plant hormones

28/08/14 BIOL2156/2334 Plant Science 1

PLANT HORMONES 1INTRO; AUXINS & GA

Nitin Mantri

School of Applied Sciences

RMIT

Room 223.1.28

Tel. 03 9925 7152

Email: [email protected]


INTRODUCTION

� Definition

� Plant hormones are natural and synthetic compounds that:

• elicit growth, differentiation or metabolic responses

• are active at very low concentrations• liquids 10-3–10-11 M

• gases 0.01 – 1 ppm v/v


TYPES OF PLANT HORMONES

5 discovered and isolated for a long time

� auxins 1880 Darwin

� gibberellins (GA) 1926 Kurosawa/Brien

� cytokinins 1956 Skoog & Miller

� abscisic acid (ABA) 1965 Wareing/Aldicott� ethylene

� 1924 fruit ripening� 1970s general - Osborne


TYPES OF PLANT HORMONES

� ‘florigen’? 1940s� makes plants flower � Polypeptide (100 amino acids)

� salicylic acid 1990s� involved in protection against pathogens� transmitted from tissue attacked to others� primes remote tissue against attack� increases compounds inhibitory to pathogens� not now regarded as a ‘hormone’


AUXINS -HISTORY OF DISCOVERY

� Darwin’s experiments (1880)

• phototropism in Avena (oat) coleoptiles

• tip bent towards light

• if tip excised or covered with foil → no response to light

• conclusion: tip contained light receptor


Darwin’s experimentTip bends to light; not if covered by foil.

light light

foil foil



� Boysen-Jenson’s experiment (1910)� tip excised and replaced with gelatin block

on coleoptile� response to light� conclusion: stimulus from tip was diffusible


Boysen-Jenson’s experimentStimulus is diffusible.

light light

gelatin block



� Went’s experiment (1926)

• many tips excised and placed on gelatin blocks in dark

• gelatin cut into small blocks

• blocks placed on decapitated coleoptiles

• response to light� conclusion: stimulus from tip was chemical


Went’s experimentStimulus is diffusible chemical.

light

gelatin block (dark)

coleoptile tips


STRUCTURE OF AUXINS

� structure of natural compounds very similar

4-Cl-IAA(4-chloroindole-3-acetic acid)

IAN(indoleacetonitrile)

IAA(indoleacetic acid)

PAA(2-phenylacetic acid)


STRUCTURE OF AUXINS

� synthetic compounds may lack a ring

IBA(indolebutyric acid)

NAA(naphthaleneacetic acid)

Dicamba(2-methoxy-3,6-dichlorobenzoic acid)

2,4-D(dichlorophenoxyacetic acid)

http://upload.wikimedia.org/wikipedia/commons/6/63/Kwas_naftylooctowy.svg

http://en.wikipedia.org/wiki/File:2,4-Dichlorophenoxyacetic_acid_structure.svg

http://upload.wikimedia.org/wikipedia/commons/a/a5/Dicamba.png


2. MODELSGeneral features:

hormone synthesis or

release system

hormone

hormone receptor

activation or derepression of

promotor

new mRNA

new protein

Model of Gene Regulation



STRUCTURE OF AUXINS� key to activity in molecules

� distance between carboxyl group and partial +ve charge on molecule must be 5.5 Å

� activity when bound to plasma membrane and ER (endoplasmic reticulum)

� Auxin binds to receptors� ABP1 (auxin binding protein 1) on plasma membrane� F-box TIRI protein (part of the ubiquitin ligase

complex)� TIRI: when auxin binds, Aux/IAA repressors are marked

for destruction� ubiquinated by the SCF complex� destroyed by the proteasome� release of ARFs (auxin response factors)� ARFs start transcription of specific genes


HORMONE PROMOTERS


MOLECULAR ACTIVITY

� Production – growing tissues

� Transport – in phloem, basipetal only

� Mechanism – binds to ER, then plasmalemma

� cell takes in more K+ ions� movement of H+ ions to cell wall (balances

charge)


MOLECULAR ACTIVITYacidic conditions in middle lamella (mostly

Ca pectate)Ca-pectate bonds broken (gel -> sol)microfibrils not held firmly by ML gel cell wall extensibleintake of water due to extra K+ ionscell expansion

