Plant Hormones (GAs, CK, ABA, Ethylene and 3rd Groups of Plant Hormones) ....Part III

Plant Hormones
Part III
Dr Ashok Kr Yadav
Professor Horticulture [Fruit Science]
Guru Kashi University, Bathinda, India
Mail: yadavakdr@gmail.com
Mobile: 7300511143, 8869847446

Discovery of Plant Hormones
The discovery of plant hormones, also known as phytohormones, unfolded through a series of
key experiments and observations, with auxin being the first to be identified. Charles Darwin's
work on plant phototropism (movement in response to light) in the late 1800s laid the
groundwork, suggesting the existence of a "transmissible substance" that influenced plant
growth. Later, scientists like Peter Boysen-Jensen, Frits Went, and others furthered this research,
leading to the isolation and identification of auxin and other plant hormones like gibberellins,
cytokinins, and abscisic acid.
Here's a more detailed look at the historical points:
 Early Observations and the Darwin's Work (Late 1800s):
 Charles Darwin and his son Francis studied the bending of canary grass coleoptiles (the
protective sheath of a grass seedling) towards light. They observed that the tip of the
coleoptile was responsible for perceiving light, and a signal was transmitted downwards to
cause bending. This led them to propose the existence of a substance that regulated this
movement.
 Boysen-Jensen's Contribution (1910):
 Peter Boysen-Jensen conducted experiments that involved placing a block of gelatin between
the tip and the rest of the coleoptile. When the tip was exposed to light, the coleoptile still

 Cholodny-Went Hypothesis (1920s):
 Independently, Frits Went and Nikolai Cholodny proposed that the bending of plant
shoots towards light was due to the asymmetric distribution of a growth-promoting
substance in the coleoptile.
 Isolation and Identification of Auxin (1926-1934):
 Fritz Went isolated the "growth substance" from oat coleoptiles and demonstrated its
ability to induce curvature in a dose-dependent manner. Later, in the 1930s, scientists
like Kögl, Haagen-Smit, and Erxleben isolated and identified the first auxin as indole-3-
acetic acid (IAA).
 Discovery of Other Hormones:
 Following the identification of auxin, other plant hormones were discovered,
including gibberellins (affecting stem elongation), cytokinins (affecting cell division
and differentiation), and abscisic acid (regulating dormancy and stress responses).
 In essence, the discovery of plant hormones was a gradual process, starting with
observations of plant behavior and culminating in the identification of specific
chemical messengers that regulate various aspects of plant life.

Here's a more detailed timeline:
 1880:
 Charles Darwin and his son Francis conducted experiments on coleoptiles (sheaths of young grass leaves) and
observed their bending towards light, suggesting the existence of a transferable influence.
 1910:
 Peter Boysen-Jensen demonstrated the existence of a mobile substance by showing that a translucent barrier did
not prevent phototropic bending.
 1926-1928:
 Frits Went isolated and characterized auxin, demonstrating its role in plant growth and development.
 1934:
 F. Kögl identified indole-3-acetic acid (IAA) as a naturally occurring auxin.
 1955:
 Kinetin, a synthetic cytokinin, was discovered, marking a significant advancement in understanding this class
of plant hormones.
 1960s:
 Abscisic acid and ethylene were isolated and characterized.
 1970s:
 Brassinosteroids were identified as a new class of plant hormones.
 Later:
 Jasmonates, salicylic acid, and strigolactones were recognized as additional plant hormones.

Plant Hormones
Plant hormones, also known as phytohormones, are organic substances
produced by plants that regulate growth, development, and various
physiological processes at extremely low concentrations. These
signaling molecules, synthesized in very small quantities within the
plant, influence a wide array of functions from embryogenesis to
responses to environmental stimuli.
Plant hormones are chemical compounds found in a plant's body in a
very low concentration. They mainly derive from indole (auxins),
terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic
acid) and gases (Ethylene).
These are produced in every part of the plant and circulate throughout a
plant’s body.
Each hormone has its own role in a plant’s body. Different hormones
carry out different functions individually or in combination with other
hormones.
Hormones play a very important role in a plant’s body and facilitate
processes like vernalization, phototropism, seed germination, dormancy
etc. with the help of external factors like sunlight, water and oxygen.
Crop production is controlled by the application of synthetic hormones.

Key Characteristics
The word hormone is derived from Greek Harmao, meaning set in motion or
pushing the growth. Early in the study of plant hormones, "phytohormone" was
the commonly used term, but its use is less widely applied now.
Plant hormones affect gene expression and transcription levels, cellular division,
and growth.
They are naturally produced within plants, though very similar chemicals are
produced by fungi and bacteria that can also affect plant growth.
Both natural hormones and many synthetic compounds are used in agriculture as
plant growth regulators (PGRs) to regulate the growth of cultivated plants, weeds,
and in vitro-grown plants and plant cells.
Plant hormones are not nutrients, but chemicals that in small amounts
promote and influence the growth, development, and differentiation of cells
and tissues.

Plant Hormones (GAs, CK, ABA, Ethylene and 3rd Groups of Plant Hormones) ....Part III

Gibberellins Part III
[Principles, chemistry and Functions]
Gibberellins are diterpenoid acids synthesized by the terpenoid pathway in plastids and then modified in the
endoplasmic reticulum and cytosol until they reach their biologically active form. All are derived via the ent-
gibberellane skeleton but are synthesized via ent-kaurene. The gibberellins are named GA1 through GAn in order of
discovery. Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA3 .Gibberellins are
tetracyclic diterpene acids. There are two classes, with either 19 or 20 carbons. The 19-carbon gibberellins are
generally the biologically active forms. They have lost carbon 20 and, in place, possess a five-member lactone bridge
that links carbons 4 and 10. Hydroxylation also has a great effect on its biological activity. In general, the most
biologically active compounds are dihydroxylated gibberellins, with hydroxyl groups on both carbons 3 and 13.
Gibberellic acid is a 19-carbon dihydroxylated gibberellin.
Landsberg erecta
[Arabidopsis plant]

Chemistry
 All are derived via the ent-gibberellane skeleton but are synthesized via ent-kaurene. The gibberellins are named
GA1 through GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally
characterized, is GA3. As of 2020, there are 136 GAs identified from plants, fungi, and bacteria. Gibberellins are
tetracyclic diterpene acids. There are two classes, with either 19 or 20 carbons.
 Some gibberellins have the full complement of all 20 carbons of gibberellin skeleton. These are called as C20 – GAs e.g., GA12,
GA27, GA53 etc. Others have lost one carbon (20th carbon) to metabolism and contain only 19 carbon atoms. These are called
as C19 – GAs e.g., GA1, GA3, GA20 etc. (Fig. 17.16). Besides this, there are other minor variations in basic structure of different
GAs such as number and positions of – OH and methyl groups and state of oxidation at C – 20 (in case of C20 – GAs). All GAs
have – COOH group at 7th carbon position.
 In general, C19 – GAs appear to be more active biologically than C20 – GAs. In addi
tion, those GAs with 3- β -hydroxylation, 3- β
-1, 3 dihydroxylation or 1, 2-unsaturation are generally more active while those with both 3-β-OH and 1, 2-unsaturation such
as GA3 exhibit the highest biological activity. GA3 is however, rare in higher plants. GA1 and GA20 (both C19 – GAs) are perhaps
the most important biologically active GAs in higher plants.

