Presentation contains biosynthetic pathway of 5 major phytohormones. Biosynthesis of Auxin, Gibberellins, Cytokinin, Ethylene and Abscisic acid.

1. 1
Dept. of Plant Molecular Biology and Biotechnology,
CPCA, S. D. Agricultural University.
Prepared by: Darshan T. Dharajiya
Ph.D. (Plant Molecular Biology and Biotechnology)

2. Content
• Biosynthesis of Auxin
• Biosynthesis of Gibberellins
• Biosynthesis of Cytokinin
• Biosynthesis of Ethylene
• Biosynthesis of Abscisic acid
• References

3. 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.

4. • 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

6. 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.

7. 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.

8. 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.

9. • 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.
Biosynthesis Pathways

10. • 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 Pathways

11. Biosynthesis of AuxinBiosynthesis of Auxin

12. • 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).
Biosynthesis Pathways

13. • 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.
Biosynthesis Pathways

15. 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.

16. 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.

17. • 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).
Biosynthesis of Gibberellins

18. • 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.
Biosynthesis of Gibberellins

19. • 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.
Biosynthesis of Gibberellins

20. • 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.
Biosynthesis of Gibberellins

21. Biosynthesis of Gibberellins

22. • 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).
Biosynthesis of Gibberellins

23. • 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.
Biosynthesis of Gibberellins

26. 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.

27. 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.

28. • 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

29. 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.

31. 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.

32. Ethelene BiosynthesisEthelene Biosynthesis

34. 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.

35. 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.

36. • 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.′
Biosynthesis of Abscisic acid

37. • 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.
Biosynthesis of Abscisic acid

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