Model of Plant cells held by Ca-pectate & microfibrils


microfibrilsCa-pectatePlasmalemmaCell wall


MOLECULAR ACTIVITYrigid cell wall (permeable)

vacuole2 membranes (semi-permeable)

� auxin activates K+ pump into cells

� H+ ions leave cell

� acidic conditions in middle lamella

� Ca-pectate bonds broken

� middle lamella gel->sol

� microfibrils not held firmly

� cell wall extensible

� extra K+ ions attract water by osmosis

� cell expands


PHYSIOLOGICAL ACTIVITIES 1. Cell enlargement

� e.g. fruit swells due to auxins produced by developing seeds

� maximum growth at optimum concentration

� maximum growth inhibited by� - less than optimum� - greater than optimum

� also abnormal growth at >optimum


PHYSIOLOGICAL ACTIVITIES 1. Cell enlargement� optimum level depends on tissue

� e.g. stems > buds > roots

-100

-50

0

50

100

10**-11 M

10**-10 M

10**-9M

10**-8M

10**-7M

10**-6M

10**-5M

10**-4M

10**-3M

roots

buds

stems


Normal buttercup

Sprayed with auxin herbicide (MCPA)

http://www.biog1105-1106.org/demos/105/unit5/synthauxins.html

Dandelions sprayed with 2,4-D

Auxin weedkillers


PHOTOTROPISM

•reversible bending of plant organs in response to directional light

•active wavelengths = blue light (400-500 nm)•receptors = carotenes


Darwin’s experimentTip bends to light; not if covered by foil - auxin stimulates growth on dark side

light light

foil foil

auxin


PHYSIOLOGICAL ACTIVITIES2. Tropisms

• directional bending in response to stimulus

� e.g. phototropism – in unidirectional light� auxin transported laterally to ‘dark’ side� [auxin] on ‘dark’ side > [auxin] on ‘light’ side� [auxin] on ‘dark’ side closer to optimum for shoots� cells on ‘dark’ side expand more than those on

‘light’ side� bending towards light


PHYSIOLOGICAL ACTIVITIES2. Tropisms

� e.g. geotropism in roots placed horizontally� auxin transported laterally to lower side� [auxin] on lower side >[auxin] on upper side� [auxin] on lower side > optimum for roots

(low)� cells on lower side expand less (inhibited) than

those on upper side� bending towards gravity


� Plant shoot tip bends against gravity over time


PHYSIOLOGICAL ACTIVITIES3. Apical dominance

� apical bud → production of high [auxin]

� [auxin] in buds near apical bud >optimum → inhibits growth

� → pyramidal growth, e.g. conifer trees

� uses – prevention of potato sprouting in storage by high [auxin]


Pyramidal growth in conifers

Metasequoia glyptostroboides in Royal Botanic Gardens, Melbourne


PHYSIOLOGICAL ACTIVITIES4. Prevention of abscission

� auxin produced by young, active leaves → diffuses back to stem

� if leaves cease supply of auxin to stem → formation of abscission layer

→ leaf abscission

� uses – prevention of premature fruit drop, e.g. apple


PHYSIOLOGICAL ACTIVITIES 5. Direction of translocation

� phloem transport → areas with high [auxin]

→ inputs of sugars, amino acids etc.

� uses - plant pathogens form auxins� e.g. fungi, bacteria

→ attract sugars etc.

→ ‘green island’ effect


PHYSIOLOGICAL ACTIVITIES6. Enzyme effects

� [auxin] in tissue � control of activity of citrate condensing enzyme

in Krebs cycle


PHYSIOLOGICAL ACTIVITIES7. Organ/tissue differentiation

� morphogenesis (light->irreversible change)� uses – rooting of stem cuttings� stem cuttings transport auxins at base of stem in

phloem� accumulation of auxins leads to differentiation

of adventitious roots� add hormone rooting powder at correct strength

to base of cuttings -> rooting


PHYSIOLOGICAL ACTIVITIES8. Ethylene production

� application of IAA to area → ethylene produced

� 2 types of plant� auxin enhances ethylene effects

• e.g. water plants, paddy rice

� auxin antagonises ethylene effects• e.g. most crop plants


ReferencesFurther reading (books)� Attwell, B.J., Kriedemann, P.E., Turnbull, C.G.N. (Eds).

