abscisicacid.pptxah.pdf by Mohd ahrar hussain

This presentation explores the critical role of abscisic acid (ABA) as a key plant hormone in regulating various physiological processes essential for plant growth and survival. We will examine how ABA acts as a signaling molecule in response to environmental stressors such as drought and salt, influencing processes like stomatal closure, seed dormancy, and root development. Through a detailed analysis of ABA’s mechanisms of action, its impact on gene expression, and its applications in agricultural practices, we aim to highlight the significant contributions of this hormone to improving plant resilience and productivity in changing climates. Join us to uncover the fascinating interactions between abscisic acid and plant life!

1. Name-Mohd Ahrar
Hussain
Roll no-22ibot27
Course-Plant
physiology
Semester-5th
SUBMITTED
TO:DR.VIKAS
SHRIVASTAVA

2. TOPIC-
ABSCISIC ACID

3. INTRODUCTION
It plays a crucial role in regulating various functionof plant growth,
development, and stress responses.
Detected in mosses but absent
in liverworts.
Several fungi genera produce
ABA as a secondary
metabolite.
Found in every major plant
organand living tissue,
including root capto apical
bud.
Synthesized in nearly all cells
that contain chloroplastsor
amyloplasts.
Abscisic acid has been found to be a ubiquitous plant hormone in vascular
plants.

4. HISTORY
•Early studies on growth inhibitors were conducted by scientistsin the 1940s showed compounds
in plants that influenced leaf drop (abscission) and bud dormancy, but the exact substances
weren’t identified.
•Lars Hambergisolated substance known asdorminfrom sycamore leavescollected in early
autumn, when the trees were entering dormancy.
•Philip wareingand eaglein the early 1960s confirmed that dorminto a bud would induce
dormancy.
•Frederick Addicottand his team discoveredcompound responsible for abscission of cotton fruit.
Two compound were isloatedand are called abscisinI and abscisinII.
•By 1965, researchers discovered that dorminand abscisinII were chemically identical.
•Later the compound was officially renamed Abscisic Acid (ABA).

5. CHEMICAL STRUCTURE
•15-carbon compound.
•orientation of the carboxyl group at carbon 2
determines the cis and trans isomers of ABA.
•all the naturally occurring ABA is in the cis form.
•Due to an asymmetric carbon atom at position 1′
in the
ring
Senantiomer (natural form)and R enantiomer
(non-natural form) formed.
•S enantiomer isresponsible for fast ABA
responses.
•Both Enantiomers are active in long-term
responses.
•chemical formula-C₁₅H₂₀O₄
•molecular weight -264.32 g/mol (aprox.)

6. Biosynthetic Pathway
The pathway begins with
Isopentenyl diphosphate (IPP),​
IPP is converted into Farnesyl
diphosphate, a C15
compound,
Farnesyl diphosphate
undergoes several reactions to
form Zeaxanthin(by enzyme
Zeaxanthin Epoxidase),
Zeaxanthin is then converted
to all-trans-Violaxanthin,
Violaxanthin is converted to
the C40 compound 9′-cis
neoxanthin, which is then
cleaved to form
xanthoxal(xanthoxin), by 9-cis-
epoxycarotenoid dioxygenase,
Xanthoxalis exported to cystol
where it is further processed to
ABA-aldehydebya short-chain
dehydrogenase/reductase
(SDR),
Then it is isoxidized to form
Abscisic Acid (ABA) by
abscisic aldehyde oxidase,

7. ABA INACTIVATION/
DEGRADATION
•conjugation to another moleculeform
ABA-β-D-glucosyl ester
•oxidation to form phaseicacid and then
dihydrophaseicacid

8. Abscisic Acid (ABA) Signaling Mechanism
•Abscisic acid (ABA) signaling involves kinase and phosphatase
activities.
•PP2C (Protein Phosphatase 2C): Dephosphorylates (inactivates)
proteins.
•SnRK2 (SNF1-related Protein Kinase 2): Phosphorylates proteins to
activate ABA responses.
•Absence of ABA (Inactive State)
•PP2C dephosphorylates SnRK2, keeping it inactive.
•Result: No ABA response is initiated.
•Presence of ABA (Active State)
•ABA binds to PYR(PYRABACTIN RESISTANCE1)/PYL(PYR1 -
LIKE)/RCAR (REGULATORY COMPONENTS OF ABA
RECEPTORS) receptors.
•These receptors interact with PP2C, inhibiting its phosphatase
activity.
•With PP2C blocked, SnRK2 is phosphorylated and activated.
•Result: ABA response pathway is initiated.

9. ABA Transport
•ABA travels through both types of
plant vessels, but it’s more
common in the phloem.
•When ABA is applied to leaves, it
moves both up to the shoots and
down to the roots.
•If the phloem is damaged (e.g., by
girdling), ABA does not
accumulate in the roots, indicating
that it is primarily transported
through phloem sap.
•ABA made in the roots can travel
up to the shoots viaxylem during
stress.
•Normally,low ABA concentration
in xylem sap, but in dry conditions,
can rise to very high levels.

