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FAQs
Q1. What are auxins?
Ans: The term ‘‘auxin’’ has been derived from a Greek word
‘‘auxein’’ that means ‘‘to increase’’ or ‘‘to grow’’. All natural
and synthetic compounds that stimulate elongation in excised
coleoptiles and stem sections or have activities similar to
principal auxin (IAA) are considered as auxins. A characteristic
structural feature of all auxins is all active auxins is a molecular
distances of 0.5nm between a fractional positive charge
on aromatic ring and a negative charge on carboxyl group.
Q2. How were auxins discovered?
Ans: Charles Darwin and his son Francis Darwin while studying
the bending of coleoptiles towards the source of unilateral light
found that coleoptile tip was responsible for perceiving the light
and it produced some signal that was transported down to the
zone of elongation were the event of bending occurred. Paal
in 1919 confirmed that the signal was chemical in nature. He
excised coleoptiles tips in dark and exposed them to light. He
replaced these coleoptiles asymmetrically on side of the cut
stem which resulted in bending in absence of unilateral light.
The presence of growth factor in the tip of oat coleoptiles was
demonstrated by Frits Went in 1926. He excised coleoptile tips
and placed them on gelatin blocks. The growth factor diffused
into the gelatin blocks from the cut tips. These blocks if placed
asymmetrically on decapitated stems induce bending in absence
of unilateral light. The principal auxin indole-3-acetic acid
(IAA) was later isolated from human urine by Kogl in 1935.
Q3. What are auxin analogues?
Ans: Auxin analogues or synthetic auxins are compounds
that have physiological activity similar to natural auxins.
They are not synthesized by the plants but can stimulate
cell elongation in coleoptiles and stem sections. They are
widely used as selective herbicides against the broad leaved
dicot weeds. Other commercial uses of synthetic auxins are:
they promote flowering in pineapple, prevent fruit and leaf
drop, induce parthenocarpy and promote rooting of cuttings.
Synthetic auxins include 2,4 dichlorophenoxy acetic acid
(2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T).
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Q4. What is ‘‘Agent Orange’’?
Ans: Agent Orange is an auxin defoliant that was used
by the US military to defoliate forests in South Vietnam.
It is a mixture of
2,4 dichlorophenoxy acetic acid (2,4D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T).
Q5. Highlight the role of auxins in plant growth and development?
Ans: Auxins are a chemically diverse group of plant hormones
characterised by their ability of stimulating elongation in
coleoptiles and stem sections. In addition to that auxin are
involved in regulating tropic movements like bending of
plants towards light (phototropism) and growth response
of plant with respect to gravity (gravitropism). Auxin can
induce parthenocarpy in some plants like tomato, promotion
of flowering in pineapples, initiation of adventitious rooting in
stem cuttings, cell division in callus in presence of cytokinin
and auxin also can prevent premature fruit or leaf drop,
delay leaf and fruit abscission. Synthetic auxins find their
role as selective herbicides against broad leaved weeds.
Q6. What is parthenocarpy?
Ans:
event
Development
of
seedless
fruits
without
the
of
fertilization
is
called
parthenocarpy.
Q7. Describe how auxins stimulate cell elongation?
Ans: Auxin are characterised by their ability to stimulate
cell elongation in coleoptiles and stem sections. Auxin
causes a five to tenfold increase in the growth rate mainly
by increasing the extensibility of the cell wall. Auxin induces
acidification of the cell wall by increasing proton extrusion
from cytoplasm into the cell wall through H+-ATPases (proton
pumps). At acidic pH the wall loosening proteins called
expansins loosen the cell wall by weakening the hydrogen
bonds between the polysaccharide components of the cell
wall. The overall cell enlargement takes place in three steps:
I. Cell absorbs water through osmosis.
II. Water absorption results in the development of the
turgor pressure because of the rigidity of the cell
wall.
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III. Biochemical loosening of the cell wall by expansins
allows the cell to expand in response to turgor pressure.
Q8. How does auxin mediate bending of shoots towards the
light?
Ans: Growth movement of plants in response to the direction of
light is called phototropism. When illuminated with a unilateral
source of light plants bend in the direction of light. Auxin plays
an important role in this bending response. Auxin is polarly
transported in plants from tip to the base. But on illumination
with unilateral light auxin is transported laterally instead of
being transported basipetally. Auxin is transported to the
shaded side of the shoot inducing more elongation of cells on the
shaded side with the result shoot bends in the direction of light.
Q9. What is abscission and how is auxin involved in leaf
abscission?
Ans: Shedding of leaves, flowers or fruits by the plant is known
as abscission. These organs abscise due to the formation of
abscission layer in the abscission zone at the base of petiole or
fruit stalk. Young leaves and fruits have high concentration of
auxin that prevents formation of abscission zone. As these organs
mature the levels of auxin decline in them. Decrease in auxin
levels allows the formation of abscission layer. The cell walls of
the cells in the abscission layer get dissolved and become weak.
Soon the leaves break off from the plant in this region and fall.
Q10. What are synthetic auxins and why are they used as
herbicides in cereal fields?
Ans: Synthetic auxins are the compound that have physiological
activities similar to IAA but are not naturally present in plants.
They are synthesized in the laboratories. They are widely used as
herbicides against broad leaved weeds. Monocots are not affected
by synthetic auxins as they can inactivate them by conjugation.
That is why synthetic auxins are commercially used to kill dicot
weeds in cereal fields. Some important synthetic auxins used
as herbicides are 2,4 dichlorophenoxy acetic acid (2,4-D) and
2,4,5-trichlorophenoxy acetic acid (2,4,5-T) and dicamba.
Q11. List some applications of synthetic auxins that are
commercially important?
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Ans: Synthetic auxins can be used to induce parthenocarpy in fruits
like apple and tomato, promote flowering in pineapple, stimulate
fruit development, prevent premature fruit fall and initiate
adventitious rooting in stem cuttings. Auxins can be used to promote
vascular differentiation and morphogenesis in tissue culture.
Q12. What are natural auxins?
Ans: Auxins synthesized by the plants themselves are
called as synthetic auxins. Indole-3-acetic acid is the
principal auxin plants. It was first discovered by Kogl in
1935 in human urine. There are other auxins in plants as
well. 4-chloroindole-3-acetic acid is present in peas; where
as mustard and corn contain indole-3-butyric acid (IBA).
Q13. What is apical dominance?
Ans: Inhibition of lateral bud growth in plants by growing
apical bud is called apical dominance Apical dominance is
mediated by the auxin synthesised in the apical bud. Removal
of apical bud relieves lateral buds from apical dominance
and stimulates growth in them, whereas application of
auxin to the decapitated stem induces apical dominance/
Q14. What is the role of auxin in lateral root development?
Ans: Auxin promotes root growth at low concentrations (10-710-13 M) but at higher concentrations it inhibits the elongation
of the primary root. However, auxin at higher concentrations
promotes initiation of lateral and adventitious roots. Lateral
roots originate from the cells in the pericycle. Auxin stimulates
the division of patches of cells in the pericycle that develop
into a root apex. The root apex so formed grows through the
cortex and the epidermis and elongates to form a lateral root.
On the basis of study of phenotypes in a series of mutants
it has been found that IAA (auxin) is required to maintain
cell division and viability of cells during lateral root growth.
Q15. What are statoliths?
Ans:
Statoliths
are
amyloplasts
that
function
as
gravity
sensors
in
roots.
The
specialized
cells
in
which
they
occur
are
called
as
statocytes