PLANT PHYSIOLOGY
Az Agrármérnöki MSc szak tananyagfejlesztése
TÁMOP-4.1.2-08/1/A-2009-0010
Auxin
Overview
1. The discovery of auxin: the first plant growth
hormone
2. Chemical structure and biosynthesis of auxin
3. Auxin transport
4. Auxin signal transduction pathway
5. Effects of auxin on growth and development
1. The discovery of auxin: the first plant growth
hormone
1.1. Charles Darwin and his son Francis studied the plant
growth toward light in the 19th century
1.2. Boysen-Jensen (1913), Paál (1919) and Went (1926)
demonstrated the presence of a growth-promoting
chemical in the tip of oat coleoptiles
Growth stimulus is produced in the coleoptile tip and passes through
gelatin but not through water-impermeable barriers
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 547.
The growth promoting stimulus has chemical in nature
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 547.
The active growth promoting substance can diffuse into a gelatin block,
and coleoptile-bending assay for quantitative auxin analysis
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 547.
2. Chemical structure and biosynthesis of auxin
2.1. The chemical structure of auxins, such as indole-3acetic acid (IAA), is relatively simple
2.2. IAA is synthesized in meristems, young leaves, and
developing fruits and seeds
2.3. Multiple pathways exist for the biosynthesis of IAA
2.4. IAA is degraded by multiple pathways
(A)
(B)
Sructures of naturally occuring (A) and synthetic (B) auxins
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 548.
Tryptophan dependent pathways of IAA biosynthesis in plants
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 428.
Biodegradation of IAA: (A) the peroxidase, and (B) the
nondecarboxylation routes
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 431.
Conjugation and degradation of IAA
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 550.
3. Auxin transport
3.1. Polar transport requires energy and is gravity
independent
3.2. A chemiosmotic model has been proposed to explain
polar transport: the acid growth hypothesis
3.3. Inhibitors of auxin transport block auxin efflux
The standard method for measuring polar auxin transport
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 551.
Adventitious roots grow from the basal ends of grape hardwood
cuttings, and shoots grow from the apical ends
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 552.
The chemiosmotic model for polar auxin transport
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 553.
Sructures of synthetic (A) and natural (B) auxin transport inhibitors
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 558.
4. Auxin signal transduction pathway
4.1. The plasma membrane auxin-binding protein (ABP1)
appears to function as an auxin receptor
4.2. Calcium and intracellular pH are possible signaling
intermediate
4.3. Auxin-induced genes fall into two classes: early and
late
A model for auxin regulation of transcriptional activation of early
response genes by auxin
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 561.
5. Effects of auxin on growth and development
5.1. Auxins promote growth in stems and coleoptiles,
while inhibiting growth in roots
5.2. The minimum lag time for auxin-induced elongation is
ten minutes
5.3. Auxin induced proton extrusion increases cell
extension
Typical dose-response curve for IAA-induced growth in oat coleoptile
section
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 563.
Comparison of the growth kinetics of oat coleoptile and soybean
hypocotyl sections
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 565.
Kinetics of auxin-induced elongation and cell wall acidification in maize
coleoptiles
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 566.
5. Effects of auxin on growth and development
5.4. Phototropism is mediated by the lateral redistribution
of auxin
5.5. Gravitropism involves lateral redistribution of auxin
5.6. Auxin regulates apical dominance
Time course of growth on the illuminated and shaded sides of a
coleoptile
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 567.
Current model for the redistribution of auxin during gravitropism in
maize roots
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 574.
Auxin supresses the growth of axillary buds in bean plants
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 450.
5. Effects of auxin on growth and development
5.7. Auxin promotes the formation of lateral and
adventitious roots
5.8. Auxin delays the onset of leaf abscision
5.9. Auxin promotes fruit development
Root morphology of Arabidopsis (A-C) wild type and alf1 mutant
seedlings (D-F) on hormone-free medium
Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 451.
The strawberry fruit growth is regulated by auxin produced by achenes
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 578.
Summary
Auxin was the first hormone to be discovered in
plants. The most common naturally occuring form of
auxin is indole-3-acetic acid (IAA). IAA is synthesized
primarily in the apical bud and transported polarly to
the root. Roles of auxin in higher plants are:
regulation of elongation growth in young stems and
coleoptiles, phototropism, gravitropism, apical
dominance, lateral-root formation, leaf abscision,
and fruit development
Questions
• Why is it necessary for a hormone to be rapidly turned
over?
• Can you suggest the physiological advantage of the
accumulation of auxin conjugates in some seeds?
• How is the polar auxin transport accomplished?
• What are the major physiological role of auxin?
THANK YOU FOR YOUR ATTENTION
Next lecture:
Gibberellins and cytokinins
• Compiled by:
Prof. Vince Ördög
Dr. Zoltán Molnár