Transcript 06 Auxin - goldsword.com

Chapter 19


Principal means of intercellular
communication
What are they?


Chemical messengers produced in one place
and modulate processes elsewhere
Biosynthesis & Metabolism
What is it?
 What does it do
 How does it do it?

2




3
Endocrine hormone – transported to sites of
action distant from the site of synthesis
Paracrine hormone – act on cells adjacent to
source of synthesis
Animals – endocrine only
Plants – both endocrine and paracrine
hormones

Hormones (animal context)
Naturally occurring molecules with profound
influence on physiology
 Synthesized in discrete organs or tissues
 Transported from the site of synthesis to the
site of action
 Control physiological processes in a
concentration-dependent manner

4

Plant hormones are different …..
Synthesis -- more diffuse, occurring in many cell
types simultaneously
 May act in the same cells or tissues where
produced
 Relationship between concentration and the
degree/ type of bioactivity is not always linear
 Occur in much smaller concentrations
 Groups of structurally similar compounds that
can elicit the same biological effect

5


Not all stimulators – inhibitory effects
Response depends on how “read” by target
tissue


6
Same hormone different responses depending
on when or where!
Rarely act alone -- crosstalk


Amount available depends on rates molecules
enter and leave the “hormone pool”
Enter –
de novo synthesis
 Retrieval of hormone from storage
 Transport from somewhere else


Leave
Oxidation/ degredation
 Synthesis of deactivated conjugate

8


Hormones -- initially discovered as glandular
secretions that influenced physiology but ….
The search for plant compounds
began as early as the mid 1700’s
 first real research by Darwin


1880 pub – The Power of Movement in Plants
1910 -- shown to be Chemical by Boysen-Jensen
 First isolated in 1928 by Went

9

6 generally recognized plant hormone
families
Auxins
 Giberellins
 Cytokinins
 Ethylene
 Abscisic Acid
 Brassinosteroids

10

Other signaling molecules (hormones?)
Jasmonic acid
 Salicylic acid
 Systemin (short [ca. 18 AA] polypeptide)
 Florigen, the elusive [Raven says it’s been
discovered!]
 Strigolactone
 Flavonoids
 …..

11

Pineapple shoot – 6 μg of IAA/kg

Comparable to a needle in 20 Metric Tons of Hay!
Prefix
12
Abbreviation
centi
c
10-2
0.01
milli
m
10-3
0.001
micro
μ
10-6
0.000001
nano
n
10−9
0.000000001
pico
p
10−12
0.000000000001
femto
f
10−15
0.000000000000001
atto
a
10−18
0.000000000000000001
zepto
z
10−21
0.000000000000000000001
yocto
y
10−24
0.000000000000000000000001
0.000001M
1 μM
0.00001M
0.0001M
0.001M
0.01M
0.1M
1M
Potassium Permanganate
Dilution
ergo, it’s a hormone!
14


Auxins – Most common -- indole-3-acetic acid
(IAA)
IAA concentrations in plants are generally




15
1 to 100 mg/kg (fresh weight)
5.7-570 nanomoles (0.0057-0.57 μM) in leaves, but even
higher in seeds.
Single corn grain 308 pM 4 days after germination
Shoot 27 pM – required 10 pM/hr to support growth

Defined – activity similar to IAA
Promote cell elongation in coleoptile and stem
sections
 Cell division in callus cultures in the presence of
cytokinins
 Formation of adventitious roots on detached
stems and leaves
 Other developmental phenomena

16



Natural & Synthetic auxins
Principal function -- promotion of cell
elongation.
Activity of auxins -- manipulated to
create herbicides …HOW? WHY?




17
2,4-D
2,4,5-T
Dicamba
Biosynthesis associated with rapidly
dividing and growing tissues … aka
…..


SAM and young leaves 1° sites
RAM – especially as roots elongate


Fruits and seeds ….

18
Dependant on shoot for most of the auxin
Synthesized or transported?



Structurally related to tryptophan
Synthesis occurs by two primary
pathways in plants.

Tryptophan-dependent pathway

Tryptophan-independent pathway
Can be bound (conjugated) or free

19
Bound – hormonally inactive


Effective signals must be transient and not
accumulate over time
Concentration controlled by
de novo synthesis
 Inactivation by conjugation with

Sugars
 Amino acids

Inactivation by oxidation via IAA oxidase
 Transport away from target cells

20

Plant axes exhibit apex-base
polarity.
Structural polarity of plant
dependant of polarity of auxin
transport
 Unidirectional transport -from apex to base
 Only hormone with polar
transport!
 Terms relative!

