and 2,4,5-T
• Contaminated with the dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)
•Dioxins are
• fat-soluble and very stable
•carcinogenic
• lead to serious impairment of development, reproduction, immune function …