GA and ABA Interaction Gibberellins

2. Hormonal Regulation Of Plant Growth And Development
PP-504
Submitted To
Dr.M.M. Burondkar

3. Presented By
Vivek Rama Suthediya
Reg. No. : ADPM/19/2680
Discipline : Genetics And Plant Breeding

4. Topic
Gibberellins

5. Introduction:
 Gibberellins (GAs) are plant hormones that are
essential for many developmental processes in plants,
including seed germination, stem elongation, leaf
expansion, trichome development, pollen maturation
and the induction of flowering .
 Hence, mutant plants that are deficient in GA exhibit
a dwarf and late-flowering phenotype, and treating
these plants with GA restores normal growth.

6. Origin and Discovery:
 First recognised in 1926 by a Japanese scientist, Eiichi
Kurosawa, studying bakabae, the ‘foolish seedling’
disease in rice.
 First isolated in 1935 by Teijiro Yabuta and Sumuki,
from fungal strains ‘Gibberella fujikuroi’ provided by
Kurosawa.
 Yabuta named the isolate as gibberellin.

7. Biosynthesis In Plants:
 Gibberellins biosynthesis pathway; residing in 3
different cellular compartments :
Plastid
Endoplasmic reticulum
Cytoplasm
 There are three stages of GA biosynthesis :
Stage 1
Stage 2
Stage 3

8. Stage 1:
 In stage 1, isopentenyl diphosphate is converted (not
shown) to geranylgeranyl diphosphate (GGPP), which
is then converted to ent-kaurene via ent-copalyl
diphosphate in plastids.

9. Stage 2:
 In stage 2, which takes place on the plastid envelope
and endoplasmic reticulum, ent-kaurene is converted
to GA12. In many plants, GA12 is converted to GA53 by
hydroxylation at C-13.
 In most plants, the 13-hydroxylation pathway
predominates, although in Arabidopsis and some
others, the non-13-OH-pathway is the main pathway.

10. Stage 3:
 In stage 3, in the cytosol GA12 or GA53 is converted, via
parallel pathways, to other GAs.
 This conversion proceeds with a series of oxidations at C-
20, resulting in the eventual loss of C-20 and the formation
of C19-GAs.
 In the non-13-hydroxylation pathway, this leads to the
production of GA9.
 GA9 is then oxidized to the bioactive GA4 by a 3β-
hydroxylation reaction. In the 13-hydroxylation pathway,
GA53 is sequentially oxidized at C-20, leading to GA20,
which is then 3β-hydroxylated to yield bioactive GA1.
 Finally, hydroxylation at C-2 converts GA4 and GA1 to the
inactive forms, GA34 and GA8, respectively.

12. Mode of Action:
 The gibberellin (chiefly GA,) combines with a receptor on
the outer surface of plasma-membrane of aleurone layer
cell
 The GA-receptor complex interacts with a heterotrimeric
G protein (also situated on the surface of plasma
membrane) and initiates two separate signal transduction
pathways;
(a) a calcium (Ca2+) independent signal transduction
pathway which involves cyclic GMP (cGMP) as signaling
intermediate (secondary messenger) leading to the
expression of a-amylase gene and
(b) a calcium (Ca2+) dependent signal transduction
pathway which involves calcium, calcium binding protein
calmodulin and a protein kinase as signalling
intermediates (secondary messengers) leading to the
stimulation of secretion of a-amylase and other hydrolytic
enzymes from cells of aleurone layer into the endosperm
for starch degradation.

13. .
 A GA signalling intermediate is activated.
 The activated GA signalling intermediate goes into the
nucleus and binds with DELLA repressor proteins (parts of
GAI and RGA) and also SPY (not shown in the figure).
 DELLA repressors and SPY are degraded or inactivated.
 Due to inactivation or degradation of these repressors,
GA-MYB & other genes are switched on. GA-MYB m-RNA
goes into the cytosol for translation and a GA-MYB protein
(a transcription factor) is synthesized.
 GA-MYB protein enters into nucleus and binds with
promotor genes for a-amylase and other hydrolytic
enzymes.
 α-amylase gene and other genes that encode other
hydrolytic enzymes are transcribed
 α-amylase m-RNA moves out from nucleus to rough ER in
cytoplasm for translation process and α-amylase protein
(enzyme) is synthesized.
 α-amylase proteins (enzymes) are secreted via Golgi-
bodies.

15. Site:
 Young leaves, roots and developing seeds (developing
endosperm ) and fruits.
Transport :
 Transport occurs through xylem, phloem, or cell to cell.
 Phloem seems to be most important transport route.
 Transport is non polar.

16. .

17. Physiological Role in Plants:
 It induces maleness.
 Promotes growth of dwarf plants.
 Possess pollenicide effect.
 Replaces chilling and light requirement of plants.
 Promotes seed germination.
 Used for breaking of dormancy.
 Delays senescence of fruits.
 Enhances seedless fruits.
 Promotes stem elongation.
 Accelerates flowering in long day plants.
 Intensifies transpiration, photosynthesis and respiration.

18. Practical uses in agriculture and
horticulture:
1.BOLTING AND FLOWERING:-
 Induce bolting(shoot elongation) &
flowering
 promote the growth of short day condition
 form rosette require cold treatment stem
elongation & flowering.
 Eg. Rudbekia speciosa & Hyoscyamus niger
2.ELONGATION OF INTERNODE:-
 Promote the elongation of internode{the
loosening of cell wall for stretching}
 The target of the gibberellin action is at
intercalary meristem.
 Eg. XGT (Xyloglucan trans glycosylase).

