CHAPTER: 15(B)
Plant Hormones: Auxin
Auxin:
•
Auxins
were the first of the major plant hormones to be discovered.
•
These
are a class of organic compounds which are mainly responsible for bringing
about cell elongation in shoots.
•
They
may be produced in plants as a result of metabolism or they may be of synthetic
origin.
•
The
principal natural auxin is indole acetic acid (IAA).
•
Auxins
have a cardinal role in coordination of many growth and behavioral processes in
the plant's life cycle and are essential for plant body development.
•
All
auxins are occurred at the meristematic apices of root, shoot, buds, leaves,
cotyledons, bacteria, yeast, fungi etc.
•
There
are several forms of auxins: Auxin –a, auxin-b, hetero auxin and related auxin are:
IAA, IBA, IPA, NAA, 2, 4-D etc.
Biosynthesis of Indole Acetic
acid (IAA):
•
Tryptophan
is the primary precursor of IAA in plants. The IAA can be formed from
tryptophan by different pathways.
- Indole-3-Pyruvic
acid pathway:
•
The
amino acid tryptophan loses the amino group by deamination or transamination to
form indole-3-acetic acid which then loses CO2 to form indole
-3-acetaldehyde. The oxidation of indole-3-acetaldehyde results in the
formation of IAA.
- Tryptamine
pathway:
•
This
is the next way to synthesis IAA. In this process the tryptophan is first of all
decarboxylated to form tryptamine, which is then oxidised as well as deaminated
to form indoleacetaldehyde. The oxidation of indole-3-acetaldehyde results in
the formation of IAA.
- Indole-3-aceto-nitrile
pathway:
•
In
this pathway, tryptophan is converted into indole-3-acetaldoxime and
indole-3-acetonitrile. The enzyme nitrilase is involved in this pathway.
Indole-3-acetonitrile is converted to indole-3-acetic acid.
- Indole-3-acetamide
(IAM) pathway:
•
It
occurs only in some bacteria.
•
The
tryptophan is first converted by the enzyme trp monoxygenase to
Indole-3-acetamide which is then converted to IAA by the action of the enzyme
IAM hydrolase.
Physiological
Role of Auxin:
•
Auxin
promotes nuclear activities in cell enlargement.
•
IAA
increases the plasticity of cell wall.
•
Responsible
for initiation and promotion of cell division.
•
Help
to prevent apical dominance.
•
Promotes
the cambial activity.
•
Useful
in tissue culture.
•
Involve
in gene expression
•
Help
in protein synthesis.
Practical
Application of Auxin in Agriculture:
- Apical
Dominance:
•
The
influence of terminal bud in supressing the growth of laterals growing
immediately below it is termed as apical dominance. If however, the apical bud
is substituted by auxin , the axillary buds are inhibited which enhance its own
growth.
- Prevention
of Abscission Layer:
•
The
premature drop of fruits as apple, citrus, and pear can be prevented to a great
extent by spraying the tree with a dilute solution of 2, 4-D, IAA, NAA or some
realetd auxin.
- Germination:
•
Auxin
(IAA, IBA, and NAA) widely used to break seed dormancy and increase
germination.
- Root
Initiation:
•
In
vegetative propagation (cutting roots/shoots), auxins can be applied to cutting
serves to induce more rooting.
- Flowering:
•
Auxin
generally inhibit flowering but enhances early flowering in pineapple, berries
and barley by spraying NAA.
- Parthenocarpy:
•
Parthenocarpy
is the phenomenon of development of seedless fruits (without pollination &
germination). If a flower bud is emasculated and auxin is applied to the stigma
of the flower a seedless fruit develops.
- Tissue
Culture:
•
Auxin
is one of the important components in tissue culture lab. It is requires for
the media formulation which helps in rooting and shooting.
- Weed
Control:
•
Auxin
like 2, 4-D spray can remove the weed.
- Sex
Expression:
•
The
spray of auxins increases the number of female flowers in cucurbits. It helps
to prevent sterility in plants.
- Respiartion:
•
Auxin
has been found to stimulate repiration.
0 comments:
Post a Comment