10. DEVELOPMENTAL
AND
PHYSIOLOGICAL
ROLE

11. STOMATAL CLOSURE
Simplified model for ABA signaling in
stomatal guard cells. The net effect is the
loss of potassium (K+) and its anion (Cl–or
malate2–) from the cell. ROS, reactive
oxygen species; CPK, Ca2+-dependent
protein kinase; OST1, OPEN STOMATAL1
protein kinase; PP2C, protein phosphatase
2C; RBOH, respiratory burst oxidase
homolog

12. ABA Levels in Seeds Peak during
Embryogenesis
Abscisic Acid (ABA) Levels in Seeds
•Early Embryogenesis: ABA levels are low.
•Mid-to Late Embryogenesis: ABA levels peak (broad peak of accumulation).
•Seed Maturity: ABA levels gradually decrease.
Genetic Control of ABA and Hormonal Balance in Seeds
•Seed Coat: Derived from maternal tissues.
•Zygote and Endosperm: Formed from both parents.
•Zygotic Genotype:
•Controls ABA synthesis in the embryo and endosperm.
•Essential for dormancy induction.
•Maternal Genotype:
•Controls the major, early peak of ABA.
•Helps suppress vivipary (premature germination) during mid-embryogenesis.

13. Role of ABA in Desiccation Tolerance
•ABA plays a crucial role in enabling desiccation tolerance in developing
seeds.
•Desiccation can severely damage membranes and other cellular
components.
•In the mid-to late stages of seed development, high levels of ABA lead to
specific mRNA accumulation in embryos.
•These mRNAs encode Late-Embryogenesis-Abundant (LEA) proteins.
•LEA proteins are believed to contribute to desiccation tolerance.
•ABA treatment can induce the synthesis of LEA proteins in young
embryos and vegetative tissues.
•Most LEA proteins are synthesized under the regulation of ABA.

14. ABA Promotes the Accumulation of Seed
Storage Protein
•ABA (Abscisic Acid) is crucial in mid-to-late embryogenesis, promoting the accumulation of storage
compounds like proteins.
•High ABA levels may impact the synthesis and translocation of essential sugars and amino acids.
Studies in ABA Mutants
•Showed ABA has no direct effect on sugar translocation.
•ABA’s primary role is likely in storage protein synthesis.
•Exogenous ABA enhances storage protein levels in cultured embryos
•ABA-deficient/insensitive mutants show reduced storage protein levels.
•Indicates ABA as one of multiple signalsfor storage protein gene expression.
•ABA not only regulates the accumulation of storage proteins, also maintains mature seeds in a
dormant state until environmentalconditions improve.

15. Role of ABA and GA in Seed Dormancy
•ABA (Abscisic Acid)promotes dormancy, helping seeds remain inactive.
•GA (Gibberellins)promotes germination by encouraging growth and breaking
dormancy.
•The ABA to GA Ratio
•The balance, or ratio of ABA to GA, determines whether a seed remains dormant or
begins to germinate.
•High ABA Ratio→ Dormancy is maintained.
•Low ABA Ratio→ Germination is promoted.
•Environmental factors (like light, temperature, and water) can alter ABA and GA levels,
thus affecting the ABA ratio and influencing dormancy or germination.
•By adjusting this ratio, plants can control seed dormancy in response to environmental
conditions, enhancing survival in unfavorable conditions.

16. ABA Inhibits Precocious Germination and Vivipary
•Precocious Germination:Early germination without dormancy
•Vivipary:Germination while still attached to the parent plant
•Immature embryos germinate prematurely if removed and cultured.
•Adding ABA to the culture prevents this premature germination.
•ABA levels naturally rise in mid-to-late seed developmentto keep embryos in their non-
germinating state and acts as a natural inhibitor of premature germination.
•Genetic mutants (e.g., vp2, vp5, vp7, vp14) show vivipary due to ABA deficiency.
•Applying ABA partially prevents vivipary in these mutants.
•ABA deficiency leads to vivipary, showing ABA’s role in suppressing germination.

17. ABA Accumulates in Dormant Buds
•ABA (Abscisic Acid) was initially considered the main dormancy-inducing hormone
•ABA accumulates in dormant buds, decreasing upon exposure to low temperatures
•However, ABA levels do not always directly correlate with dormancy level
•Inconsistencies in ABA levels and dormancy may result from interactionsbetween
ABA andother hormones
•Bud dormancy is likely controlled by the balance between Growth inhibitors (like
ABA) andGrowth promoters (like cytokininsand gibberellins).

18. ABA Inhibits GA-Induced Enzyme Production
•ABA inhibits the synthesis of hydrolytic enzymes needed for breaking down seed
storage reserves.
•Gibberellic acid (GA) stimulates the aleurone layer in cereal grains to produce α-
amylase and other hydrolytic enzymes for breakdownbreak down stored resources in
the endosperm during germination.
•ABA inhibits GA-dependent enzyme synthesis by preventing the transcription of α-
amylase mRNA.
•Mechanisms of ABA Action:
•Mechanism 1: VP1 protein acts as a transcriptional repressor for certain GA-regulated
genes (Hoecker et al., 1995).
•Mechanism 2: ABA represses the expression of GA-MYB, a transcription factor required
for GA-induced α-amylase expression

19. Roots and shoots growth
ABA has different effects on the root and shoot growth, and the effects are
strongly dependent on the water status of the plant.
Under low water potential, when ABA levels are high, the endogenous
hormone exerts a strong positive effect on root growth by suppressing
ethylene production, and a negative effect on shoot growth. Endogenous
ABA acts as a signal to reduce shoot growth only under water stress
conditions.

20. REFERENCE
• Plant physiology and development by Taiz and
Zeiger ( 6th edition )
• Plant physiology (3rd edition ) by Taiz and
Zeiger
• Life Science fundamental and practices
Pathfinder (ninth edition)