The document summarizes auxin transport and synthetic auxins. It discusses how auxin moves polarly from cell to cell through chemiosmotic transport, driven by influx and efflux carriers like PIN proteins. Several compounds can inhibit this transport. Synthetic auxins are widely used and mimic the effects of natural auxin (IAA), finding applications in agriculture to promote germination, rooting, flowering and fruit set while also acting as herbicides. The mechanisms of auxin-induced growth include cell wall loosening through proton pumping and an increase in extensibility.
3. “Went” developed the coleoptile curvature test for auxin and
discovered that auxin moves mainly from apical to basal end.
This type of unidirectional transport is termed as “Polar transport”.
Polar transport of auxin is observed in all plants including
bryophytes,ferns and also in higher woody plants. Polarity in the
transport of auxin is the main cause for the plant to exhibit apical
base structural polarity.
“Polar transport requires energy and is gravity independent”
Auxin transport
4.
5. CHEMIOSMOTIC MODEL
Polar transport proceeds from cell to cell fashion,
rather than symplast ie;auxin exit the cell through plasma membrane,
diffuses across compound middle lamella and enters to other cell
through plasma membrane.
Loss of auxin from cell-Auxin efflux
Entry of auxin into the cell-Auxin uptake
The overall chemiosmotic model requires energy in the form of
ATP.
Chemiosmotic model for polar auxin transport, auxin uptake is driven
by the proton motive force across the plasma membrane, auxin efflux
is driven by the membrane potential.
The chemiosmotic model includes two steps:
a)Auxin influx
b)Auxin efflux
6. Auxin influx
It is the first step in polar transport of auxin. According to this model,
auxin can enter plant cells by either of two mechanisms
1)Passive nutrition of the protonated form from any direction across
the phospholipid bilayer
2)Secondary active transport of the dissociated form(IAA-) via 2H+-
IAA-) symporter.
The dual pathway of auxin uptake arise because the
passive permeability of the membrane to auxin depends strongly on the
apoplastic PH.
The undissociated form of IAA in which the carboxyl
group is protonated readily diffuses through lipid bilayer,(Where as
dissociated form is negatively charged and does not cross the
membrane).
7. Auxin efflux
Auxin(IAA) enters the cytosol, which has PH of approximately
7.2,nearly all of it will be dissociated to ionic form, As the membrane is
less permeable to IAA than to IAAH,IAA tends to accumulate in
cytosol. However much of the auxin that enter to cell escapes via
auxin anion efflux carriers.
The main component of auxin-anion efflux carrier complex
were PIN proteins. These proteins are localized in the direction of
auxin transport.
PIN’s are integral membrane proteins with weak carrier
transport activity that can fascilitate transport of auxin by interacting
with other proteins.
8. INHIBHITORS OF AUXIN INFLUX AND EFFLUX
Several compounds acts as auxin transport inhibhitors.They
are
NPA(1-N-naphthylphthalamic acid)
CPD(2-carboxyl phenyl-3-phenyl propane-1,3 dione),
TIBA(2,3,5-triiodobenzoic acid)
NOA(1napthoxy acetic acid)
NPA,CPD and TIBA are auxin efflux
inhibitors(AEIs).They block polar transport of auxins by preventing
auxin efflux. Some AEIs such as TIBA inhibit polar transport of
auxins by competing with auxin at efflux carrier site and interfering
by binding the regulatory site.
9. Non-polar transport of Auxins through phloem
Auxin also transport in plants by nonpolar transport through
phloem.
Auxins along with other components of phloem can move
up and down the plant with higher velocities than the polar
transport.
Although loading and unloading of auxin into phloem is
carrier mediated ,translocation in phloem is passive and
driven by source-sink forces.
10. Auxin transport mediated by multiple
mechanisms
Expression of gene encoding auxin transport proteins
is regulated in a tissue specific fashion in response to
developmental and environmental conditions. Indeed expression
of some auxin transport genes is regulated by auxin itself.
1)Protein phosphorylation
Flavones such as Quercitin , Kaemferol and isoflavones such
as genestein are naturally occurring plant compounds that
inhibhit the activity of certain kinases and phosphatases.
Flavones have been shown to displace AEIs from membrane
binding sites.Quercitin & Kaemferol inhibit hydrolysis of ATP to
ADP.
11. 2)Protein trafficking
The polar localization of auxin transport proteins
involves the secretion of vesicles to specific sites on
the plasma membrane of auxin conducting cells.
Experiments shown that localization of PIN efflux
facilitator proteins and AUX1 influx carriers is
regulated by trafficking mechanisms that involves
endocytotic cycling between the plasma membrane
and endosomal compartments.
12. Mechanism of action of Auxins
O Auxin was discovered as the hormone involved in the
bending of coleoptile towards the light.
O Auxin promotes growth in the stems and coleoptiles, while
inhibition of growth in roots.
O Auxins synthesized in the shoot apex is transported
basipetally to the tissues below.
O Because of the level of endogenous auxin in the elongation
region, a normal healthy plant is nearly optimal for growth.
O Auxin rapidly increases the extensibility of the cell wall.
13. Plant cell gets expands in 3 steps
1) Osmotic uptake of water across the plasma
membrane is driven by the gradient in water
potential.
2) Turgor pressure builds up because of rigidity of
cell wall.
3) Biochemical wall loosening occurs allowing the
cell to expand in response to the turgor
pressure.
14. Auxin induced proton extrusion
involves both activation and synthesis
Auxin could increase the rate of proton
extrusion by 3 possible mechanisms.
1) Activation of pre existing plasma membrane
H+ ATPase.
2) Synthesis of new plasma membrane H+
ATPase.
3) Increased H+ ATPase abundance on the
plasma membrane.
15. H+ ATPase Activation
Addition of auxin to isolated plasma
membrane vesicles from tobacco cells causes a
small stimulation (20%) of ATP driven proton
pumping activity, raising the possibility that auxin
can directly activate the H+ ATPase.
16. H+ -ATPase activation synthesis and
secretion.
O The ability of Protein synthesis inhibitors such as
cycloheximide rapidly inhibit auxin induced proton
extrusion and growth suggest that auxin might also
stimulate proton pumping by increasing the
synthesis of H+ ATPase.
O A three fold stimulation by auxin can observe if a
mRNA for the plasma membrane H+ ATPase is
there in the non vascular tissues of the coleoptile.
17. Synthetic Auxins
O There are many synthetic auxins commercially
available today that are used for many purposes.
O Auxins are synthesized artificially to commence
the physiological responses similar to IAA.
O Most of this synthesized auxins are used as
herbicides in horticulture and agriculture.
24. Factors affecting biological activity of
compounds of auxins.
The biological activity of a compound depends not only on
it’s chemical structure and the specific manner by which it
reacts at some site within the plant but also on external and
internal factors.
1) The external environment.
2) The ability of the substance to penetrate the cuticle or
cell membrane.
3) Translocation within the plant to the site or sites of
action.
4) Mode of inactivation within the plant.
5) Availability of ATP or other nucleotides.
6) Metal or co-factor requirements if involved in enzymatic
reactions.
25. Agricultural uses of Auxins.
The auxin type plant growth regulators comprises the oldest
compounds used in agriculture. IAA, itself is not much useful in
agriculture because it is rapidly broken down to inactivate products by
light and microorganisms. Nevertheless, a number of synthetic auxins
are utilized in agriculture.
1) Germination.
2) Rooting of cuttings.
3) Flowering .
4) Stimulation of fruit set.
5) Parthenocarpy.
6) Chemical thinning.
7) Prevention of premature fruit drop.
8) Tissue and organ culture.
9) Herbicidal action.