21


Transport
independent of the
orientation of the
plant tissue
Basipetal transport
Shoots  vascular
 Roots  non-vascular


Polar transport
Requires energy
 Gravity independent

22

Polar transport primarily cell-to-cell via
symplast
Export – auxin efflux
 Import – auxin uptake of auxin influx




23
Requires metabolic energy – depressed in
absence of O2, sucrose or metabolic inhibitors
Velocity 3mm/hr – faster than diffusion;
slower than phloem translocation
Specific  protein carriers


24
Chemiosmotic
potential
Can be
chemically
inhibited


Auxins regulate gene-expression of auxin
transport proteins
Ethylene affects auxin transport


25
Activity and abundance of uptake and efflux
transporters  lateral root development
Other hormones regulate expression of
auxin transporter genes

Cell Elongation

Promotes growth in stems & coleoptiles


Outer tissues the target


10-5 or 10-6 M (or less) is optimum
In the absence of auxin, central tissues elongate faster!
Inhibits growth in roots
Low concentrations (10-10 to 10-9) promote growth
 Higher concentrations (10-6) inhibit growth


Minimum lag time is 10 minutes …..
Remember 10-6 is micromolar …..
26

Increases the extensibility of the cell wall
Osmotic uptake of water
 Turgor pressure builds
 Biochemical wall loosening
 Cell expands in response to turgor pressure


27
Acid-growth hypothesis  H+ ions

Acid-growth hypothesis  H+ ions

Predictions …..





28
Acid buffers should promote short-term growth
Auxins should increase rate of acidification
Neutral buffers inhibit auxin-induced growth
Anything that promotes acidification should stimulate
Something (aka The Wall
Loosening Factor) has an acidic
pH optimum

Plant Tropisms – Growth responses
Phototropism  instrumental in discovery
 Gravitropism
 Thigmotropism
 Hydrotropism
 Chemotropism
 Thigmomorphogenesis – irrespective of
direction


29
Nastic Movements – Movement responses


Mediated by lateral redistribution
Darwins -- sites of perception and response
were different
Coleoptiles sites of high auxin concentration
 Ability to perceive a directional light source
 Ability to transport auxin laterally in response


30
Phototropins (flavoproteins) are the
photoreceptors (blue light)


Lateral Transport  Differential Growth
Acidification of apoplast

31
Increases auxin transport



Lateral redistribution
Tip – produces auxin; tissues below tip can sense!
No gravity gradient!



32
Amyloplasts (contain starch) sense gravity
Statoliths & Statocytes
How?
Gravitropism
Shoots
Shoots
Orthogravitropic +, Orth = Straight
Plagiogravitropic 0-900
Plagio = oblique
Diagravitropic 900
Dia = through
Plant Physiology By Hopkins & Huner 2004

Sites of gravity perception
Shoots & coleoptile – starch sheath around
vascular tissues
 Roots – root cap



Mechanism poorly understood
Role of ER?

35
Nodal ER – 5-7 rough sheets like
flower petals

Starch-statolith hypothesis
Amyloplasts only organelles that sediment
 Sed-rate closely correlated with time to
perceive gravity
 Starchless mutants – slower response



36
Starch independent mechanisms may also exist!
Response may involve changes in pH
37
Gravitropism
Roots
http://www.youtube.com/watch?v=I7TmX0rrNRM
Notice lateral roots
Radish

Thigmotropism – touch
http://www.youtube.com/watch?v=_SIJ4ov_FxA
 Rapid response – encircling in 5-10 minutes
 Long-lasting response -- brief contact induces
response that lasts for days
 Response occurs in light; plants touched in the
dark remember.

39


40
Hydrotropism – water
Chemotropism – chemical signals
Hydrotropism

Hydrotropism: soil equally saturated with water

Roots – Triticum
sp. Wheat
subterranean
roots
Moist
Dry
Chemotropism

Videos?

Nastic movements -- Movement
unrelated to direction of stimuli.