History of Gibberellins
 The effect of gibberellins had been known in Japan since early 1800, where certain rice plants were found to suffer from
bakane or bakanae (foolish seedling) disease. Such rice plants were thin, pale green, spindle shaped, longer by 50% than the
healthy plants, and were sterile. The disease was found by Hori (1918) and Kurosawa (1926) to be caused by a fungus,
Gibberella fujikori. The fungus is the perfect stage of Fusarium moniliforme.
 Kurosawa also found that the sterile filtrate of the fungus also caused appearance of disease symptoms in uninfected rice
seedlings. The active substance was separated and named gibberellin by Yabuta (1935). Yabuta (1938) also prepared
crystalline form of gibberellin (it actu
ally consisted of six gibberellins).
 Japanese work came to light only after World War II. Gibberellic acid or GA3 was isolated in pure form by Brian et al in 1955.
Cross (1961) worked out the structure of gibberellic acid, GA3.
 Gibberellins, also known as gibberellic acid, are a class of tetracyclic diterpenoid plant hormones primarily responsible for
regulating growth and development. They are characterized by a tetracyclic (four-ringed) structure and are acidic in nature.
Gibberellins influence various processes, including stem elongation, seed germination, flowering, and fruit development.

Bioactive GAs
 The bioactive Gibberellins are GA1, GA3, GA4, and GA7.There are three common
structural traits between these GAs: 1) hydroxyl group on C-3β, 2) a carboxyl
group on carbon 6, and 3) a lactone bridge inbetween carbons 4 and 10.
 The 3β-hydroxyl group can be exchanged for other functional groups at C-2
and/or C-3 positions. GA5 and GA6 are examples of bioactive GAs without a
hydroxyl group on C-3β.
 The presence of GA1 in various plant species suggests that it is a common
bioactive GA.
Gibberellin A1 (GA1) Gibberellic acid (GA3) ent-Gibberellane ent-Kaurene

Gibberellins (GAs) are a class of tetracyclic diterpenoid plant hormones
characterized by a 6-5-6-5 fused ring structure, also known as the gibberellin
skeleton, are essential phytohormones for plant growth and development.
The more than 136 GAs discovered to date have been classified into two groups
based on their number of carbon atoms: C19-GAs (with one carboxylic group at
the C-7 position, e.g., GA20 and GA9) and C20-GAs (with two carboxylic groups
at the C-7 and C-19 positions, respectively, e.g., GA12 and GA53).
GAs have also been classified into two groups based on other chemical
structural criteria: [1] 13-H GAs (hydrogen at the C-13 position) and [2] 13-OH
GAs (hydroxyl group at the C-13 position).
Plant biologists consider GA4, GA1 (also known as 13-OH GA4), GA7, and GA3
(also known as 13-OH GA7) to be the only common bioactive GAs in flowering
plants.
The bioactivity of GA1 in plants is 1000-fold lower
∼ than that of GA4
(Eriksson et al., 2006).

Gibberellins Biosynthesis
Gibberellin biosynthesis is a multi-step process that primarily occurs in plastids,
the endoplasmic reticulum, and the cytoplasm of plant cells. It begins with the
precursor isopentenyl pyrophosphate (IPP) and involves a series of enzymatic
reactions that ultimately lead to the formation of various gibberellin (GA)
hormones, which regulate plant growth and development.
Here's a breakdown of the key stages:
1. Plastid Stage:
• The biosynthesis starts with the formation of isopentenyl pyrophosphate (IPP)
from acetyl-CoA through the mevalonate pathway.
• IPP is then converted into geranylgeranyl diphosphate (GGPP), a precursor for
many terpenoids.
• Two enzymes, ent-copalyl diphosphate synthase (CPS) and ent-kaurene
synthase (KS), located in plastids, convert GGPP into the tetracyclic hydrocarbon
ent-kaurene.

Early and intermediate
steps of GA biosynthesis
in higher plants (green
arrows) and the fungus
Fusarium fujikuroi (red
arrows). In plants, ent-
kaurene is synthesised in
plastids, predominately
via the methylerythritol
phosphate pathway,
while in fungi, it is
biosynthesised from
mevalonic acid.
Conversion of ent-
kaurene to GA12 and
GA53 (plants) and GA14
(fungi) is catalysed by
membrane-associated
cytochrome P450
monooxygenases.
Arrows running through
structures indicate
multiple steps catalysed
by single enzymes

 Generalized scheme of
gibberellin (GA) biosynthesis and
deactivation in higher plants.
 GA biosynthesis starts in the
plastids and is followed by the
production of GA12 in the
endoplasmic reticulum.
 In the cytoplasm GA12 is
processed by GA20ox and GA3ox
enzymes in two separate
branches to produce bioactive
GAs (gray circles).
 The non-13-hydroxylation yields
GA7 and GA4, whereas the 13-
hydroxylation yields GA5, GA6,
GA3, and GA1.
 The GA2ox enzymes deactivate
precursors and bioactive GAs.
Abbreviations: GGDP,
geranylgeranyl diphosphate; CPS,
ent-copalyl diphosphate synthase;
KS, ent-kaurene synthase; KO,
ent-kaurene oxidase; KAO, ent-
kaurenoic acid oxidase.

 Diagrammatic
representation of
biosynthesis of
gibberellins in plants.
 CPS: entCopalyl
diphosphate synthase;
KS: ent-Kaurene
synthase; KO: ent-
Kaurene oxidase; KAO:
entKaurenoic acid
oxidase; GA 20-ox:
gibberellin 20-oxidase;
GA 3-ox: gibberellin 3-
oxidase; GA 2-ox:
gibberellin 2-oxidase

2. Endoplasmic Reticulum Stage:
• Ent-kaurene is then transported to the endoplasmic reticulum (ER).
• A series of oxidation reactions catalyzed by cytochrome P450 monooxygenases
(P450s) converts ent-kaurene into GA12, a key intermediate in GA biosynthesis.
3. Cytoplasm Stage:
• GA12 is further metabolized in the cytoplasm into various bioactive GAs through the
action of enzymes like GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox).
• These bioactive GAs, such as GA1 and GA4, regulate various aspects of plant growth,
including stem elongation, seed germination, and flowering.
4.Regulation of GA Biosynthesis:
• GA biosynthesis is tightly regulated by both developmental and environmental cues.
• Factors like light, temperature, water, and nutrient status can influence the
expression of genes involved in GA synthesis.
• For example, short photoperiods can advance the peaks of GA20 and GA1,
promoting flowering in some plants.

