Ethylene and Polyamines: Plant Growth Regulators
UNIT 20 – Ethylene and Polyamines
Definitions
Methionine: An amino acid formed from S-adenosylmethionine (SAM), a precursor in metabolic pathways, including ethylene and polyamine biosynthesis.
Ethylene: A developmental regulatory molecule (hydrocarbon, C2H4) in plants and animals. It has a simple chemical structure and is active in gaseous form. Its effects occur at very low concentrations and are evident in virtually all life cycle stages.
Triple Response: Ethylene’s effect in seedlings, causing reduced stem elongation (hypocotyl), increased lateral expansion (modification of cellulose microfibrils), and hook formation on the stem (diageotropic growth).
ACC Synthase: An enzyme catalyzing the conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC) in ethylene biosynthesis. This activity is often the limiting regulatory step.
ACC Oxidase: An enzyme catalyzing the oxidation of ACC to ethylene, the final step in ethylene synthesis.
Malonyl-ACC: A conjugated compound synthesized by ACC N-malonyltransferase. This synthesis is an alternative to ACC oxidation, serving as a storage form of the ethylene precursor and controlling ACC concentration.
Climacteric: In fruits, a period of significant respiratory increase associated with maturation’s end. It’s a phase transition between maturation and senescence.
Ethylene Biosynthesis
The first stage is the formation of S-adenosylmethionine (SAM) from methionine, catalyzed by S-adenosylmethionine synthase. The first specific step is SAM conversion to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS), the limiting regulatory step. ACS can be inhibited by aminoethoxy-vinyl-glycine (AVG) or amino-oxoacetic acid (AOA). ACS has a short half-life due to substrate-induced inactivation. The final stage is ACC oxidation to ethylene by ACC oxidase (ACO), requiring iron ions, ascorbate, and oxygen. CO2 stimulates ACO. An alternative is malonyl-ACC synthesis by ACC N-malonyltransferase, an irreversible storage form of ACC. Methionine is recycled via the Yang cycle, forming ethylene from ATP’s ribose ring while regenerating methionine.
Regulation of Ethylene Biosynthesis
Self-regulation: Ethylene activates its production (autocatalysis) in various tissues, especially during fruit ripening and flower senescence. Above a threshold, ACS and ACO activity increase, boosting ethylene production. Ethylene can also inhibit its biosynthesis by reducing ACS and ACO activity and increasing ACC-malonyltransferase activity.
Auxins: Induce ethylene synthesis by increasing ACC synthase activity.
Environmental Stresses: Drought, injury, infection, and freezing stimulate ethylene synthesis by activating ACS and ACO.
Developmental Stage: Influences ethylene synthesis, such as during fruit ripening.
Bioassay for Ethylene Content
Gas Chromatography: Ethylene is quantified by isolating plant material, incubating it in dark conditions at 25°C, extracting solutions, and injecting them into a gas chromatograph. Ethylene quantities are determined by comparing peak areas with a standard.
Ethylene’s Effect on Fruit Ripening
Ethylene increases CO2 concentration in climacteric fruits by enhancing respiration, substrate availability, ATP, and enzymatic activity. It:
- Increases permeability, releasing degradative enzymes and accelerating metabolic processes.
- Increases protein content, providing more enzymes.
- Induces fruit and leaf abscission.
Ethephon on Mutant Tomatoes:
- Never ripe mutant: No effect, as it doesn’t respond to ethylene.
- Ripening inhibitor mutant: Similar effects as wild type, accelerating maturation.
- Wild type: Normal maturation acceleration.
Ethylene in Leaf Abscission
Ethylene promotes enzymes that hydrolyze cell wall polysaccharides, causing separation and leaf abscission.
Ethylene-Induced Triple Response in Seedlings
Ethylene causes reduced elongation, increased lateral development, and horizontal growth, resulting in shorter, thicker hypocotyls and a closed plumula hook. This protects the apical meristem during emergence. The hook’s curvature is due to uneven ethylene distribution, affecting auxin distribution and growth direction. Light modulates ethylene’s action, restoring normal growth. Ethylene alters cellulose microfibril deposition in the cell wall.
Ethylene Uses in Agriculture
Ripening Fruits: Accelerates maturation, produces off-season fruits (apples, tomatoes), and degreens citrus.
Rooting and Propagation: Stimulates root formation and seed germination.
Flowering: Induces femininity, flowering in Bromeliaceae, and flower abscission.
Fruit Set and Development: Induces abscission of flowers and young fruits (thinning), and leaf abscission for mechanical harvesting.
Compounds Releasing Ethylene
- Ethephon (2-chloroethylphosphonic acid) – ECA, Ethrel, CEPH, Florel, Bromeflor
- Alsol (2-chloroethyl-tris-(2-methoxyethoxy) silane) – CGA 15,281
- DMNP (5-chloro-3-methyl-4-nitro-1H-pyrazole)
Mechanism of Ethylene Action
Ethylene is sensed by a receptor in the plasma membrane, transmitting a signal to an ETR1 protein dimer.
Polyamines
Nature and Role
Polyamines are polycationic molecules with amino groups, present in most living organisms. They are related to arginine, ornithine, and glutamic acid. Their role as phytohormones is debated due to their high endogenous concentrations and unclear long-range transport. They affect growth, development, senescence, and stress responses.
Effects on Growth and Development
- Morphogenesis and Cell Division: Involved in later stages, inducing somatic embryogenesis in vitro. Spermidine affects flower formation, putrescine marks root formation, and they stimulate fruit development and stem elongation.
- Delayed Senescence: Stabilizes membranes, interacts with nucleic acids, controls protein structure and enzymatic activity, inhibits chlorophyll breakdown, and delays fruit ripening.
- Macromolecule Biosynthesis: Influences biosynthesis.
- Stress Minimization: Acts as antioxidants and membrane stabilizers.