19. 3.PARTHENOCARPY:-
 Induces parthenocarpy{seedless &
fleshy fruits} in fruit-like tomatoes,
grapes,etc.
4.BREAKING OF BUD DORMANCY
 It is done by gibberellin treatment
 Eg: In potato tubers, this is treated by
gibberellins.

20. 5.MALTING OF BARLEY:-
 Gibberellin treatment
increases the α-Amylase
content of germinating
 Such grains give good quality
malt to brewing industries
 Eg: barley grains
6.HIGH SUGAR YIELD:-
 Growing of sugarcane
enhances the growth of
internodes and increases the crop
productivity & sugar yield of
crop

21. 7.CONTROL OF PLANT GROWTH:-
 To avoid the manual pruning the plant growth
regulators like MH,CCC & ethrel are
 Used for arresting apical growth of plant
 It is cheap method. There is no need of labours.
8.MODIFICATION OF SEX EXPRESSION:-
 It can induced to produce male or female flower in
large numbers
 This is known as modification of sex expression
 Ethrel, maleic, hydrozide(MH), cycocel(CCC), Tri-
iodo-benzoic acid(TIBA) are used.

22. .
9.FAST GROWTH OF SEEDLINGS:-
 GA3 is used to promote their growth rate(Applied to shoot tips)
 It also applied in rootstocks of mango, citrus, etc.
 Lead to large production in a short period.
10.BREAKING OF SEED AND BUD DORMANCY:-
 Treatment of seeds with 100ppm GA3 is useful to overcome seed dormancy
 Eg: Citrus, cherry, grapes, pears, plums, apple, peach, annona, tomato,etc.

23. Interaction With Other Hormones:
GA and Auxin Interaction:
 Gibberellins and auxins are found to induce cell elongation, parthenocarpy and
metabolic activities including RNA and protein synthesis.
 But GA and auxins have their own specific effects on different tissues of the
plant body.
 While GA promotes internode elongation, overcomes genetic dwarfism,
induces amylase synthesis in aleurone cells; it can substitute cold treatment or
far red treatment.
 IAA does not elicit any of these effects. On the other hand, auxins impose
apical dominance, induce adventitious roots and induce cellulase synthesis.
 On the other hand GAs doesn’t elicit any of the auxins’ responses.
 However, in the case of cell elongation though GA and IAA have independent
actions in promoting the growth of etiolated normal pea stem sections, if both
the hormones are provided together, their total effect is just additive but not
synergistic.

24.  On the contrary, if internodes of dwarf pea stems are treated
with either GA or IAA, the promotive effect on stem segments is
very little, but if both are provided together the stimulation in
terms of growth is highly pronounced and the total effect is
synergistic.
 The above observation indicates that gibberellins need auxins
for synergistic activity.
 This particular conclusion is further substantiated by the fact
that a decapitated internodal segment does not respond to GA
treatment alone but if the apical meristem or an agar block
containing auxin is placed on the decapitated segment,
elongation of the internode is stimulated in the presence of GA.
 This is because apical meristems do synthesize auxins, which
interact with gibberellins to bring about the combined
effect. Recent studies in in vivo and invitro system strongly
suggest that GA has an important role in promoting the
biosynthetic pathway of auxin, thus in the presence of GA, the
levels of auxins increase.
 Probably GAs enhances the rate of IAA synthesis.

26. GA and ABA Interaction:
 Plant hormonal interactions are fascinating for the simple reason that
many of them act antagonistically and some cooperatively and few
synergistically.
 Some of the interactions are concentration dependent. GA and Auxin
induce parthenocarpy; Cytokinins and auxins act antagonistically in
Auxin induced new root formation.
 Induction of dormancy and breaking dormancy are two opposite
development pathways in seeds.
 ABA can induce dormancy and GA can break the dormancy.
 Cold treatment induces vernalization in some plants in ABA
dependent manner, but vernalization can be broken by GA.

28. Role in Plant Processes:
Seed Germination:
 Cereal grains of Zea mays, sorghum, Hordium, Oryza etc
have a distinct layer around the endosperm called aleurone
layer, the cells of which are rich in protein granules.
 During germination, aleurone cells become active and
with time, they secrete enzymes into endospermous tissue,
where the reserve starch gets degraded to glucose and the
same is utilized by the growing embryos.

30. Flowering :
 Though GA is known to induce bolting and flowering in
long day plants, the effect of GA on a short day plants like
sorghum bicolor is very interesting.
 In these plants, GA3 stimulates flowering even under non
inductive conditions.
 But in combination with far red light treatment GA’s effect
is synergistic. The hormone also brings about marked
increase in the number of spikelets and glumes.
 Along with these promotive responses, GA also increases
the number of floral structures like ovaries and promotes
fertilization, but inhibits stamen development.

32. Stem Elongation:
 The transcriptional factors GAI and RGA act as
inhibitors or repressors of transcription of those genes
that leads to growth with the result that growth is
suppressed.
 SPY is also a negative regulator or repressor which acts
upstream of GAI and RGA in GA-signal transduction
chain and plays inhibitory role directly or by
enhancing the effects of GAI and RGA.
 In presence of GA, these repressors are deactivated or
degraded so that transcription of genes occur that
leads to stem elongation (growth).

34. References:
 www.plantcellbiology.masters.grkraj.org
 www.learnpick.in
 www.biologydiscussion.com