 Gibberellin biosynthetic
pathway in Fusarium
fujikuroi, Sphaceloma
manihoticola and
Phaeosphaeria sp L487.
Common steps for these fungi
are indicated as blue arrows.
Specific steps for F. fujikuroi
are indicated as black arrows,
whereas specific steps for
Phaeosphaeria sp. L487 are
indicated as green arrows.
 The main route followed by F.
fujikuroi after the synthesis of
GA12-aldehyde is indicated
with yellow arrows. Enzyme
names are preceded by the
letter F (for F. fujikuroi), S (for
S. manihoticola) or P (for
Phaeosphaeria sp L487),
which correspond to the
initials of the fungi where
they are present (adapted
from Bömke & Tudzynski,
2009)

 Simplified GA metabolic pathway in flowering plants.
All enzymes mapped to the GA metabolic pathway,
except GA13ox from Tripterygium wilfordii, were
verified with enzymatic assays and the chemical
profiling of transgenic plants. P450s (ER membrane-
associated proteins) in this pathway are highlighted in
blue, while, 2-oxoglutarate-dependent dioxygenases
(2-ODDs, cytosolic proteins) are highlighted in purple.
All dashed lines indicate enzymatic a step that has not
yet been characterized at the genetic level in any
flowering plant.
 The genes/enzymes characterized from Arabidopsis
and rice are marked in black and blue, respectively.
The carbon backbone of GA12 is labeled with numbers,
and the bioactive GAs (GA1, GA3, GA4, GA7, and
DHGA12) are marked in Red. The Arabidopsis GA-
deficient mutants (ga1 to ga5) used for map-based
cloning are shown in parentheses next to the
corresponding genes.
 CPS, ent-copalyl diphosphate synthase; GAMT, GA
methyltransferase; GAS2, gain of function in ABA-
modulated seed germination 2; GGPPS, geranylgeranyl
diphosphate synthase; KAO, ent-kaurenoic acid oxidase;
KO, ent-kaurene oxidase; KS, ent-kaurenoic acid
synthase; MEP, 2-C-methyl-D-erythritol 4-phosphate.

Functions of Gibberellins
Gibberellins (GAs) are a class of plant hormones that play a fundamental role in regulating several aspects of plant
growth and development. They are involved in processes such as stem elongation, seed germination, leaf expansion,
flowering, and fruit development. Here’s an overview of the roles of gibberellins in plant growth and development:
1. Stem Elongation and Cell Expansion:
• Stimulating Cell Growth: Gibberellins promote stem elongation by stimulating cell division and elongation. They
help cells grow longer and increase the overall length of stems and other plant organs.
• Cell Wall Loosening: Gibberellins activate enzymes that break down the rigid components of the cell wall, allowing
cells to expand and elongate.
2. Seed Germination:
• Breaking Seed Dormancy: Gibberellins are critical for breaking seed dormancy and initiating seed germination.
They activate the enzymes that degrade stored food in the seed, allowing the embryo to grow.
• Radicle Growth: Gibberellins promote the growth of the embryonic root (radicle) during germination.
3. Leaf Expansion:
• Promoting Leaf Growth: Gibberellins influence the expansion of leaves, leading to larger leaf size and enhanced
photosynthesis.
4. Flowering and Fruit Development:
• Floral Initiation and Growth: Gibberellins play a role in promoting flowering by influencing the differentiation of
floral buds. They also influence the growth and development of flowers.
• Fruit Growth and Seed lessness: Gibberellins influence fruit growth, affecting fruit size and development. They
can also help in the development of seedless fruits.

Schematic representation of detrimental
effects of stresses and their mitigation
by gibberellins in plants.
(a) Various deleterious effects caused by
different stresses which leads to
alteration in various physiological
parameters and enhance production of
Reactive oxygen species (ROS) species
in different cellular compartments of the
cell and cause oxidative stress which
ultimately leads to DNA damage, lipid
peroxidation, enzyme inactivation etc. in
plants.
(b) The gibberellic acid mediated stress
alleviating strategies in plants by
enhancing production of various stress
defensive genes and antioxidants which acts
as ROS scavengers and reduce ROS
accumulation and leads to better growth of
plants
[Reactive oxygen species (ROS) in plants are molecules containing oxygen that
are highly reactive and can cause damage to cellular components.]

5. Dormancy Break in Buds and Bulbs:
Bud Dormancy Break: Gibberellins help in breaking bud dormancy in certain plants,
promoting new shoot growth and branching.
Bulb Sprouting: Gibberellins promote the sprouting of bulbs after a period of dormancy.
6. Apical Dominance and Lateral Branching:
Reducing Apical Dominance: Gibberellins reduce apical dominance by promoting lateral bud
growth and branching, resulting in a bushier plant.
7. Stress Response:
Stress Mitigation: Gibberellins help plants cope with various stresses, including drought and
salinity, by regulating growth and stress-responsive genes.
8. Male Sterility and Fertility:
Fertility in Flowers: Gibberellins play a role in regulating the fertility of flowers, influencing
the development of reproductive structures.
9. Interaction with Other Hormones:
Synergy with Auxins: Gibberellins often work in synergy with auxins, another class of plant
hormones, to regulate various growth processes, striking a balance between cell division and
cell elongation.

Cytokinins: History, Chemistry and Functions
 The idea of specific substances required for cell division to occur in plants actually dates back to the Swiss physiologist J.
Wiesner, who, in 1892, proposed that initiation of cell division is evoked by endogenous factors, specifically a proper balance
among them.
 Austrian plant physiologist, G. Haberlandt, reported in 1913 that an unknown substance diffuses from the phloem tissue
which can induce cell division in the parenchymatic tissue of potato tubers.
 In 1941, Johannes Van Overbeek found that the milky endosperm of immature coconut also had this factor, which stimulated
cell division and differentiation in very young Datura embryos.
 Jablonski and Skoog (1954) extended the work of Haberlandt and reported that a substance present in the vascular tissue
was responsible for causing cell division in the sith cells.
 Miller and his co-workers (1954) isolated and purified the cell division substance in crystallized form from autoclaved herring
fish sperm DNA.
 This active compound was named as kinetin because of its ability to promote cell division and was the first cytokinin to be
named. Kinetin was later identified to be 6-furfuryl-amino purine. Later on, the generic name kinin was suggested to include
kinetin and other substances having similar properties.
 The first naturally occurring cytokinin was isolated and crystallized simultaneously by Miller and D.S. Lethum (1963–65)
from the milky endosperm of corn (Zea mays) and named Zeatin. Lethem (1963) proposed the term cytokinins for such
substances.
 Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are
involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.

Chemistry and structure
 In 1956, Miller isolated the first cell-division inducing factor
from autoclaved herring sperm DNA which they named kinetin
and identified as 6-(furfurylamino) purine. It was later
revealed that kinetin is not a natural component of DNA but is
its breakdown product. Synthetic kinetin is also available as a
powerful promoter of cell division.
 In 1963, Letham isolated 6-(4-hydroxy-3-methylbut-trans-
2-enylamino) purine from immature kernels of Zea mays and
named it ‘zeatin’. Other active cytokinins widespread in
plants are N6
, N6
-dimethylallyIaminopurine or N6
(Δ2
–
isopentenyl) adenine (i6
ADE) and its ribosyl derivative N6
(Δ2
-
isopentenyl) adenosine (i6
A).
In plants, cytokinins are the primary class of cytokines, and
they are broadly categorized into two chemical types:
adenine-type cytokinins and phenylurea-type
cytokinins. Adenine-type cytokinins, such as kinetin,
zeatin, and 6-benzylaminopurine, are the most common
and are primarily synthesized in roots. Phenylurea-type
cytokinins include compounds like diphenylurea and
thidiazuron (TDZ). These chemical differences impact their
activity and roles within the plant.

