LSM3261_Lecture 6--- Growth Responses and Regulation of Growth

8/3/2019 LSM3261_Lecture 6--- Growth Responses and Regulation of Growth

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LSM 3261 Life Form and Function

Growth Responses and Regulation of Growth


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Lecture 6 Topics

Why learn about growth and development?

Phototropism

Gravitropism

Thigmotropism

Plant hormones and development

Classical hormones: auxins, gibberellins, cytokinin,ethylene, abscisic acid

Light signals and plant development

Photo eriodism Phytochrome

Responses to herbivores and pathogens


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Reference

Solomon, E.P., L.R.Ber and D.W. Martin.

2011. Biology. 9th ed.


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Why Learn about Growth and

Development in this Module?

Module is about structure and function

ruc ures n p an s grow an eve op

through growth responses and regulation of

grow w c are n uence y e p an sinternal conditions (e.g., genetic makeup) and

s gna s rom e env ronmen ex erna

environment) Structures are correlated with functions


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Tropisms

Tropism = Directional growth in response to anexternal stimulus such as light, gravity or touch

Results in an irreversible change in a plant

part May be positive ornegative, depending of

whether movement is toward (+) or away (),

v y, u u Under hormonal control (covered later)

Types

Phototropism rav trop sm

Thigmotropism


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Phototro ism Directional growth of a plant caused

li ht

Most growing shoot tips grow toward

photosynthesis

detected by a photoreceptor family What is thecom lementar

phototropins = Light-activated color of blue?

-

enzymes)

in the blue-light signaling pathway


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Gravitropism=gravitys direction

Positive gravitropism means movingtoward the Earths center; negative = inopposite direction

Root ti s usuall have + ravitro ism

Shoot tips usually have gravitropism

Negative (top roots)an pos ve o om

roots) geotropism in

tiger orchid

speciosum)


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Gravitropism egu a e y =heavy starch-filled granules)

that settle toward the bottomofspecial cells within the

root cap in response to

Amyloplasts reorient asthe root changes direction

However, recent studies ofArabidopsis thaliana indicate

a amy op as - ac ngmutant plants still show +

geotropism More studies needed to

clarify the mechanism for What can cause a root to changegrav rop sm

its orientation as it grows throughthe ground?


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A Corn (Zea mays) Seedling:


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Thigmotropism Thigmotropism = Growth response to a mechanical

stimulus such as contact with a solid object

Twining and curling of stems and tendrils


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Plant Hormones 1

Plant hormone = An organic compound that acts as achemical signal eliciting a variety of responses that regulategrowt an eve opment

Effective at extremely low concentrations (usually


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Plant Hormones 2


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How Plant Hormones Act

Read up model of a signal transduction


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Auxins

Auxin = A plant hormone involved in

development, such as stem elongation,

,

cuttings, e.g., indole acetic acid (IAA) is

Etymology: Greekauxein, to increase

CH2COOH

N

Indole acetic acid (IAA)


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Auxins DiscoveryFrancis Darwin

(1848 1925)

Charles and Francis= Son of CharlesDarwin; hisassistant

evidence for this

n

Shaded differentCharles Darwin 1809 1882

coleoptile of canary

Concluded that aCanary grasssu s ance was passe

from the tip to the

ower par s, caus ng

to bend


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Coleo tile

= Protective sheath

young stem of some

Protects the plumule

germination

o Canary grass

(Phalariscanariensis)

o Oat (Avena sativa)Maize seedling

o Maize (Zea mays)


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The Darwins and

Phototropism Opaque covers on the tip

caused a loss of

Covers on the lowerportion of the stem had

no effect on bendingtoward light

e p was remove ,no bending

produced in the tip isresponsible for

p o o rop sm


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Digression: Research Specialization

or t ose o you contemp at ng researc as a career Conventional wisdom is to specialize to establish your niche

op sc en s s researc e var ous areas, e.g.: Charles Darwin: Theory of evolution by natural selection, emotions in

humans and animals, earthworms, barnacles, power of movement inplants (just covered), orchids, climbers, human evolution, sexualselection, carnivorous plants, etc.