Directional movements

44
Nyctinasty
Thigmonasty
Heliotropism

Nearly every stage of life cycle

Morphology depends on directed movement
 maintains root-shoot polarity

Loss of components  severe embryonic defects
Polarity established in early embryogenesis
 Effect depends on the identity of target tissue

45




Apical Dominance
Floral bud development
Phyllotaxy
Lateral root formation


Vascular development



46
Inhibits primary root formation – stimulates lateral and
adventitious roots
Below developing buds and young leaves
Leaf abscission
Fruit formation

Apical dominance


Apical bud inhibits growth of lateral buds
But … how?


diminished concentration?
Nope – post decapitation
concentration increases!


47
Radioisotope labeled
apical auxin not present
Source: xylem and
interfasicular
schlerenchyma
48

Vascular differentiation
activated in the Spring by IAA production by
young, developing leaves
 Xylem/Phloem ratio depends upon relative
amounts of IAA to GA

IAA > GA  Xylem
 IAA < GA  Phloem

49

Leaf abscission

Auxin concentrations highest in young
developing leaves (q.v., vascular differentiation)


50
Removal of the leaf blade promotes abscission
Ethylene plays a role

51
Fruit Development – produced by
developing seeds



52
The principal function of auxins is the promotion
of cell elongation.
This cell elongation contributes not only to typical
vertical growth, but to the directional growth
associated with phototropism and gravitropism
(see Ch. 23).
The cell elongation response to auxin increase
with concentration but is saturable.
53

54
Figure 18.5 (individual panels in four slides)



Auxins promote the cellular differentiation of
vascular tissue, particularly in shoots.
The regeneration of vascular tissue after
wounding is also mediated by auxin.
The differentiation of vascular tissue is
concentration dependent:


55
Phloem sieve tubes differentiate under low auxin
concentrations.
Xylem differentiation occurs with higher auxin
concentrations.

56
Figure 18.6



57
Auxins establish apical dominance over the
axillary buds on the stem.
The auxins produced by the apical meristem are
transported down the stem, where they slow or
suppress mitosis and cell expansion in the buds.
Removing the apex of the plant, such as when a
plant is pruned, releases the axillary buds from
inhibition, allowing their growth until one of the
buds establishes itself as the new apical meristem.


58
Auxins promote cell expansion.
The effect of auxins is related to the pH
decrease observed during cell enlargement.

The role of auxins in cell wall extensibility is
currently described by the acid-growth
hypothesis.
A docking protein anchors a membrane-bound
auxin binding protein (or ABP1) receptor to the
plasma membrane.
 When auxin binds to ABP1, a signal
transduction cascade is activated which
activates enzymes, including phospholipase A2
(PLA2).

59

The role of auxins in cell wall extensibility is
currently described by the acid-growth
hypothesis.
The products of PLA2, lysophospholipids and
fatty acids, trigger a protein kinase cascade.
 The protein kinase cascade activates the proton
ATPase, which releases protons into the cell
wall space.

60

The role of auxins in cell wall extensibility is
currently described by the acid-growth
hypothesis.
The decreased pH is optimal for the function of
cell wall-loosening enzymes such as expansins.
 When expansins loosen the walls, the turgor
pressure within the cell causes the cell to
expand.

61

62
Figure 18.8



63
The acid-induced growth
of plants is transient,
ceasing with 30 to 60
minutes.
The maintenance of
growth is also mediate by
auxins, but through
different mechanisms.
A component of these
mechanisms is the
induction of gene
expression by auxins.

Figure 18.9

Auxins induce the transcription of a set of genes
called the primary auxin responsive genes,
which include:



64
Small upregulated auxin RNAs (SAUR)
AUX/IAA
SAUR genes are produced rapidly (<3 min.) in
tissues in response to auxin, sometimes
asymmetrically in association with gravitropic
responses.



65
AUX/IAA genes are induced over longer time
frames by auxin (up to 30 min.)
The members of this gene family act as
transcriptional regulators, influencing
transcription through interaction with auxin
response factors (ARFs).
A repressor protein called Transport Inhibitor
Response 1 (TIR1) is also involved and acts to
derepress transcription of auxin responsive genes.

The activation of a auxin responsive gene
occurs via the following steps:
TIR1 has a binding site that accepts both auxin
and the AUX/IAA protein.
 When auxin and AUX/IAA bind, the TIR1
complex binds to the SCF complex.
 The AUX/IAA protein is ubiquinated, tagging it
for degradation by the ubiquitin-26S
proteaosome pathway.