Here's a more detailed look:
1. Adenine-type Cytokinins:
Structure:
These cytokinins are characterized by an adenine base modified with an isoprenoid side chain, often
attached at the N6 position.
Examples:
 Zeatin: A naturally occurring cytokinin found in many plant species.
 Kinetin: An early synthetic cytokinin.
 6-Benzylaminopurine (6-BAP): Another synthetic cytokinin widely used in plant tissue culture.
Synthesis:
Primarily synthesized in the roots, but also produced in other actively dividing tissues like the cambium.
2. Phenylurea-type Cytokinins:
Structure:
These cytokinins contain a phenylurea group instead of the isoprenoid side chain found in adenine-type
cytokinins.
Examples:
 Diphenylurea: A synthetic cytokinin.
 Thidiazuron (TDZ): A potent synthetic cytokinin with a thiazolyl urea structure.
Activity:
Generally considered more potent than adenine-type cytokinins in certain applications, such as inducing
shoot formation in tissue culture.
Other Considerations:
Phytocytokines:
A broader category of plant peptides, some of which are involved in plant immunity and may overlap with
or be considered analogous to animal cytokines.
Dual Roles:
Some plant hormones initially identified for development, stress response, or other functions have been
shown to play roles in plant immunity, and vice versa.
Microbial Cytokinins:
Interestingly, some microbial molecules have been shown to mimic or interact with plant cytokinin
signaling pathways.

 Cytokinins, a class of plant
hormones, share a core structure
based on a modified adenine
molecule with a side chain attached
to the N6 position.
 This side chain is crucial for
determining the specific cytokinin's
activity and can be either isoprenoid
or aromatic. Naturally occurring
cytokinins often feature isoprenoid
side chains, while synthetic ones
may have aromatic or phenyl urea

Chemical structures of cytokinins (CKs); adenine-type and phenylurea-type cytokinins

Cytokinin types. Cytokinins are N⁶-derivatives of adenine either with isoprenoid or aromatic side chains. A hydroxyl group
further differentiates zeatins from isopentenyladenine and topolins from benzyladenine. Further, kinetin is an aromatic cytokinin
and 6-(3-methylpyrrol-1-yl)purine is at the borderline of isoprenoid and aromatic cytokinins, because it has an aromatic side
chain but originates from trans-zeatin. Methoxytopolins exist in analogous isomers as topolins.

Cytokinins Biosynthesis
Cytokinin biosynthesis primarily involves the conversion of adenine nucleotides into active cytokinin forms through a series of
enzymatic steps. The process begins with the transfer of an isopentenyl group from dimethylallyl diphosphate (DMAPP) to the N6
position of ATP or ADP, catalyzed by the enzyme isopentenyl transferase (IPT). This forms isopentenyladenosine-5 -
′
monophosphate (iPMP). Further modifications, including hydroxylation and removal of the ribose-5 -monophosphate group by
′
the Lonely Guy (LOG) enzyme, result in active cytokinin forms like isopentenyladenine (iP) and zeatin.
Here's a more detailed breakdown:
1. Initial Substrates:
The process starts with adenosine-5 -phosphates like ATP or ADP.
′
2. Isopentenyl Transfer:
The enzyme IPT catalyzes the transfer of an isopentenyl group from DMAPP to the N6 position of ATP or ADP, forming iPMP.
3. Hydroxylation:
The isopentenyl side chain of iPMP is hydroxylated to form zeatin-type cytokinins, specifically zeatin riboside-5 -monophosphate (ZMP).
′
4. Activation to Free Base Forms:
The LOG enzyme removes the ribose-5 -monophosphate group from iPMP and ZMP, releasing the active free base forms of cytokinins, iP and
′
zeatin.
5. Further Modifications:
Additional modifications and conversions can occur, leading to various cytokinin derivatives.
There are two main pathways for cytokinin biosynthesis in plants: the adenosine phosphate-IPT pathway, which primarily produces trans-zeatin
isomers, and the tRNA-IPT pathway, which produces cis-isomers through tRNA degradation. IPT is a key enzyme in both pathways, catalyzing the
initial prenylation of adenosine phosphates.

Key Steps in Cytokinin Biosynthesis
The biosynthesis of cytokinins primarily involves the conversion of adenine nucleotides to cytokinin
forms through several enzymatic steps. Here’s a simplified overview of the biosynthesis pathway:
1. Initial Substrates
 Adenosine-5 -phosphates:
′ The process begins with adenine nucleotides like ATP (adenosine
triphosphate) or ADP (adenosine diphosphate).
2. Isopentenyl Transfer
 Isopentenyl Transferase (IPT): This enzyme catalyzes the first committed step in cytokinin
biosynthesis. IPT transfers an isopentenyl group from dimethylallyl diphosphate (DMAPP) to the N6
position of ATP or ADP, forming isopentenyladenosine-5 -monophosphate (iPMP).
′
3. Hydroxylation
 Cytokinin Hydroxylase: The isopentenyl side chain of iPMP is hydroxylated to form zeatin-type
cytokinins. This involves the conversion of iPMP to zeatin riboside-5 -monophosphate (ZMP).
′
4. Phosphoribohydrolase Activity
 Lonely Guy (LOG) Enzyme: LOG converts inactive cytokinin nucleotides (iPMP, ZMP) into their
active free base forms, such as isopentenyladenine (iP) and zeatin, by removing the ribose-5 -
′
monophosphate group.
5. Further Modifications
 Conjugation: Cytokinins can undergo further modifications, such as glycosylation, where sugar
molecules are added, forming O-glucosides or N-glucosides. These conjugates can be storage
forms or inactive forms that can be activated when needed.

 Proposed cytokinin biosynthesis and metabolic
pathways in Arabidopsis with numbers representing
enzymes responsible for the illustrated conversions: (1)
phosphatase, (2) 5'-ribonucleotide phosphohydrolase,
(3) adenosine kinase, (4) adenosine nucleosidase, (5)
purine nucleoside phosphorylase, (6) adenine
phosphoribosyltransferase, (7) cis-trans isomerase, (8)
zeatin reductase, (9) cis-hydroxylase, (10) LOG
phosphoribohydrolase (Modified from [125, 126, 146,
225]).
 Abbreviations: HMBDP: (E)-4-hydroxy-3-methyl-but-2-
enyl diphosphate; DMAPP: Dimethylallyl pyrophosphate;
MEP: Methylerythritol phosphate pathway; MVA: the
mevalonate pathway; CYP735A: CK transhydroxylase; iP:
Isopentenyladenine; Z: trans Zeatin; cZ: cis Zeatin; DZ:
Dihydrozeatin; [9R] iP: Isopentenyladenosine; [9R]Z:
trans Zeatin riboside; [9R] Z: cis Zeatin riboside, (cis; [9R]
DZ: Dihydrozeatin riboside; [9R-NT] iP:
Isopentenyladenosine monophosphate; [9R-NT] Z: trans
Zeatin riboside monophosphate; [9R-NT] Z: cis Zeatin
riboside monophosphate, (cis); [9R-NT] DZ:
Dihydrozeatin riboside monophosphate; Ado:
Adenosine; Ade: Adenine; AMP: Adenosine
monophosphate; ADP: Adenosine diphosphates; ATP:
Adenosine triphosphates.