Albert Einstein: photoelectric effect (Nobel Prize), general theory ofrelativity, relativistic cosmology, capillary action, critical opalescence,unified field theory, etc.

To scientists chan ed fields e. .: John Maynard Smith (19202004): aeronautical engineer then took

second degree in genetics, becoming one of the most famous, .

More current thinking: Become a deep generalist

Next best thing: collaborate, but must be able to be a team player,have social skills


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Avena (Oat) and Auxin

coleoptiles in the 1920s Placed coleoptiles on agar blocks

Phototropic = light-responding

,

Agar blocks caused increased growth on the side of the growing shoot

Frits Warmolt

Went (19031990)


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Auxin Effects

The movement of auxin is polar,(unidirectional, downward from its site

of production, usually shoot apicalmeristem) resulting in an auxin gradientthat tri ers s ecific res onses

Effects include:

oApical dominance

o ee ess ru s

oOthers


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Cell Elongation

elongation in stemsand coleoptiles

Movementof auxin

Auxins cause theincrease in cell length

on the side awa fromlight

Causes bending

Indole acetic acid(IAA) is one of themost im ortantauxins

Causes acidification,an so en ng, an soelongation withturgor pressure, of thecell wall


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Apical Dominance =

The inhibition ofaxillary bud growth by

Auxin produced in

inhibits axillary budsnear the apex from Mango tree

Leading branch then

( Mangifera indica)

the rest of the plant

Scots pine tree

(Pinus sylvestris)

Ethylene and cytokinin Do shrubs have

dominance

strong or wea ap ca

dominance?


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Seedless Fruits

flowers of certain

,

fertilization leads to

,

e.g.,Parthenocar ic = Fruit roduction without fertilization

Not all seedless fruits,

e.g.,oThomson seedless

grapes (abortion of

embryos after

fertilization, application

of gibberellin)


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Synthetic Auxins

is a naturalproduct

Synthetic = humanmanufactured, e.g.,

Naphthalene aceticac or rooinduction

-

dichlorophenoxyacetic acid (2, 4-D) =eu co er c e

2, 4, 5-

acetic acid (2, 4, 5-T)= eudicot herbicide

The US Army sprayed Agent Orange (2, 4-D + 2, 4, 5-T +

Others picloram [systemic herbicide]) on Vietnamese forests todefoliate them (toxic dioxin was a contaminant): possible linkto serious illnesses in Vietnam war veterans and Vietnamese


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Digression: Use of Scientific Research

Scientific knowledge is neither good nor bad

Its use is determined by humans

For ood e. . a riculture to im rove ieldspropagate plants, cure human diseases, nuclearpower for electricity generation

For bad, e.g., defoliate forests for war, biologicalwarfare, weapons of mass destruction

Always important to behave ethically and

morall in the a lication of science However, not always easy to decide what is

,


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Tutorial Assignment 3 and Practical 3

T urs ay, 29 ep 2011 Go first to LS Lab 7

o Tutorial Assignment 3 briefing: 2.00 to 2.45 pm

e r p o ngapore o an c ar ens:

3.00 to 5.30 pm

oDress comfortably, bring umbrella/rain coat,water, notebook, schedule, camera, insect

repellent

o Leavin from Science Drive 4 behind UniversitHall at 3.10 pm

.


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Gibberellins GAs Eiichi Kurosawa (Japan) in 1926

studied a disease of rice (Oryzasativa that caused exa eratedgrowth caused by a funguscalled Gibberella fujikuroi

and UK showed giberellins areproduced by healthy plants

can be extraordinary Causebolting rapid growth of

-plants, prominent in cabbage-related species (Brassicaceae)

e s are s mu a e o e onga eAND divide

Stimulate flowering andRice mutant which

germ na on n many p an s

Can bypass requirement forvernalization in many plants

gibberellinNormal rice

li i


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Vernalization=

exposure to low temperatures,

e.g., germination or flowering

Adaptation to prevent

germination or flowering in

GAs can substitute for the low

tem erature treatment

Can induce flowering inbiennials in first year, e.g., carrot

or perennials, e.g., plum blossom

(Prunus mume) needs 550800

hours of chillin Can induce seed germination in

many temperate species, e.g.,

apple

B l i


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Bolting

In the first yearIndian blanket

(Gaillardia

pulchella) grows as arose e o eaves

from a shortened

In the second year ofrowth boltin

occurs (right) with

stem elongation

Is the result of

gibberellin

pro uc on


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CytokininsCytokinins promote