(1999). Plants in Action. Macmillan Education Australia Pty Ltd, South Yarra, Melbourne, Australia.

� Bidwell, R.G.S. (1979). Plant Physiology, 2nd edn. Macmillan, New York, USA.

Specialist reviews on auxins� Giovannoni, J. (2001). Molecular biology of fruit

maturation and ripening. Annu. Rev. Plant Physiol. Plant Mol. Biol. 52, 725-749.

� Delker, C., Raschke, A., and Quint, M. (2008). Auxin dynamics: the dazzling complexity of a small molecule’s message. Planta 227:929–941.

08/28/14 BIOL1043/BIOL2051 Plant Science 41

PLANT HORMONES 1Auxins and GA

� auxins 1880 Darwin

� ethylene 1924/70 Osborne

� gibberellins (GA) 1926 Kurosawa/Brien

� abscisic acid (ABA) 1965 Wareing/Aldicott

� cytokinins 1956 Skoog, Miller

� ‘florigen’?/phytochrome 1940s


GIBBERELLINS -HISTORY OF DISCOVERY� 1926 Kurosawa (Japan)- foolish rice disease

� infected by fungus Gibberella fujikuroi� excessive elongation of internodes� plants grew tall and fell over

� 1934 extraction of chemical from fungus• called gibberellin (gibberellic acid – GA3)

� 1956 extraction of gibberellin from plants (bean seeds)

� now >136 known - each plant has about 15


STRUCTURE OF GIBBERELLINS

� >136 forms

� all very like gibberellic acid (GA3)

� differences only in side chains

� now produced commercially by growth and extraction of fungus

GA3(gibberellic acid)


MOLECULAR ACTIVITY

� Production� meristematic parts of plants

� Transport� xylem and phloem� bi-directional


MOLECULAR ACTIVITY� Mechanisms

• cell expansion• acidification of cell wall• in auxin-insensitive tissues

� induction of enzymes• alteration of RNA transcription

� inhibited by anti-GA synthesis compounds� e.g. AMO, Arest



� ‘bolting’ of rosette plants

� rapid elongation of stem (+ flowering)

� e.g. cabbage, radish, lettuce, beet



� increase in internode length in dwarf varieties

• reduced GA content

• dwarf + GA -> tall

Left +GA Right -GA



� application of anti-GA compounds to keep plants short

� e.g. liliums, azaleas, chrysanthemums

� application of GA to increase tenderness and length

� e.g. celery, sugar cane



� production of:� parthenocarpic/� larger/� widely spaced fruit

� e.g. mandarin orange, peach, grape


PHYSIOLOGICAL ACTIVITIES2. Release from dormancy

� some seeds require certain environmental conditions to germinate – replaced by GA

� e.g.lettuce cv. Grand Rapids – light required� e.g. barley, alpine ash – cold required

(vernalisation, stratification)

� buds -> spray with AMO (anti-GA) � -> no sprouting in storage� e.g. winter twigs, potatoes


PHYSIOLOGICAL ACTIVITIES3. Overcoming juvenility

� reduction in period of maturity required before first flowering in woody plants

� Uses� tree breeding programs� increasing wheat yield


Increase in wheat yield 1860-1978


PHYSIOLOGICAL ACTIVITIES4. Flowering

� in long-day/cold-requiring plants

• e.g. rosette plants

• normally require long days for flowering

• GA can replace in some


PHYSIOLOGICAL ACTIVITIES 5. Enzyme induction

� imbibition in cereal grains� embryo releases GA to aleurone layer� aleurone layer releases or produces amylases� endosperm starch hydrolysed into sugars� sugars -> embryo (energy source)

� system is very sensitive only to GA� used as bioassay for GA



� uses:brewing(malting)� barley steeped� to digest starch � before yeast added

(needs sugars)

� +GA -> faster rate of malting


Use of barley α-amylase bioassay to measure GA concentration

Top row: unknown concentration of GA in starch agar

Bottom row (left to right): 10-5 M, 10-6 M, 10-7 M, 10-8 M GA in starch agar

Barley endosperm-halves incubated for 48 h at 25°C. Plates then stained with I2/KI to show starch (blue-black reaction)


PHYSIOLOGICAL ACTIVITIES7. Organ/tissue differentiation

� induction of flowering in long-day/cold-requiring plants

� overcoming juvenility