Cytokinin Metabolism
 Cytokinins are a class of N6-adenine derivatives that
can be divided into isoprenoid and aromatic cytokinins
on the basis of side-chain structure. Isoprenoid
cytokinins are widely present in plants, and their
synthesis and metabolism have been studied
extensively.
 Isoprenoid cytokinins mainly include iP-, tZ-, cZ-, and
DZ-type cytokinins. The initiation step of isoprenoid
cytokinin biosynthesis is catalyzed by IPTs, which are
classified into three types: ATP/ADP-IPT, AMP-IPT, and
tRNA-IPT.
 ATP/ADP-IPT is present mainly in plants, whereas AMP-
IPT is present mainly in bacteria and non-plant
eukaryotes, with tRNA-IPTs possessing a much broader
taxonomic distribution. In this review, we mainly focus
on cytokinin synthesis in plants. ATP/ADP-IPT catalyzes
cytokinin synthesis using dimethylallyl diphosphate
and ATP/ADP as substrates to produce iP nucleotides,
including iP riboside 5 -triphosphate (iPRTP)
′ and iP
riboside 5 -diphosphate (iPRDP),
′ tRNA-IPT catalyzes
the synthesis of cis-hydroxy isopentenyl tRNAs, which
are transformed into cZ riboside 5 -monophosphate.
′

Significance of Cytokinins in Plant Biology
1. Shoot and Root Development
 Shoot Initiation: Cytokinins promote the formation of shoots in tissue culture and influence shoot apical
meristem activity.
 Root Growth: While cytokinins generally inhibit root growth, they promote root hair formation and lateral root
development.
2. Delay of Senescence
 Leaf Longevity: Cytokinins delay leaf senescence by maintaining chlorophyll levels and photosynthetic activity.
3. Nutrient Mobilization
 Source-Sink Relationship: Cytokinins regulate the allocation of nutrients by influencing source-sink relationships,
enhancing nutrient mobilization to growing tissues.
4. Stress Responses
 Abiotic Stress: Cytokinins can modulate plant responses to abiotic stresses such as drought, salinity, and
temperature extremes.
Conclusion
Cytokinin biosynthesis is a complex and highly regulated process essential for plant growth and development. By
understanding the pathways and regulation of cytokinin production, researchers can better manipulate these
hormones to enhance plant growth, improve crop yields, and develop stress-resistant plants. Advances in
biotechnology continue to shed light on the intricate mechanisms governing cytokinin biosynthesis, offering new
opportunities for agricultural innovation.

Abscisic acid (ABA) :Chemistry and Biosynthesis
Abscisic acid (ABA) is a sesquiterpenoid, meaning it is a 15-carbon molecule derived
from three isoprenoid units. In plants, ABA is a crucial hormone that regulates various
physiological processes, including stress responses, development, and senescence. It
plays a key role in seed dormancy, germination, and adaptation to environmental stresses
like drought and salinity.
Key points about ABA as a sesquiterpenoid:
Structure:
• ABA is a monocyclic sesquiterpenoid carboxylic acid.
Biosynthesis:
• ABA is synthesized in plants from carotenoids, with a key step involving the cleavage
of a C40 carotenoid by the enzyme NCED (9-cis-epoxycarotenoid dioxygenase).
Function:
• ABA is a primary plant hormone involved in responses to abiotic stresses like drought,
salinity, and low temperatures. It also regulates seed dormancy and germination, as
well as stomatal movement.
Ubiquity:
• ABA is found in almost all plant cells with chloroplasts or amyloplasts, and its core
signaling pathways are conserved across land plants.
Beyond plants:
• While ABA primarily functions as a signaling molecule in plants, it is also found in
some bacteria, fungi, and various metazoans

ABA Biosynthesis
Abscisic acid (ABA) is an isoprenoid plant hormone, which is synthesized in the
plastidal 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway; unlike the structurally
related sesquiterpenes, which are formed from the mevalonic acid-derived precursor
farnesyl diphosphate (FDP), the C15 backbone of ABA is formed after cleavage of C40
carotenoids in (MEP) pathway. Zeaxanthin is the first committed ABA precursor; a
series of enzyme-catalyzed epoxidations and isomerizations via violaxanthin, and final
cleavage of the C40 carotenoid by a dioxygenation reaction yields the proximal ABA
precursor, xanthoxin, which is then further oxidized to ABA. via abscisic aldehyde.
Abamine has been designed, synthesized, developed and then patented as the first
specific ABA biosynthesis inhibitor, which makes it possible to regulate endogenous
levels of ABA.

Biosynthesis of ABA
 Plants synthesize ABA using the
carotenoid pathway initiated from
β-carotene (C40). Processes that
convert C40 to xanthoxin (C15) all
take place in plastids, and in the
cytoplasm, ABA2 and AAO3 convert
xanthoxin into ABA. Among these
processes, conversion of 9 -cis-
′
neoxanthin and 9 -cis-violaxanthin
′
to xanthoxin by NCEDs is a rate-
limiting step in ABA biosynthesis.
 ABA catabolism is controlled by
both ABA conjugation and catalytic
hydroxylation. Abscisic acid can be
glucosylated into ABA-GE by
UGT71C5, whereas AtBG1 and
AtBG2 can transform ABA-GE to
active ABA. ABA can be catalyzed to
phaseic acid (PA) by CYP707As,
which in turn is catalyzed to
dihydrophaseic acid (DPA) by PA
reductase (PAR).

Pathway of abscisic acid
biosynthesis (modified
from Nambara and
Marion-Poll, 2005).
Sources: ZEP: Arabidopsis-
Ataba1/npq2/los6, N.
plumbaginifolia-Npaba2,
Rice-Osaba1; NCED: Maize-
vp14, Tomato-notabilis,
Arabidopsis-Atnced3;
ABA2: Arabidopsis-
Ataba2/gin1/isi4/sis4;
AAO3: Tomato-sitiens,
Arabidopsis-aao3; MoCo:
Tomato-flacca, N.
plumbaginifolia-Npaba1,
Arabidopsis-Ataba3/los5/gi
n5.

 The biosynthesis and metabolism pathways of
abscisic acid (ABA) in higher plant. The C40
zeaxanthin is first converted into all-trans-violaxanthin
under the catalysis of zeaxanthin epoxidase (ZEP).
 Then, all-trans-violaxanthin can be converted to 9’-
cis-violaxanthin or 9’-cis-neoxanthin in two pathways
under the catalysis of neoxanthin synthase (NSY) and
the unknown isomerase.
 The enzyme 9-cis-cyclocarotenoid dioxygenase
(NCED) cleaves both 9’-cis-neoxanthin and 9’-cis-
violaxanthin into xanthoxin.
 Eventually, xanthoxin is mainly converted into abscisic
aldehyde and further forms ABA, which are catalyzed
by a short-chain alcohol dehydrogenase (ABA2) and
abscisic aldehyde oxidase (AAO/ABA3) respectively.
 ABA metabolism involves two main pathways:
hydroxylation and glycosylation, which are catalyzed
by the cytochrome P450 monooxygenase (CYP707A)
and ABA glucosyltransferase (GT) respectively.
 Red letters represent enzymes, and blue letters
represent cellular components divided by dashed
line. ABA-GE, ABA-glucose ester; BG, β-glucosidase;
VDE, violaxanthin de-epoxidase.