= Plant hormone involved in variousaspects of plant growth and

Normal tobacco plant

cytokinesis (cell division)

,

of senescence, and differentiation

plant tissues in culture

cytokinin and auxin High ratio of cytokinin to auxin promotes

shoot formation

Low ratio of cytokinin to auxin promotes

In apical dominance, auxin inhibits axillary

bud formation; cytokinin promotes it

Cytokinins delay cellular aging, or

senescence High cytokinin transgenictobacco plant


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and Auxin

Control of

activityby auxin

and c tokinin

Determination of thedifferentiated tissue

type depends on the

ratio of auxin to

cy o n n

h l 1


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Ethylene 1

Ethylene gas (C2H4) isproduced by plants

= Gaseous plant hormone

involved in various aspects ofplant growth and development

such as leaf abscission and

fruit ripening Is critical to many plant

functionso Inhibits cell elongation

o Promotes see germ nat on

o Promotes apical dominance

o Fruit ripening

4

wrappers are needed for keeping fruitsfrom ripening in transit

E h l 2


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Ethylene 2

Ripening fruit produceethylene, so a ripe fruit maypromo e r pen ng o near yfruit

green then ripened with ethylene

o Ripe banana in a bag with otherfruit ma romote ri enin of the

second fruito One rotten apple spoils the wholebarrel: because of ethylene

Leaf abscission indeciduous plant promoted ast e et y ene : aux n rat o ncells at the base of the

fall (autumn)

E h l 3


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Ethylene 3

Thigmomorphogenesis =Plant developmental

responses to mechanical

stimuli, e.g., wind, rain,contact with passing animals

Mechanically disturbed

plants produce additionalethylene whicho Inhibits cell elongation

o Enhances cell wall thickeningJuniper trees

a es s ems s or er an

thicker so more resistance

o amage

Ab i i A id (ABA)


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Abscisic Acid (ABA)

Abscisic acid is present in large amounts in dormant seeds;

concentration drops as it is washed out

Gibberellin or cytokinins used to overcome dormancy

Promotes plant changes during water stress

Causes stomata to close, saving water

In a abscisic acid-free maize mutant, a

seed germinates while still on the plant


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Ph t i di d Fl i


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Photoperiodism and Flowering

Photoperiodism = Anyresponse of a plant to the

re at ve engt s o ay g t

and darkness in the duration

Plants types

=- -

Long-day (= short-night)

-

Day-neutral

Short Day PlantsChrysanthemum


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Short-Day Plantsy

Flowerwhen night is longerthan a critical period (long-n g t p ant)

Typically flower in late summero nsett a

Cocklebur

Long Day Plants


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Long-Day Plants

Flower when night ish r r h n ri i l

period (short-night plant)

Flower in late s rin or

Black-eyed Susan

early summer

thaliana

p nac

Types of Short and Long Day Plants


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Types of Short- and Long-Day Plants

Types

period of daylight (all or none effect depending on duration

of daylight per day)o Quantitative or facultative: flowering accelerated by

specific period of daylight per day

o o easy o scr m na e e ween e wo ypes ecause a

photoperiodic requirement may be more pronounced under, . .,

photoperiodic requirement under one temperature and have a

facultative photoperiodic requirement under anothertemperature

o Better to treat these two categories as two ends of a

con nuum


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-

Long-Day

Plants The behavior can

be modified by a

flash of red light

= long-

night

This indicates theim ortance of a

p ant

red-sensitive

pigment system =short-

night

plant

I t di t D Pl t


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Intermediate-Day Plants

Flower when exposed to an

Sugar cane

o eus

,

typical sugar cane plant to flower in the town of Pontianak(Indonesia) (situated exactly at the equator)?