66

The activation of a auxin responsive gene
occurs via the following steps:
The degradation of the AUX/IAA protein
derepresses the gene.
 The gene is transcribed, with the resulting
mRNA translated into an auxin-responsive
protein.
 The ability of a molecule to bind with TIR1 and
derepress gene expression is one of the defining
factors that makes a molecule an auxin.

67

68
Figure 18.10


The transport of auxin from sites of
synthesis contributes to the control of plant
growth and development.
The transport of auxin in the shoot is
basipetal and occurs by two mechanisms:
Some auxin moves in the phloem.
 Most auxin is transport be a complex cell-to-cell
polar transport mechanism.

69

70
Figure 18.11

Transport of auxins in roots occurs in both
directions:



71
An acropetal stream of auxin travels through the xylem
parenchyma in the stele to the root tip.
A basipetal stream reverses the direction of auxin flow
upwards through the cortical region.
The gradients of auxin created by polar transport
contribute to numerous developmental processes.


72
The mechanism of polar auxin transport
involves a carrier-mediated, secondary
active transport mechanism.
Evidence for this transport mechanism
comes in part from inhibitor studies,
including those with phytotropins such as
TIBA, morphactin, and NPA.

73
Figure 18.12

The current chemiosmotic model for polar
auxin transport has the following features:
A proton motive force serving as the driving
force for secondary active transport of IAA.
 An IAA influx carrier at the top of the auxintransporting cell.
 An efflux carrier at the base of the auxintransporting cell.

74

The steps involved in the transport of IAA
into and out of the cell are as follows:
In the cell wall space at the top of the
transporting cell, the slightly acidic pH causes
~80% of the IAA to deprotonate to form IAA-.
 The IAA enters the cell via the influx carrier
(encoded by the AUX1 gene), with a smaller
amount of the protonated IAAH entering by
simple diffusion.

75

The steps involved in the transport of IAA
into and out of the cell are as follows:
As the cell cytoplasm is near pH 7.0, all of the
IAA will deprotonate to form IAA-.
 The efflux carriers that mediates the transport
of IAA- from the cell (encoded by genes of the
PIN family) are located only at the cell’s base.
 The specific positioning of the efflux carrier
insures that IAA moves in a polar direction.

76

77
Figure 18.13



78
The PIN genes contribute not only to polar auxin
transport, but contribute to other aspects of plant
development as well.
During embryogenesis, the localization of PIN
proteins in the cell membrane help establish the
apical-basal axis.
Patterns of PIN protein distribution also
contribute to lateral root initiation and tropic
growth responses (Ch. 23).

79
Figure 18.4



80
Understand the concept of hormones,
particularly with respect to plant hormones
Recognize the range of chemical compounds
considered to be “auxins”
Know the tryptophan dependent and
independent pathways for auxin synthesis



81
Know the roles for auxins, particularly with
respect to plant growth and development
Understand how auxins activate auxin
responsive genes
Know how auxins are transported in plants
and the relationship of auxin transport to
auxin function and to plant development

82
Figure 18.1

Other natural auxins include:
Indole-3-ethanol
 Indole-3-acetaldehyde
 Indole-3-acetonitrile
 Indole-3-butyric acid (IBA)
 4-chloroIAA
 Phenyl acetic acid (PAA)

83

As auxin activity is based upon structure, several
synthetic auxins have been made, including:


The auxinic activity of synthetic compounds can
also be manipulated to create herbicides:



84
Naphthalene acetic acid (NAA)
2,4-D
2,4,5-T
Dicamba
85
86
87
88





89
Chemical Nature: Indole-3-Acetic Acid (IAA) – principal
naturally occuring auxin. Synthesized via tryptophandependent and tryptophan independent pathways
Sites of Biosynthesis: primarily in leaf primordia and
young leaves and in developing seeds
Transport: both polarly (unidirectionally) and nonpolarly
Effects: Apical dominance; tropic responses; vascular
tissue differentiation; promotion of cambial activity;
induction of adventitious roots on cuttings; inhibitions of
leaf and fruit abscission; stimulation of ethylene
synthesis; inhibition or promotion (in pineapples) of
flowering; stimulation of fruit development
First found: coleoptiles




90
Chemical Nature: Gibberellic acid, a fungal produce, is the
most widely studied. Synthesized via the terpenoid
pathway
Sites of Biosynthesis: in young tissues of the shoot and
developing seeds. It is uncertain whether synthesis also
occurs in roots
Transport: probably transported in the xylem and phloem
Effects: hyperelongation of shoots by stimulating both cell
division and cell elongation, producing tall, as opposed to
dwarf plants; induction of seed germination; stimulation
of flowering in long-day plants and biennials; regulation of
production of seed enzymes in cereals.




