MOLECULAR AND BIOCHEMICAL STEPS IN BIOSYNTHESIS OF ETHYLENE IN PLANT

Course No. PP - 504

Course Title - Hormonal regulation of plant growth and development

Submitted to,

Dr. M.M.Burondkar

Prepared by,

Name- CHAVAN MAHADEO RAJARAM

Reg. No.- ADPM/15/2419

Class- Jr. M. Sc. Agriculture

Genetics and Plant Breeding

Ethylene Ethylene is a gaseous plant hormone that affects

developmental processes and fitness responses, including

germination, flower and leaf senescence, fruit ripening, leaf

abscission, root nodulation and responsiveness to stress and

pathogen attack.

Ethylene is the plant hormone mainly responsible for

ripening of fruits. It is gas, CH2=CH2or C2H4 at room temperature and is

produced in minute quantities by plant tissues & active at low concentrations(well below 1 part in 10 million of air).

H H \ / C = C / \ H H

Ethylene is not easily soluble in aqueous medium and

volatilizes into the intercellular space from where it is released

in the outer atmosphere.

The meristems and the nodes are the chief sites for the

production of ethylene.

Chemical Formula of ethylene -

In 1901, Dimitry Neljubow demonstrated that the gas ethylene was the active factor which caused leaves to drop from trees that were near leaking gas mains. Plants produce ethylene in response to stresses

such as drought, flooding, mechanical pressure, injury, and infection.

Ethylene production also occurs during fruit ripening and during programmed cell death.

Ethylene is also produced in response to high concentrations of externally applied auxins.

• Ethylene instigates a seedling to perform a growth maneuver called the triple response that enables a seedling to circumvent an obstacle.

• Ethylene production is induced by mechanical stress on the stem tip.

• In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally.

• As the stem continues to grow horizontally, its tip touches upward intermittently.– If the probes continue to detect a solid object above,

then another pulse of ethylene is generated and the stem continues its horizontal progress.

– If upward probes detect no solid object, then ethylene production decreases, and the stem resumes its normal upward growth.

• It is ethylene, not the physical obstruction per se, that induces the stem to grow horizontally.– Normal seedlings growing free of all physical

impediments will undergo the triple response if ethylene is applied.

• Arabidopsis mutants with abnormal triple responses have been used to investigate the signal transduction pathways leading to this response.–Ethylene-insensitive (ein) mutants fail to

undergo the triple response after exposure to ethylene.

Site of synthesis:-• It is naturally produced in plants & synthesized from

any part of the plant.• The production of ethylene in accelerated at the time

of ripening of fruits, leaf fall, senescing flowers, injuries, drought stress etc.

• Maturing fruits, meristematic region, nodal region are the sites of actively synthesis .

Synthesis :-• Ethylene get synthesised naturally in mature fruits,

aging leaves etc.• Physiological stress like - wounding, bruising,

flooding, chilling, temperature, cutting etc.

Biosynthesis Pathway of Ethylene

Methionine ATP Ado Met Synthatase ADP+Pi S-Adenosyl Methionine(AdoMet) ACC Synthase 1-Amino cyclopropane-1 –Carboxylic acid(ACC) ½ O2 ACC Oxidase HCN+O2

Ethylene

The chief precursor of ethylene is the sulphur containing amino acid methionine.

Biosynthesis of Ethylene1) Pathway of ethylene synthesis is given by Yang 19802) The amino acid methionine is precursor of ethylene and ACC (1-

aminocyclopropane - 1 carboxylic acid ) serves as an intermediate between methionine and ethylene.

3) When methionine is conjugated with adenosine gives rise to SAM (S - adenosyl methionine) in presence of enzyme adomate synthetas and it used a molecule of ATP .

4) SAM in presence of amynocyclopropane carboxylic acid synthetas (ACC synthetase) break down into amynocyclopropane carboxylic acid and s - methylthioadenosine .

5) Aminocyclopropane finally converted into ethylene in presence of ACC oxidase.

• Precursor - Methionine

Cont…….

• Adam and Yang (1977) have worked out the pathway for the biosynthesis of ethylene from amino acid methionine.

• Methionine is activatedv by ATP to give rise to S-adenosilmethionine (SAM)in the presence of an enzyme methionine adenosyl transferase.

o The compound SAM in presence of enzyme ACC synthetase break into S-Methelthioadenosine (MTA) and aminocyclopropane carboxylic acid (ACC).

ACC is finely converted into ethylene.o This reaction is presumed to be triggered by ethylene

forming enzyme( EFE).

OUTLINE OF SYNTHESIS OF ETHYLENE

CH3-S-CH2-CH2-CH(NH2)-COOH(Methionine)

CH3-S-CH2-CH2-CHOMethanol

CH3-S-CH2-CH2-CO-COOH4-Methyl mercepto-2-oxo butyric

acid CH2=CH2

ETHYLENE

FMN+LightTransminase

Peroxidase +H202Peroxidase +H202

Transport

• In comparison to ACC synthase and ACC oxide, less is known about ethylene perception and single transduction, because of difficulties in isolation and purifying ethylene.

• Ethylene released by tissue and diffuses in the form of gas through intracellular space .

• Transport by diffusion.

Mechanism of ethylene actionMembrane permeability – • Ethylene acts by changing permeability of the cell

membrane. • Ethylene is lipid soluble, hence it gets dissolved in lipid

membrane and alters the permeability of membrane.Nucleic acid and protein metabolism - Some responses

like fruit ripening & abscission are due to gene expression and protein synthesis. Synthesises specific mRNA and enzymes.

Regulation of auxin metabolism – Reduces biosynthesis of auxins and inhibits its transport.

MECHANISM OF ACTION

• Increased permeability increases RNA and Protein synthesis.

• It also accelerates the secretion of variety of enzymes like polygalactourinase, cellulase and phospholipases they cause softening and degradation of cell wall.

• α-amylase enzyme it results in hydrolysis of starch into

sugars.

• High permeability increases respiration thereby hastens fruit ripening process.

ELIMINATING SOURCES OF ETHYLENE • Specific anti-ethylene compounds: STS,

I-MCP • Inhibition of ethylene biosynthesis:

AVG and AOA• Activated or brominated charcoal • Catalytic oxidizers: Platinized asbestos• Chemical removal: KMnO4 (Purafil)

Anti-ethylene• Ethylene is used commercially to ripen tomatoes,

bananas, pears and a few other fruits. Ethylene gas is used to do this and it is a postharvest use.

• There are commercial liquid products that release ethylene..

• These are only used preharvest. There are several anti-ethylene chemicals. Silver thiosulfate (STS) is used on flowers. AVG (trade name ReTainTM) blocks ethylene synthesis.

• It is a liquid that is applied preharvest. The fruit

(plant) will not produce much ethylene, so there is not

an ethylene response. However, the plant can respond

to ethylene from another source.

• In contrast, 1-MCP blocks ethylene binding to its

receptor. It is applied postharvest. The fruit (plant)

may still produce some ethylene but there is no

response to any ethylene, regardless of source.

References

Textbook of plant physiology – V. Verma Plant physiology – S.N.Pandy and B.K.Sinha Internet website www.plant physiol.org www.plant cell Internet www.cab direct.

Thank you