Auxins, Abscisic Acid and Ethylene

Auxins derive their name from the Greek word αυξειν ("to grow/increase"). Auxin participates in phototropism, geotropism, hydrotropism and other developmental changes. The uneven distribution of auxin, due to environmental cues, such as unidirectional light or gravity force, results in uneven plant tissue growth. Auxin generally governs the form and shape of plant body, direction and strength of growth of all organs, and their mutual interaction.

 

 

<Native auxins>

 

 

Funtions of auxins:

• Stimulates cell elongation
• Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture
• Stimulates differentiation of phloem and xylem
• Stimulates root initiation on stem cuttings and lateral root development in tissue culture
• Mediates the tropistic response of bending in response to gravity and light
• The auxin supply from the apical bud suppresses growth of lateral buds
• Delays leaf senescence
• Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission
• Can induce fruit setting and growth in some plants
• Involved in assimilate movement toward auxin possibly by an effect on phloem transport
• Delays fruit ripening
• Promotes flowering in Bromeliads
• Stimulates growth of flower parts
• Promotes (via ethylene production) femaleness in dioecious flowers
• Stimulates the production of ethylene at high concentrations

 

 

 

 

 

 

Abscisic acid is a single compound unlike auxins. Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for winter, ABA is produced in terminal buds. This slows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season. ABA also inhibits the division of cells in the vascular cambium, adjusting to cold conditions in the winter by suspending primary and secondary growth.

 

 

Functions of Abscisic acid:

• Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
• Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
• Induces seeds to synthesize storage proteins.
• Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase.
• Has some effect on induction and maintanance of dormancy.
• Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.

 

Ethylene, unlike the rest of the plant hormone compounds, is a gaseous hormone. Like abscisic acid, it is the only member of its class. Of all the known plant growth substance, ethylene has the simplest structure. It is produced in all higher plants and is usually associated with fruit ripening and the tripple response.

 

H₂C=CH₂

FUNCTIONS OF ETHYLENE:

• Stimulates the release of dormancy.
• Stimulates shoot and root growth and differentiation (triple response)
• May have a role in adventitious root formation.
• Stimulates leaf and fruit abscission.
• Stimulates Bromiliad flower induction.
• Induction of femaleness in dioecious flowers.
• Stimulates flower opening.
• Stimulates flower and leaf senescence.
• Stimulates fruit ripening.