Day-Neutral Plants


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Day Neutral Plants

Flowering not initiated in

response o seasona c anges

in day- and night-length

ower n response o o er

external or internal stimuli

Not All Hormonal Responses


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Not All Hormonal Responses

re u y arac er ze Long-day tobaccop an cot anasylvestris) is grafted

onto a day-neutraltobacco plant(Nicotiana tabacum)will induce lon -da

behavior, causingboth plants toflower whenexposed to long days

An unknown

,is thought to beresponsible for theresponse

Phytochrome Detects


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y

The photoreceptor pigment for

photoperiod is called phytochrome =

Family of about 5 blue-green pigment

proteins (each with own gene)

structures not

examinable!

complementary color of red?

There are 5 kinds of these pigments

in all vascular plants studied so far

phyA, phyB, phyC, phyD and

h E in rabido sis thaliana

phyB important at all stages;

others important over narrow

ranges o e e cyc e

phyA antagonizes phyB: in long-

day plants, phyB inhibits

flowering while phyA may induce

flowering

Electromagnetic Spectrum


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Electromagnetic Spectrum e -a sor ng nm p y oc rome r a erna es w ar-

red-absorbing (730 nm) phytochrome (Pfr)

Phytochrome Shifts between 2 Forms


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Phytochrome Shifts between 2 Forms

Red light (660 nm) converts Pr to Pfr, and far-red light (730 nm)converts Pfr to Pr

Conversion of Pr to Pfr triggers or inhibits physiological responseslike flowering

Phytochrome Effects


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Phytochrome Effects

Shade avoidance

Germination

eep movemen s n eaves

Shoot dormancy

Leaf abscission ,

fruits, and leaves

Shade Avoidance 1


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Shade Avoidance 1

= Response in which plants tend togrow taller when shaded to compete

Neighboring plants absorb more

Lamina is like a light filter

Shade Avoidance 2


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Increasing the ratio of far-red to red light increases

In tomatoes, increased far-

fruit production

Germination


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Some species have seeds with a light requirement

involving phytochrome

M x li h i h hi h r d f r-r dratio to germinate

Examples

o emperate ec uous orest n spr ng n as tt e s a e,

hence good time for such seeds to germinate

o oneer ree spec es n e rop cs germ na e on y un eropen conditions where such species thrive

Circadian Rhythms


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y Nearly all organisms display an internal clock

Approximates 24-hour cycle

, ,

etc.

In plants circadian rhythms common ause open ng c os ng o s oma a Movements of leaves called sleep movements

Rain tree (=pukul lima)

(Samanea saman)

ean aseo us

vulgaris) plant

During the day (left)

At night (right)

Settin the Biolo ical Clock


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Settin the Biolo ical Clock

- -

implicated in setting the biological clock for many plants

homologous with those of clock proteins in fruit flies,

,

the fruit fly and mouse

o og ca c oc s are very mpor an n coup ng e

activity of plants to important events, e.g.,

o Flower opening must be coordinated with pollinators

Plant Responses to Herbivores and Pathogens


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Plant Responses to Herbivores and Pathogens

Plants have an innate immune

system to fight local infections, but

this immune system is unlike thosein animals

A signal transduction pathway is

tr ggere w en a mo ecu e rom a

pathogen or herbivore binds to a

A hypersensitive response results

and this seals off the infected areacausing it to die to create a necrotic

lesion (dead area) to prevent or

slow the spread of the infection

S stemic Ac uired Resistance SAR


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S stemic Ac uired Resistance SAR

Plant also sends signals from the site of

oca ze n ec on roug e p oem

These lead to SAR = Buildup of long-

lasting enhanced resistance throughout

the plant

SAR is broad spectrum, heighteneddefense against many pathogen types

and not only the causal pathogen

Such hormone-si nallin moleculesinclude:

o Jasmonic acid

o

Methyl salicylate (oil of wintergreen)

LSM3261_Lecture 6--- Growth Responses and Regulation of Growth

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