Abscisic Acid Biosynthesis: Steps
 Abscisic acid is a type of metabolite known as isoprenoids, or terpenoids. Isopentenyl (IDP) is a five-carbon (C5)
precursor molecule from which it is derived.
 Originally, it was assumed that all isoprenoids are synthesized from mevalonate (MVA) until recently a secondary
pathway has been identified for the synthesis of IDP, initially, in certain eubacteria and finally in higher plants
(Nambara and Marion-Poll, 2005).
 Various enzymes are involved which utilizes β-carotene to synthesize ABA. In 1960, ABA was isolated and
identified from cotton balls. Many plant varieties are capable of producing mutant ABA.
 Identifying such mutants along with their physiochemical properties has improved our knowledge of the
biosynthesis pathway in other plant species.
 Conversion of β-carotene to ABA is mediated via number of enzyme-catalyzed steps.
 The abiotic stress which entails triggering of assorted ABA bio-synthetic genes corresponding to zeaxanthin
oxidase (ZEP), 9-cis-epoxycarotenoid dioxygenase (NCED), ABA-aldehyde oxidase (AAO) and molybdenum
cofactor sulfurase (MCSU) might be because of calcium dependent phosphorylation pathway (Tuteja, 2007).
 Zeaxanthin is a trans-isomer form produced through cyclic hydroxylation of all-trans-lycopene via carotene.
 The initial step includes the synthesis of cis-isomers of violaxanthin and neoxanthin, each cleaved to generate
C15 precursor of ABA.
 Regardless of the truth that ABA has 15 carbon atoms, it isn’t emanating immediately from the C15
sesquiterpene precursor, farnesyldiphosphate (FDP) in plants.
 However, it occurs via the cleavage of C40 carotenoids emerging from the MEP (2-C-methyl-d-erythritol-four-
phosphate) pathway (Nambara and Marion-Poll, 2005).

Discovery
Abscisic acid (ABA) was discovered in the 1960s, independently by several research groups, as a plant hormone
involved in various processes, particularly those related to stress responses and dormancy. Initially, it was identified
as a growth inhibitor, with researchers focusing on its role in abscission (shedding of leaves and fruits) and bud
dormancy. It was later recognized as a key player in a broader range of plant responses to environmental stress and
in regulating seed dormancy.
Here's a more detailed look at the discovery:
 Early investigations:
 In the early 1960s, researchers like F.T. Addicott and his team at University of California, Davis were studying the
abscission of cotton fruits and identified a substance they called "Abscisin II," which promoted leaf and fruit
shedding. Around the same time, other scientists like Philip Wareing and his group were studying bud dormancy
in maple trees and identified a substance they named "dormin".
 Chemical Identification:
 Chemical analysis revealed that Abscisin II and dormin were the same substance. This compound was eventually
named abscisic acid (ABA).
 Stress Hormone Role:
 Further research established that ABA plays a crucial role in plant responses to various environmental stresses,
such as drought, salinity, and extreme temperatures. It is now considered a key regulator of these responses,
including stomatal closure, which helps plants conserve water under stress.
 Dormancy and Development:
 ABA is also involved in regulating seed dormancy and germination, as well as bud dormancy, ensuring that seeds
and buds remain dormant until favorable conditions for growth arise.

Locations and timing of ABA biosynthesis
Synthesized in nearly all plant tissues, e.g., roots, flowers, leaves and stems
Stored in mesophyll (chlorenchyma) cells where it is conjugated to glucose via uridine
diphosphate-glucosyltransferase resulting in the inactivated form, ABA-glucose-ester .
Activated and released from the chlorenchyma in response to environmental stress, such as
heat stress, water stress, salt stress.
Released during desiccation of the vegetative tissues and when roots encounter soil
compaction.
Synthesized in green fruits at the beginning of the winter period
Synthesized in maturing seeds, establishing dormancy
Mobile within the leaf and can be rapidly translocated from the leaves to the roots (opposite
of previous belief) in the phloem
Accumulation in the roots modifies lateral root development, improving the stress response
ABA is synthesized in almost all cells that contain chloroplasts or amyloplasts

Functions of Abscisic Acid
Abscisic acid (ABA) is a plant hormone that plays a crucial role in various metabolic processes, including seed
dormancy, stomatal regulation, and responses to abiotic stress. It influences the expression of genes involved in
stress responses and metabolic pathways related to growth and development.
Seed Dormancy and Germination:
ABA is a key regulator of seed dormancy, preventing premature germination until environmental conditions are
favorable. It also plays a role in dormancy release during germination by influencing ABA synthesis and
degradation pathways.
Stomatal Closure:
ABA triggers stomatal closure in response to water stress, reducing water loss through transpiration.
Abiotic Stress Responses:
ABA accumulates under various abiotic stresses like drought, salinity, and extreme temperatures, inducing stress-
protective mechanisms.
Fruit Ripening:
ABA influences fruit ripening by regulating the expression of genes involved in color, flavor, and texture
development.
Metabolic Processes:
ABA affects various metabolic processes, including carbohydrate metabolism, lipid metabolism, and amino acid
metabolism, which are crucial for plant growth and stress responses.

Role of ABA in stress conditions
Abscisic acid (ABA) is a key phytohormone that plays a crucial role in plant stress tolerance, particularly in
response to abiotic stresses like drought, salinity, and extreme temperatures. It acts as a signaling molecule,
triggering various physiological and biochemical changes that help plants adapt and survive under stressful
conditions.
Here's a more detailed look at ABA's role:
1. Stomatal Closure:
• One of the primary roles of ABA is to induce stomatal closure, which reduces water loss through transpiration during
drought stress.
• This helps plants conserve water and survive in water-limited environments.
• Stomatal closure also provides a defense mechanism against microbes by limiting their entry through stomatal pores.
2. Gene Expression and Stress Response:
• ABA triggers the expression of various stress-related genes.
• These genes encode proteins that help plants cope with different abiotic stresses, such as drought, salinity, and cold.
• Transcription factors like NAC, MYB, and bZIP are involved in mediating the expression of these stress-related genes.
3. Osmotic Adjustment:
• ABA promotes the accumulation of compatible solutes (osmolytes) in plant cells.
• These solutes help maintain cell turgor pressure under stress conditions, preventing cellular dehydration and damage.

4. Antioxidant Defense:
• ABA activates antioxidant defense mechanisms in plants.
• This helps scavenge reactive oxygen species (ROS) produced under stress,
protecting cellular components from oxidative damage.
5. Seed Dormancy:
• ABA plays a vital role in regulating seed dormancy, preventing premature
germination under unfavorable conditions.
• It helps seeds remain dormant until favorable conditions for germination arise.
6. Cross-Talk with Other Hormones:
• ABA interacts with other phytohormones like auxins, cytokinins, and gibberellins
to regulate plant growth and development under stress.
• These interactions fine-tune the plant's response to different stress conditions.
• In essence, ABA acts as a central regulator of plant stress responses, enabling
plants to adapt and survive under adverse environmental conditions.

