BIO1PS 2012Plant Science

Lecture 4Hormones Pt. I

Dr. Michael EmmerlingDepartment of BotanyRoom 410m.emmerling@latrobe.edu.au

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 …