PLANT PHYSIOLOGY

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 19^th 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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 547.

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

2. Chemical structure and biosynthesis of auxin

2.1. The chemical structure of auxins, such as indole-3- acetic 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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 548.

Sructures of naturally occuring (A) and synthetic (B) auxins (A)

(B)

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 428.

Tryptophan dependent pathways of IAA biosynthesis in plants

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 431.

Biodegradation of IAA: (A) the peroxidase, and (B) the nondecarboxylation routes

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 550.

Conjugation and degradation of IAA

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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 551.

The standard method for measuring polar auxin transport

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 552.

Adventitious roots grow from the basal ends of grape hardwood cuttings, and shoots grow from the apical ends

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 553.

The chemiosmotic model for polar auxin transport

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 558.

Sructures of synthetic (A) and natural (B) auxin transport inhibitors

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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 561.

A model for auxin regulation of transcriptional activation of early response genes by auxin

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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 563.

Typical dose-response curve for IAA-induced growth in oat coleoptile section

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 565.

Comparison of the growth kinetics of oat coleoptile and soybean hypocotyl sections

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 566.

Kinetics of auxin-induced elongation and cell wall acidification in maize coleoptiles

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

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 567.

Time course of growth on the illuminated and shaded sides of a coleoptile

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 574.

Current model for the redistribution of auxin during gravitropism in maize roots

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 450.

Auxin supresses the growth of axillary buds in bean plants

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

Source: Taiz L., Zeiger E. (2002): Plant Physiology. p. 451.

Root morphology of Arabidopsis (A-C) wild type and alf1 mutant seedlings (D-F) on hormone-free medium

Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 578.

The strawberry fruit growth is regulated by auxin produced by achenes

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