hormones aux in

Auxin

One of the first hormone responses inplants was the observation that lightaffected the direction of growth ofplant coleoptiles in germinating seeds.

In the early 1900's, Fritz Went and anumber of other researchers showedthat these effects could be induced byplant extracts, that were subsequentlyshown to contain the plant hormoneindoleacetic acid (IAA) (see Chapters2 & 9).

Auxins are commonly used as an aid torooting cuttings.

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Auxins promote adventitious root formation as shown with these yew (Taxus) cuttings.

Auxin

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Auxin is the most important hormone involved in rooting.

Indole-3-acetic acid (IAA)

Indole-3-butyric acid (IBA)

Naturally occurring

Synthetic

-Naphthalene acetic acid (NAA)

2,4-diclorophenoxyacetic acid (2,4-D)

Auxin

IAA is the most abundant naturallyoccurring auxin in plants. It issynthesized from the amino acid L-tryptophan in leaf primordia, youngleaves and developing seeds.

Although naturally occurring, IAAis not commonly used inpropagation because it breaks downquickly in the plant.

Synthetic auxins like IBA and NAAare more commonly used and aremore effective.

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Auxin

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Indole-3-butyric acid (IAA) isnaturally occurring, but at very lowabundance.

It works by being converted toIAA by the plant.

It is commonly found in commercialrooting compounds.

Auxin

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-Naphthalene acetic acid (NAA) is apurely synthetic auxin.

It is chemically similar to IAA instructure but is a more effectiveauxin in promoting rooting.

It is commonly found in commercialrooting compounds and is oftencombined with IBA.

Auxin

The potassium salt form of IBAand NAA have the advantage ofbeing water soluble and notneeding a solvent.

In some cases IBA or NAAtreated cuttings can be damagedby the solvent used to dissolvethem.

In these cases, the watersoluble K-IBA and K-NAA can bea very useful alternative.

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Auxin

The herbicide 2,4-dicloro-phenoxyacetic acid (2,4-D) isalso considered a syntheticauxin, but it is seldom usedcommercially to promote rooting.

It is commonly used to inducesomatic embryogenesis in tissueculture.

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Auxin

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IAA is not used commercially asoften as synthetic auxins.

This is because it is not as stable.

IAA degrades in the light and issusceptible to destruction in theplant by IAA-oxidase.

IAA-oxidase removes the carboxylgroup (COOH) making it ineffectiveas an auxin.

CH2COOH

N

H

Acid form of IAA

Decarboxylated IAA

CH2

N

H

CO2+

H2O

COOH

Auxin

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Relative rooting response of different auxins.

Auxin

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Conjugation of IAA naturally

protects it from destruction by

decarboxylation.

Conjugation adds a sugar or an

amino acid to the carboxyl end of

the molecule.

The conjugated form can be

metabolized back to active IAA.

Indole-3-acetic acid (IAA)

Auxin

There are conjugated forms ofausin used for cutting propagation.

These are also aryl ester and arylamide forms of IAA and IBA calledphenyl-IAA (P-IAA), phenyl-IBA (P-IBA), phenyl thioester IBA (P-ITB)and phenyl amide IBA (NP-IBA).

These have been shown to beeffective alternatives to IAA andNAA and show less toxicity inanimal studies. P-ITB has labelclearance from EPA, but is still notcommonly available.

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Auxin

Auxin is produced in the apicalmeristems.

Auxin moves from cell to cell in apolar gradient (i.e., tip to base).

It moves from distal to proximal.

This is why cuttings root at the

base (proximal end) of the stem.

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Root / ShootJunction

Proximal

Distal

Proximal

Distal

Auxin

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Polar auxin transport

Auxin transport proteins arelocated at the base ofparenchyma cells.

Transport is not sensitive togravity and always moves in apolar direction.

Auxin

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Transporter proteins (PIN1)are only located at theproximal end of the cell.

Therefore, auxin can only movein one polar direction.

Mutations in the PIN proteinresult in embryos with poorlyformed meristems showing howimportant polar transport andauxin gradients are for plantgrowth and development.

hormones aux in

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