ABA and its Role in stomatal closure
A model for roles of ion channels in ABA signaling.
 ABA triggers cytosolic calcium
([Ca2+]cyt) increases. (McAinsh et al.,
1990).
 [Ca2+]cyt elevations activate two
different types of anion channels:
Slow-activating sustained (S-type;
Schroeder and Hagiwara, 1989) and
rapid transient (R-type ; Hedrich et al.,
1990) anion channels. Both mediate
anion release from guard cells, causing
depolarization.
 This change in membrane potential
deactivates inward-rectifying K+ (K+in)
channels and activates outward-
rectifying K+ (K+out) channels resulting
in K+ efflux from guard cells (Fig. 1, left
panel).
 In addition, ABA causes an alkalization
of the guard cell cytosol, which
directly enhances K+out channel
activity and down-regulates the
transient R-type anion channels. The
sustained efflux of both anions and K+
from guard cells via anion and K+out
channels contributes to loss of guard
cell turgor, which leads to stomatal
closing

ABA Signalling  A guard cell model,
illustrating the proposed
functions of ion channels in
ABA signaling and stomatal
closing (177).
 The right cell of the
stomate shows ion channels
and regulators that mediate
ABA-induced stomatal
closing.
 The left cell shows the
parallel effects of ABA-
induced [Ca2+]cyt increases
that inhibit stomatal
opening mechanisms.
[Modified with permission
from Schroeder et al, 2001
(180)].

[Closure of stomata by the effect of
abscisic acid (ABA) in the plant exposed
to the stress]

Phytohormones mediated abiotic stress tolerance in plants

Oxidative stress in plants
and its significance. Under
abiotic-stress-induced
oxidative stress, ROS
generation is the most
significant step, which leads
to the battle for equilibrium
between ROS and
antioxidant defense. This
involves substantial crosstalk
and consequences between
stress signals and plant
growth and yield reduction.
For instance, minor damage
caused by oxidative stress
can improve growth and
yield, whereas extreme
oxidative stress can
significantly reduce plant
growth and yield—modified
from Hasanuzzaman et al.
(2020a). ROS, reactive oxygen
species; H2O2, hydrogen
peroxide; O2, oxygen; ¹O2,
singlet oxygen; ³O2, triplet
oxygen; O2 -, superoxide;
∙
OH , hydroxyl radical.
∙

The role of phytohormones in
improving plant tolerance
against multiple abiotic
stresses. Under stress
conditions, phytohormones can
modulate the stress intensity in
plants by triggering defense
mechanisms and thus regulate
physio-biochemical processes
by increasing plant tolerance to
environmental stress. CK, GA,
ABA, IAA, and JA mainly play
inhibitory roles, whereas BRs,
SA, SLs, and ethylene play
stimulatory roles in improving
several physiological and
biochemical mechanisms under
stress conditions. Notably, ABA
is a primary driving force,
playing a vital role alone or
combined with other
hormones under stress.
Furthermore, CKs and auxin
play a dual role (inhibitory and
stimulatory) by regulating plant
growth and development
processes.

 Management of ROS (Reactive Oxygen
Species) metabolism and signaling in
plants under stress conditions.
Cellular ROS accumulation is
controlled by three main methods—
(1) ROS generation, (2) ROS
scavenging, and (3) ROS transport—
which maintain ROS concentrations
and produce various ROS signatures
and gradients that act as signals in
various abiotic factor-response signal
transduction pathways.
 These redox regulations lead to
coordinated changes in the plant’s
physiology, metabolome, proteome,
methylome, and transcriptome.
Dashed arrows show that ROS
generation, scavenging, and transport
can be controlled by the ‘redox state’
of plant cells under stress. Figure
based on the concept of Mittler et al.
(2004, 2022). For more information on
ROS metabolism and signaling, readers
are referred to Mittler et al. (2022).
ROS, reactive oxygen species; O2,
oxygen; H2O, water.

Overview of the main process
associated with seed
ethephon induced drought
tolerance and drought
avoidance in dryland winter
wheat.
 Seed ethephon priming
invokes drought stress
memory against drought at
tillering stage, which
increased drought
avoidance capability by
increasing root diagram,
shoot dry weight, and thus
maintained leaf water under
drought.
 Seed ethephon priming
affects ABA independent
signaling pathways that
increasing drought
tolerance by enhancing
osmotic adjustment, root to
leaf stomatal regulation,
ROS-scavaging capability

Ethylene(Ripening Hormones)
Ethylene, a gaseous plant hormone (chemical formula C₂H₄ or H₂C=CH₂), plays a crucial role in various plant
growth and developmental processes, particularly in fruit ripening, flower senescence, and stress responses. It's
the only plant hormone that exists as a gas.
Key Aspects of Ethylene Chemistry:
Structure:
Ethylene is a simple alkene, consisting of two carbon atoms connected by a double bond and each carbon atom
also bonded to two hydrogen atoms.
Biosynthesis:
Ethylene is produced in various plant tissues, including roots, leaves, stems, and fruits, with production levels
varying depending on the plant's developmental stage and tissue type.
Physiological Roles:
 Fruit Ripening: Ethylene is a key regulator of fruit ripening, triggering changes in color, texture, and flavor.
 Flower Senescence: It induces flower fading and senescence, contributing to the end of a flower's life cycle.
 Abscission: Ethylene promotes the shedding of leaves, fruits, and flowers.
 Stress Response: Ethylene is produced in response to various stresses, such as injury, drought, and pathogen
attack, triggering protective mechanisms.
 Seed Germination: It can break seed dormancy and promote germination in some species.
 Root and Shoot Development: Ethylene can influence root and shoot elongation, branching, and other
developmental processes.

Discovery
Ethylene, the simplest alkene, was likely first discovered by Johann Joachim Becher around 1669, who produced it by heating
ethanol with sulfuric acid. Later, its role as a plant hormone was recognized, particularly in fruit ripening, through observations
of its effects on stored bananas and other fruits. Dimitry Neljubow identified ethylene as the active component in illuminating
gas that caused pea seedlings to grow abnormally, solidifying its place as a plant growth regulator.
Here's a more detailed look:
 Early Discovery:
 Becher's work in the 17th century involved heating ethanol with sulfuric acid, which produced a gas later identified as
ethylene. He documented this in his book, Physica Subterranea.
 Plant Hormone:
 In the late 19th and early 20th centuries, scientists began to notice the effects of ethylene on plants. Fruit merchants were
aware that rotten fruit could speed up the ripening of other fruits. H.H. Cousins confirmed that ripened oranges accelerated
banana ripening. Dimitry Neljubow's work with pea seedlings exposed to illuminating gas (which contained ethylene) led to
the identification of ethylene as the cause of the observed growth abnormalities, such as the "triple response" (shortened and
thickened stems and horizontal growth).
 Formal Identification:
 In 1934, Richard Gane provided further evidence by discovering that plants synthesize ethylene. This discovery was crucial in
establishing ethylene as a plant hormone.
 Commercial Applications:
 Today, ethylene is widely used in the agricultural industry for ripening fruits and vegetables. While ethylene itself is a gas and
can be difficult to handle, ethephon (2-chloroethylphosphonic acid) is a common commercial source, as it is a solid that can
be easily applied in solution and converted to ethylene by plants.

