Molecular And Biochemical Steps In Synthesis Of Auxin In Plant

1. MOLECULAR AND BIOCHEMICAL STEPS IN
SYNTHESIS OF AUXIN IN PLANT
Name- CHAVAN MAHADEO RAJARAM
Reg. No.- ADPM/15/2419
Class- Jr. M. Sc. Agriculture
Genetics and Plant Breeding

2.  Derived from Greek word ‘auxin’
means “to increase” or “to grow”
 “compounds that induce cell elongation in
coleoptiles or resemble indole-3- acetic
acid (IAA) in their physiological activities
are considered as auxin”

3. Went (1926) was successful in isolating this growth
substance from Avena coleoptile tips which still
retained the growth promoting activity.
 Later went (1928) developed a biossay for
quantitative analysis of auxin.
 He suggested – The curvature of stem is
proportional to the amount of growth substance
in the block.
 This test – Avena curvature test

4.  In 1935 Kogl isolated auxintriolic acid from
urine and named it as auxin -a.
 Kogl also isolated indole-3-acetic acid (IAA)
from human urine

5. Natural Auxins :
indole-3-acetic acid (IAA)
4-chloroindole-3-acetic acid (4-Cl-IAA)
phenylacetic acid (PAA)
indole-3-butyric acid (IBA)
Type of auxin

6. 2,4-Dichlorophenoxyacetic acid
α-Naphthalene acetic acid
2-Methoxy-3,6-dichlorobenzoic acid 4-Amino-3,5,6-trichloropicolinic acid
2,4,5-Trichlorophenoxyacetic acid

7.  Free auxin :
- It is readily extractable
- Immediately utilizable in growth
 Bound auxin :
- Liberated from tissue when subjected to enzymolysis,
hydrolysis, or autolysis
- Storage form of hormone.
- synthesized –seed maturation and
- Hydrolyzed – seed germination

8. Auxin biosynthesis
Two major pathways for IAA biosynthesis have
been proposed in plants
 tryptophan (Trp)-independent
Trp-independent IAA biosynthesis,indole-3-
glycerol phosphate or indole is the likely
precursor
 Trp dependent pathways

9. Auxin biosynthesis
The indole-3-pyruvic acid pathway (IPAPathway)
The tryptamine pathway (TAM Pathway)
The indole-3-acetonitrile pathway (IAN Pathway)
 indole-3-acetamide (IAM pathway)
Mechanisms:

10. Trytophan
Tryptamine
Indole -3-acetaldehyde
Decarboxylation
Oxidation and
Deamination
Indole-3-pyruvic acid
Deamination
Decarboxylation
IAA
Oxidation
Indole-3-acetaldoxime
Indole-3-acetonitrile
IPA
TAM
IAN

11. The amino acid tryptophane losess the amino group (deamination)
 indol-3- acetic acid which thane loses CO2 to form
indol -3-acetaldehyde
The oxidation of indol -3- acetaldehyde result in the
formation of IAA
 IPA Pathway (Indol -3- pyruvic acid)

12. It occurs in those species which lack IPA pathway.
The tryptophan is first of all decarboxylated to form tryptamine.
Which is than oxidized as well as deaminated to form
indolacetaldehyde.
Nonhhebel et al (1993) have found both IPA as well as TAM
pathway in tomato.
 TAM Pathway

13.  IAN and other components of the IAN pathway have been found primarily
in the Brassicacae; therefore, this pathway has not been considered to be
of general importance.
 Trp is converted to indole-3-acetaldoxime by an enzyme activity that has
been detected in several plant species
(Ludwig-Miiller and Hilgenberg,1988).
 Indole-3-acetaldoxime conversion to indole-3 acetonitrile has been
demonstrated in plasma membranes of Chinese cabbage (Ludwig-Miiller
and Hilgenberg, 1990).
 The nitrilase that converts indole-3-acetonitrile to IAA
 The IAN Pathway

14. Abbreviations
IAA
indole-3-acetic acid
IAD
indole-3-acetaldehyde
IAM
indole-3-acetamide
IAN
indole-3-acetonitrile
IAOX
indole-3-acetaldoxime
IPA
indole-3-pyruvic acid
TAM
tryptamine
Trp
tryptophan

15.  Transport of auxin in plant
◦ The transport of auxin in plant is predominantly polar. In stems, polar
transport of auxin is basipetal i.e., it takes place from apex towards
base.
o In roots also, the auxin transport is polar but is primarily acropetal.
o Jacobs (1961) found polar transport of auxin in coleus stem sections to
be both basipetal and acropetal in the ratio of 3:1.

16. Case study
The pathway of auxin biosynthesis in plants
Yoshihiro Mano1,* and Keiichirou Nemoto2
Abstract
The plant hormone auxin, which is predominantly represented by indole-
3-acetic acid (IAA), is involved in the regulation of plant growth and
development. Although IAA was the first plant hormone identified, the
biosynthetic pathway at the genetic level has remained unclear. Two major
pathways for IAA biosynthesis have been proposed:
the tryptophan (Trp)-independent and Trp-dependent pathways. In Trp-
dependent IAA biosynthesis, four pathways have been postulated in plants:
(i) the indole-3-acetamide (IAM) pathway; (ii) the indole-3-pyruvic acid
(IPA) pathway;
(iii) the tryptamine (TAM) pathway; and (iv) the indole-3-acetaldoxime
(IAOX) pathway.

17. Plants would be expected to share evolutionarily conserved
core mechanisms for auxin biosynthesis because IAA is
a fundamental substance in the plant life cycle, although
different plant species may have unique strategies and
modifications to optimize their metabolic pathways.
Biochemical and molecular biological findings and
bioinformatics studies indicate that the best candidate for the
major pathway of IAA biosynthesis is the IAM pathway and/or
the IPA pathway
Conclusions and perspectives

18. Arai Y, Kawaguchi M, Syono K, Ikuta A. 2004.
Partial purification of an enzyme hydrolyzing indole-3-
acetamide from rice cells. Journal ofPlant Research 117, 191–
198.
Bak S, Nielsen HL, Halkier BA. 1998.
The presence of CYP79homologues in glucosinolate-
producing plants shows evolutionary conservation of the enzymes
in the conversion of amino acid toaldoxime in the biosynthesis of
cyanogenic glucosides and
glucosinolates. Plant Molecular Biology 38, 725–734.
Bak S, Tax FE, Feldmann KA, Galbraith DW, Feyereisen R.
2001.
CYP83B1, a cytochrome P450 at the metabolic branch point in
auxinand indole glucosinolate biosynthesis in Arabidopsis. The
Plant Cell
References