Biosymthesis of Plant Hormones

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Dept. of Plant Molecular Biology and Biotechnology,CPCA, S. D. Agricultural University.

Prepared by: Darshan T. DharajiyaPh.D. (Plant Molecular Biology and Biotechnology)

Content

• Biosynthesis of Auxin• Biosynthesis of Gibberellins• Biosynthesis of Cytokinin• Biosynthesis of Ethylene• Biosynthesis of Abscisic acid• References

Plant Hormones

• Plant hormones: They are organic compounds synthesized in one part of the plant and translocated to another part, where in very low concentration it causes a physiological response.

• Plant development was thought to be regulated by only five types of hormones:

1)Auxins

2)Gibberellins

3)Cytokinins

4)Ethylene

5)Abscisic acid.

Plant Hormones

Auxin

• The first plant hormone we will consider is auxin.

• In the mid-1930s it was determined that auxin is indole-3-acetic acid (IAA).

• Several other auxins in higher plants were discovered later, but IAA is by far the most abundant and physiologically relevant.

Site of Synthesis

• IAA biosynthesis is associated with rapidly dividing and rapidly growing tissues, especially in shoots.

• Although virtually all plant tissues appear to be capable of producing low levels of IAA, shoot apical meristems, young leaves, and developing fruits and seeds are the primary sites of IAA synthesis.

Biosynthesis Pathways

• Multiple Pathways Exist for the Biosynthesis of IAA.

• IAA is structurally related to the amino acid tryptophan, and early studies on auxin biosynthesis focused on tryptophan as the probable precursor.

• The IPA pathway: The indole-3-pyruvic acid (IPA) pathway

• It is probably the most common of the tryptophan-dependent pathways.

• It involves a deamination reaction to form IPA, followed by a decarboxylation reaction to form indole-3-acetaldehyde (IAld).

• Indole-3-acetaldehyde is then oxidized to IAA by a specific dehydrogenase.


• The TAM pathway: The tryptamine (TAM) pathway

• It is similar to the IPA pathway, except that the order of the deamination and decarboxylation reactions is reversed, and different enzymes are involved.

• Species that do not utilize the IPA pathway possess the TAM pathway.

• In at least one case (tomato), there is evidence for both the IPA and the TAM pathways.


Biosynthesis of AuxinBiosynthesis of Auxin

• The IAN pathway: The indole-3-acetonitrile (IAN) pathway

• Tryptophan is first converted to indole-3-acetaldoxime and then to indole-3-acetonitrile.

• The enzyme that converts IAN to IAA is called nitrilase.

• The IAN pathway may be important in only three plant families: the Brassicaceae (mustard family), Poaceae (grassfamily), and Musaceae (banana family).


• IAM Pathway: the indole-3-acetamide (IAM) pathway

• Another tryptophan-dependent biosynthetic pathway—one that uses indole-3-acetamide (IAM) as an intermediate is used by various pathogenic bacteria, such as Pseudomonas savastanoi and Agrobacterium tumefaciens.

• This pathway involves the two enzymes tryptophan monooxygenase and IAM hydrolase.

• The auxins produced by these bacteria often elicit morphological changes in their plant hosts.


Gibberellins

• In the 1950s the second group of hormones, the gibberellins (GAs), was characterized.

• The gibberellins are a large group of related compounds (more than 125 are known) that, unlike the auxins, are defined by their chemical structure rather than by their biological activity.

• Gibberellins are most often associated with the promotion of stem growth, and the application of gibberellin to intact plants can induce large increases in plant height.

Biosynthesis of Gibberellins

• Gibberellins constitute a large family of diterpene acids and are synthesized by a branch of the terpenoid pathway.

• Pathway contains three stages.• Gibberellins are tetracyclic diterpenoids made

up of four isoprenoid units. Terpenoids are compounds made up of five-carbon (isoprene) building blocks: joined head to tail.

• Stage 1: Production of terpenoid precursors and ent-kaurene in plastids.

• The basic biological isoprene unit is isopentenyl diphosphate (IPP).

• 2 IPP used in gibberellin biosynthesis in green tissues is synthesized in plastids from glyceraldehyde-3-phosphate and pyruvate.

• Once synthesized, the IPP isoprene units are added successively to produce intermediates of 10 carbons (geranyl diphosphate), 15 carbons (farnesyl diphosphate), and 20 carbons (geranylgeranyl diphosphate, GGPP).


• GGPP is a precursor of many terpenoid compounds, including carotenoids and many essential oils, and it is only after GGPP that the pathway becomes specific for gibberellins.

• The cyclization reactions that convert GGPP to ent-kaurene represent the first step that is specific for the gibberellins.

• The two enzymes that catalyze the reactions are localized in the proplastids of meristematic shoot tissues, and they are not present in mature chloroplasts.

• Thus, leaves lose their ability to synthesize gibberellins from IPP once their chloroplasts mature.


• Stage 2: Oxidation reactions on the ER form GA12 and GA53.

• In the second stage of gibberellin biosynthesis, a methyl group on ent-kaurene is oxidized to a carboxylic acid, followed by contraction of the B ring from a six- to a five-carbon ring to give GA12-aldehyde.

• GA12-aldehyde is then oxidized to GA12, the first gibberellin in the pathway in all plants and thus the precursor of all the other gibberellins.


• Many gibberellins in plants are also hydroxylated on carbon 13.

• The hydroxylation of carbon 13 occurs next, forming GA53 from GA12.

