Gibberellins

GA and ABA Interaction
Gibberellins

1. welcome

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 GA 12 . In many plants, GA 12  is converted to GA 53  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 GA 12  or GA 53  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 C 19 -GAs. In the non-13-hydroxylation pathway, this leads to the production of GA 9 . GA 9  is then oxidized to the bioactive GA 4  by a 3β-hydroxylation reaction. In the 13-hydroxylation pathway, GA 53  is sequentially oxidized at C-20, leading to GA 20 , which is then 3β-hydroxylated to yield bioactive GA 1 . Finally, hydroxylation at C-2 converts GA 4  and GA 1  to the inactive forms, GA 34  and GA 8 , respectively.

11. 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 (Ca 2+ ) 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 (Ca 2+ ) 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.

12. . 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.

13. 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.

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15. 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.

16. 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 ).

17. 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.

18. 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

19. 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.

20. . 9.FAST GROWTH OF SEEDLINGS:- GA 3 is used to promote their growth rate(Applied to shoot tips) It also applied in rootstocks of mango, citrus, etc. L ead to large production in a short period. 10.BREAKING OF SEED AND BUD DORMANCY:- Treatment of seeds with 100ppm GA 3 is useful to overcome seed dormancy Eg : Citrus, cherry, grapes, pears, plums, apple, peach, annona , tomato,etc .

21. 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.

22.  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.

23. 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.  

24. 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.

25. 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.

26. 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).

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

28. Thank you