Auxin transport

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Auxin transport,Mechanism of action & Synthetic Auxins

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

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

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).

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. PINs are integral membrane proteins with weak carrier transport activity that can fascilitate transport of auxin by interacting with other proteins.

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.

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.

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.

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.

Mechanism of action of AuxinsAuxin was discovered as the hormone involved in the bending of coleoptile towards the light.Auxin promotes growth in the stems and coleoptiles, while inhibition of growth in roots.Auxins synthesized in the shoot apex is transported basipetally to the tissues below.Because of the level of endogenous auxin in the elongation region, a normal healthy plant is nearly optimal for growth.Auxin rapidly increases the extensibility of the cell wall.

Plant cell gets expands in 3 stepsOsmotic uptake of water across the plasma membrane is driven by the gradient in water potential.Turgor pressure builds up because of rigidity of cell wall.Biochemical wall loosening occurs allowing the cell to expand in response to the turgor pressure.

Auxin induced proton extrusion involves both activation and synthesis Auxin could increase the rate of proton extrusion by 3 possible mechanisms.Activation of pre existing plasma membrane H+ ATPase.Synthesis of new plasma membrane H+ ATPase.Increased H+ ATPase abundance on the plasma membrane.

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.

H+ -ATPase activation synthesis and secretion.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.

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.

Synthetic AuxinsThere are many synthetic auxins commercially available today that are used for many purposes.Auxins are synthesized artificially to commence the physiological responses similar to IAA.Most of this synthesized auxins are used as herbicides in horticulture and agriculture.

Synthetic Auxins may be classified in to 6 groups.1)Indoles: Indole 3-butyric acid & Indole 3-propionic acid.2)Benzoic acids: 2,3,6-trichlorobenzoic acid,2-methoxy-3,6-dichlorobenzoic acid (dicamba).3)Napthalene acids: -napthalene acetic acid,-napthalene acetic acid.4)Chlorophenoxy acids: 2,4,5-Trichlorophenoxy acetic acid & 2,4-dichlorophenoxy acetic acid.5)Napthoxy acid: -napthoxy acetic acid.6)Picolinic acid: 4-amino-3,5,6-trichloropicolinic acid (tordon or pichloram)






Factors affecting biological activity of compounds of auxins.The biological activity of a compound depends not only on its chemical structure and the specific manner by which it reacts at some site within the plant but also on external and internal factors.

The external environment.The ability of the substance to penetrate the cuticle or cell membrane.Translocation within the plant to the site or sites of action.Mode of inactivation within the plant.Availability of ATP or other nucleotides.Metal or co-factor requirements if involved in enzymatic reactions.

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.

Germination.Rooting of cuttings.Flowering .Stimulation of fruit set.Parthenocarpy.Chemical thinning.Prevention of premature fruit drop.Tissue and organ culture.Herbicidal action.

Thank you Prepared by:Aswathi.ETAM/2014-04