• All the enzymes involved are monooxygenases that utilize cytochrome P450 in their reactions.

• These P450 monooxygenases are localized on the endoplasmic reticulum.

• Kaurene is transported from the plastid to the endoplasmic reticulum, and is oxidized in route to kaurenoic acid by kaurene oxidase, which is associated with the plastid envelope.



• Stage 3: Formation in the cytosol of all other gibberellins from GA12 or GA53.

• All subsequent steps in the pathway are carried out by a group of soluble dioxygenases in the cytosol.

• These enzymes require 2-oxoglutarate and molecular oxygen as cosubstrates, and they use Fe2+ and ascorbate as cofactors.

• Two basic chemical changes occur in most plants:

1. Hydroxylation at carbon 13 (on the endoplasmic reticulum) or carbon 3, or both.

2. A successive oxidation at carbon 20 (CH2 → CH2OH → CHO).


• The final step of this oxidation is the loss of carbon 20 as CO2.

• When these reactions involve gibberellins initially hydroxylated at C-13, the resulting gibberellin is GA20.

• GA20 is then converted to the biologically active form.• GA1, by hydroxylation of carbon 3.• Finally, GA1 is inactivated by its conversion to GA8 by a• hydroxylation on carbon 2. This hydroxylation can also

remove GA20 from the biosynthetic pathway by converting it to GA29.


Cytokinin

• Cytokinins are chemically related to rubber, carotenoid pigments, the plant hormones gibberellin and abscisic acid, and some of the plant defense compounds known as phytoalexins.

• All of these compounds are constructed, at least in part, from isoprene units.

• The precursor(s) for the formation of these isoprene structures are either mevalonic acid or pyruvate plus 3- phosphoglycerate, depending on which pathway is involved.

Biosynthesis of Cytokinin

• The first committed step in cytokinin biosynthesis is the addition of the isopentenyl side chain from DMAPP to an adenosine moiety.

• The plant and bacterial IPT enzymes differ in the adenosine substrate used; the plant enzyme appears to utilize both ADP and ATP, and the bacterial enzyme utilizes AMP.

• The products of these reactions (iPMP, iPDP, or iPTP) are converted to zeatin by an unidentified hydroxylase.

• The first committed step in cytokinin biosynthesis is the transfer of the isopentenyl group of dimphate (DMAPP) to an adenosine moiety.

• In both cases, DMAPP and AMP are converted to isopentenyladenosine-5 -monophosphate (iPMP).′

• As with the free cytokinins, isopentenyl groups are transferred to the adenine molecules from DMAPP by an enzyme call tRNA-IPT.

Biosynthesis of Cytokinin

Cytokinin BiosynthesisCytokinin Biosynthesis

Biosynthetic pathway for cytokinin biosynthesis. The first committed step in cytokinin biosynthesis is the addition of the isopentenyl side chain from DMAPP to an adenosine moiety. The plant and bacterial IPT enzymes differ in the adenosine substrate used; the plant enzyme appears to utilize both ADP and ATP, and the bacterial enzyme utilizes AMP. The products of these reactions (iPMP, iPDP, or iPTP) are converted to zeatin by an unidentified hydroxylase. The various phosphorylated forms can be interconverted and free trans-Zeatin can be formed from the riboside by enzymes of general purine metabolism. trans-Zeatin can be metabolized in various ways as shown, and these reactions are catalyzed by the indicated enzymes.

Ethylene

• In 1901, Dimitry Neljubov observed that dark-grown pea seedlings growing in the laboratory exhibited symptoms that were later termed the triple response: reduced stem elongation, increased lateral growth (swelling), and abnormal, horizontal growth.

• When the plants were allowed to grow in fresh air, they regained their normal morphology and rate of growth.

• The first indication that ethylene is a natural product of plant tissues.

Ethelene BiosynthesisEthelene Biosynthesis

Abscisic acid

• It is now known that ethylene is the hormone that triggers abscission and that ABA-induced abscission of cotton fruits is due to ABA’s ability to stimulate ethylene production.

• ABA biosynthesis takes place in chloroplasts and other plastids.

Biosynthesis of Abscisic acid

• The pathway begins with isopentenyl diphosphate (IPP), the biological isoprene unit, and leads to the synthesis of the C40 xanthophyll (i.e., oxygenated carotenoid) violaxanthin.

• Synthesis of violaxanthin is catalyzed by zeaxanthin epoxidase (ZEP), the enzyme encoded by the ABA1 locus of Arabidopsis.

• This discovery provided conclusive evidence that ABA synthesis occurs via the “indirect” or carotenoid pathway, rather than as a small molecule.

• Violaxanthin is converted to the C40 compound 9 -cis-neoxanthin, which is then cleaved to form ′the C15 compound xanthoxal, previously called xanthoxin, a neutral growth inhibitor that has physiological properties similar to those of ABA.

• The cleavage is catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED), so named because it can cleave both 9-cis-violaxanthin and 9 -cis-neoxanthin.′


• Synthesis of NCED is rapidly induced by water stress, suggesting that the reaction it catalyzes is a key regulatory step for ABA synthesis.

• The enzyme is localized on the thylakoids, where the carotenoid substrate is located.

• Finally, xanthoxal is converted to ABA via oxidative steps involving the intermediate(s) ABA-aldehyde and/or possibly xanthoxic acid.

• This final step is catalyzed by a family of aldehyde oxidases that all require a molybdenum cofactor.


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Science

Biosymthesis of Plant Hormones