Biosynthesis of Ethylene
Ethylene biosynthesis in plants involves a two-step enzymatic pathway. First, methionine is converted to S-
adenosylmethionine (SAM) and then to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). In the second
step, ACC is converted to ethylene, carbon dioxide, and cyanide by ACC oxidase (ACO).
Here's a more detailed breakdown:
1. Methionine to SAM:
• The amino acid methionine is converted to S-adenosylmethionine (SAM) using ATP.
2. SAM to ACC:
• SAM is then converted to ACC by the enzyme ACC synthase (ACS).
• This step is a key regulatory point for ethylene production, according to Wiley and Biology
Discussion.
3. ACC to Ethylene:
• Finally, ACC is converted to ethylene, along with carbon dioxide and cyanide,
by ACC oxidase (ACO).
4. Cyanide Detoxification:
• The cyanide produced in the final step is rapidly detoxified by conversion to β-cyanoalanine.
• The regulation of ethylene biosynthesis involves controlling the expression and activity of ACS and ACO enzymes.
Environmental factors and developmental cues can influence ethylene production by affecting the levels of these enzymes.

 Ethylene biosynthesis
with the methionine
precursor and the
intermediate
synthesized as 1-
aminocylopropane-1-
carboxylic acid (ACC) in
higher plants through
the Methionine/Yang
cycle.
 Ethylene biosynthesis
is the conversion of S-
adenosyl-methionine
(SAM) from
methionine to ACC by
ACC synthase (ACS).
Methionine is
reproduced within the
Yang cycle.
 The Yang cycle is a set
of reactions that
recycle 5-
methylthioadenosine
to methionine.

 Methionine metabolism. Overview of the methionine
cycle and other tightly coupled metabolic pathways. In
green the methionine cycle, which quickly converts
methionine to S-Adenosylmethionine (SAM) to
generate methylation potential for the cell. Enzymes in
the methionine cycle: MAT2A (methionine adenosyl
transferase), SAM1/2 (SAM synthetase),
Methyltransferases (MTs), SAHH (SAH hydrolase), HMT
(Homocysteine methyltransferase).
 SAM is the primary methyl-group donor and is
required for epigenetic regulation and other
methylation-controlled processes.
 In red the methionine salvage pathway: SAM is also
required for the synthesis of polyamines. Byproducts
of this pathway are recycled to regenerate methionine.
 In blue the transsulfuration pathway: Homocysteine is
converted to cysteine, which feeds into the generation
of glutathione (GSH) and taurine to maintain the redox
balance in the cell. In yellow the sulfate assimilation
pathway: Most fungi are able to generate methionine
from absorbed sulfate. This process is energy
demanding and consumes 2 ATP and 4 NADPH
molecules per one molecule of methionine.
(ACC)
Ethylene

Ethylene biosynthesis and
signal transduction. Met:
methionine; SAM: S-
adenosylmethionine; ACC: 1-
Aminocyclopropanecarboxyli
c Acid; SAMS: SAM
synthetase; ACSs: ACC
synthases; ACOs: ACC
oxidases; CTR1: Constitutive
triple response1; EIN2:
Ethylene-Insensitive2;
EIN3/EIL1: Ethylene-
Insensitive3/EIN3-like1;
ENAP1: EIN2 nuclear-
associated protein1; EBF1/2:
EIN3 binding F-box
protein1/2; ERFs: Ethylene-
response factors.

Figure : Compatibilities of different chemicals and biological elicitors with molecular mechanism
of delayed ripening phenomena.

Importance of Ethylene Metabolism:
• Plant Development:
• Ethylene plays a crucial role in regulating various developmental processes, including seed
germination, fruit ripening, leaf abscission, and flower senescence.
• Stress Responses:
• Ethylene can be involved in plant responses to various stresses, such as wounding, flooding,
drought, and pathogen attack.
• Metabolic Priming:
• Ethylene can influence metabolic processes, potentially leading to increased photosynthesis
and carbohydrate utilization for enhanced growth and stress tolerance.
• Secondary Metabolism:
• Ethylene can affect secondary metabolic pathways, impacting plant defense mechanisms
and other processes.
• In summary, ethylene metabolism is a complex process involving a two-step biosynthesis
pathway and intricate regulatory mechanisms. It is essential for various plant
developmental processes, stress responses, and metabolic pathways, making it a critical
hormone in plant life.

 Methionine metabolism In the methionine cycle,
homocysteine is remethylated to methionine through
the methionine synthase enzyme (MTR) with vitamin
B12 as a cofactor, or necessary catalyst for MTR
activity.
 Methionine is then converted to SAM, which donates
a methyl group and is converted to S-
adenosylhomocysteine (SAH), which is then converted
back to homocysteine.
 Histone and DNA methyltransferases transfer the
methyl group from SAM to amino acids and bases on
histone amino-terminal tails and to the 5 position of
cytosine residues in DNA .
 When MTR enzymatic activity is inhibited by reactive
oxygen and nitrogen species, BHMT uses betaine to
remethylate homocysteine to methionine.
 Betaine can be taken in through the diet or synthesized
through the oxidation of choline in mitochondria.
Abbreviations: Hcy, homocysteine; Met, methionine;
SAM, S-adenosylmethionine; SAH, S-
adenosylhomocysteine; BHMT, betaine homocysteine
methyltransferase; ROS, reactive oxygen species; RNS,
reactive nitrogen species; HMTs, histone
methyltransferases; DNMTs, DNA methyltransferases.

New Discovered Plant Hormones
Several new plant hormones have been discovered in recent years, including strigolactones and phytomelatonin. Strigolactones, originally
identified for their role in parasitic plant germination, are now known to regulate shoot branching, mycorrhizal fungi growth, and
responses to stress like drought and salinity. Phytomelatonin, initially found in animals, has also been identified in plants and is emerging
as a potential new plant hormone.
1. Strigolactones:
• Discovery:
• First identified for their role in stimulating the germination of parasitic weeds like Striga lutea.
• Role:
• They are now known to be involved in a variety of processes:
• Inhibition of Shoot Branching: Strigolactones help control the growth of new shoots, influencing the overall architecture of the plant.
• Mycorrhizal Fungi Interaction: They promote the growth of beneficial arbuscular mycorrhizal (AM) fungi in the soil, which help
plants access nutrients.
• Stress Responses: Strigolactones play a role in how plants respond to drought, salinity, and other environmental stresses
2. Phytomelatonin:
• Discovery:
• While melatonin is well-known as a hormone in animals, it has also been found in plants, where it's referred to as phytomelatonin.
• Role:
• Research suggests it may act as a new plant hormone, but its specific functions are still being investigated.
• Receptor: A receptor for phytomelatonin has been identified in plants, further supporting its role as a hormone.

Newer Plant Hormones and their Chemistry:
Strigolactones (SLs):
These are a group of carotenoid-derived compounds that play a role in seed germination of
parasitic weeds and influence plant architecture and root development. The first SL, strigol,
was identified in 1966, but it took time to fully elucidate its structure.
Brassinosteroids (BRs):
These are steroid hormones that promote cell elongation and division, influencing various
developmental processes like stem elongation and pollen tube growth. Brassinosteroids were
first isolated from rape plant pollen and are considered ubiquitous in the plant kingdom.
Jasmonic Acid (JA):
This is a lipid-derived signaling molecule involved in plant defense against herbivores and
pathogens, as well as in responses to wounding and stress.
Salicylic Acid (SA):
Another signaling molecule, salicylic acid, is crucial for systemic acquired resistance (SAR) in
plants, protecting them against pathogens.

Plant Hormones (GAs, CK, ABA, Ethylene and 3rd Groups of Plant Hormones) ....Part III

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Plant Hormones (GAs, CK, ABA, Ethylene and 3rd Groups of Plant Hormones) ....Part III