91
Chemical Nature: N6-adenine derivatives, phenyl urea
compounds. Zeatin is the most common cytokinin
Sites of Biosynthesis: primarily in root tips
Transport: transported in the xylem from roots to shoots
Effects: promotion of cell division; promotion of shoot
formation in tissue culture; delay of leaf senescence;
application of cytokinin can cause release of lateral buds
from apical dominance and can increase root development
in arid conditions




92
Chemical Nature: synthesized from a carotenoid
intermediate. Name is a minomer – has little to do with
abscission.
Sites of Biosynthesis: in mature leaves and roots,
especially in response to water stress. May be synthesized
in seeds
Transport: exported from leaves in phloem; from roots in
the xylem
Effects: stomatal closure; induction of photosynthate
transport from leaves to developing seeds; induction of
storage-protein synthesis in seeds; embryogenesis; may
affect induction and maintenance of dormancy in seeds
and buds of certain species




93
Chemical Nature: gas; synthesized from methionine.
Only hydrocarbon with a pronounced effect on plants.
Sites of Biosynthesis: in most tissues in response to
stress, especially in tissues undergoing senescence or
ripening
Transport: diffusion from its site of synthesis
Effects: fruit ripening (especially in climacteric fruits, such
as apples, bananas, and avocados); leaf and flower
senescence; leaf and fruit abscission




94
Chemical Nature: polyhydroxylated steroid compounds,
synthesized as a branch of the terpenoid pathway
Sites of Biosynthesis: throughout the plant especially in
young growing tissues
Transport: endogenous brassinosteroids act locally, at or
near their sites of synthesis
Effects: a wide range of developmental and physiological
processes, including cell division and cell expansion;
branching; vascular tissue differentiation; development of
lateral roots; seed germination; leaf senescence
95
96

97
Salicylic acid – implicated in the activation
of disease resistance following pathogen
invasion. Key in regulating thermogenesis
in members of Araceae (i.e., skunk cabbage)

98
Jasmonic acid -- derivative of linolenic
acid, activates plant defenses against insect
herbivores and microbial pathogens. In
addition, mediates response to drought,
ozone, UV radiation, and other abiotic
stresses. Transported via phloem

99
Systemin -- 18 amino acids! Mediates
response to wounding by insects (involves
protease inhibitors [interferes with protein
digestion in atticking insects, retarding
growth & development]) Protease
inhibitors accumulate not only in wounded
tissue, but also in undamaged leaves far
from site of attack. Apparently signal
jasmonic acid; biosynthesis activated by
systemin



100
Florigen -- produced in leaves; causes
flowering in the shoot apical meristem
Strigolactone -- regulates outgrowth of
lateral buds
Flavonoids -- function as both intracellular
and localized modulators of signal
transduction pathways
List of signaling agents and growth regulators
continues to expand!
101



Auxins – first to be discovered.
Master Hormone
Primarily synthesized in
meristems or actively growing
organs



102
Coleoptile apices, root tips,
germinating seeds, apical buds
Young leaves, developing
inflorescences, embryos
Actively distributed
throughout the plant body

Involved in virtually every step of plant
growth and development
Cell elongation
 Vascular differentiation
 Apical dominance -- growth of axillary buds
 Secondary root initiation
 Development of axilary buds, flowers & fruits
 Gravitational responses

103
1. Cellular Elongation
 2. Apical Dominance
 3. Abscission
 4. Differentiation of Vascular
Tissues
 5. Fruit Development
 6. Formation of Adventitious Roots

104

The pathway for tryptophan-independent
synthesis has not been fully described, but
some details have been determined:
Indole-3-acetonitrile is the precursor of IAA.
 The source of the indole-3-acteonitrile is not
known, but is derived from the glucosinolate
glucobrassicin in some plants.

105