Abscisic acid

The abscisic acid (ABA) is a phytohormone with important functions within the plant physiology. It participates in processes of development and growth, as well as in the adaptive response to stress of both biotic and abiotic types. It was discovered in the early 1960s, when its involvement in controlling seed dormancy and organ abscission was found. Today it is known that ethylene is actually the hormone that mainly intervenes in organ abscission, and that the ABA-induced organ abscission observed in cotton fruits is due to ABA’s ability to induce ethylene synthesis.

General data

Summary

[ hide ]

• 1 Biosynthesis
• 2 ABA catabolism
• 3 Functions of abscisic acid
• 4 Physiological Effects of ABA
• 5 General ABA considerations
• 6 Source

Biosynthesis

Its synthesis takes place mainly in the plastids of vascular tissues (chloroplasts) but the final stages take place in the cytosol of the cell .
It has 2 biosynthetic routes, both derived from mevalonate.

• One route involves direct cyclization of a C-15 precursor (used primarily by fungi)
• The other pathway first forms a C-40 carotenoid precursor, synthesized from isopetenyl diphosphate via the terpenoid pathway, followed by oxidative metabolism leading to the structure of C-15.

It is favored by certain environmental conditions such as:

• Drought
• Frost
• Pathogens

It is mobilized by xylem and phloem as free ABA and as ABA βD-glucopyranosides. It is a slow movement, non-polar and in all directions.

ABA catabolism

ABA levels are regulated by a continuous balance between its active and inactive forms, which is of great importance in the plant’s response to stress. This balance is achieved not only thanks to ABA synthesis and catabolism, but also to conjugation and deconjugation processes. However, synthesis and catabolism are the main mechanisms involved in the regulation of ABA levels within the plant.
ABA catabolism includes the conjugation processes that inactivate the ABA molecule. Catabolism processes include two main routes:

• Oxidative pathway: ABA is hydroxylated at position C8 ‘to generate an unstable intermediate that eventually converts to phaseic acid. ABA can also be hydroxylated at the 7 ‘and 9’ positions and form 4-dihydrophaseic acid. In the rehydration phase after water stress, it has been seen that while ABA levels decrease, those of phaseic acid increase.
• Conjugation: ABA or its metabolites can be inactivated by conjugation with another molecule. The most common conjugate is the ABA glucosyl ester (ABA-GE), which is formed through an esterification reaction carried out by a glycosyltranferase. Contrary to the oxidative pathway, inactivation of ABA by conjugation with glucose is a reversible process. The hydrolysis of ABA-GE is carried out by a β-glucosidase and results in the release of ABA. Furthermore, the ABA-GE form is not only a form of ABA storage but also of transport. ABA-GE accumulates in the vacuoles and apoplast, but is transported to the endoplasmic reticulum in response to dehydration.

Functions of abscisic acid

ABA actively participates in multiple physiological processes of the plant, such as:

• maturation of the embryo,
• the dormancy of the seed,
• vegetative growth and
• processes related to stress tolerance, both biotic and abiotic.

It is a phytohormone very associated with stress, dormancy and senescence:

• It induces alterations in the carbohydrate content, specifically Sucrose and Fructose to increase tolerance to cold.
• In saline stress, ABA is especially increased in the roots (xylem).
• In response to mechanical injury, ABA levels increase 5-fold in tomato.
• ABA inhibits growth (short days), it would be a direct relationship on development. Probably GAs, in some species, could counteract the action of ABA in this process.
• Closely related to seed dormancy.
• Stomatal opening and closing.

Physiological Effects of ABA

Encourages seed development: promotes tolerance of the embryo to desiccation and promotes the accumulation of storage proteins during embryogenesis.
It maintains the dormancy of the seeds: it is opposite to that of the gibberellins , it is a process that responds to a hormonal balance.
Inhibits the production of enzymes inducible by gibberellins.
Promotes stomatal closure in response to water stress.
Increases the hydraulic conductivity and ion flow in the roots .
Decreases the resistance to water movement through the apoplast and membranes , by modifying the properties of the membranes.
Promotes root growth and decreases apex growth at low water potentials.
It promotes senescence of the leaves: by its own effect and by stimulation of ethylene biosynthesis, and the latter also favors abscission.

General ABA Considerations

• Plants have the ability to quickly and effectively regulate ABA levels, through key enzymatic activities of biosynthesis and degradation.
• Such changes in ABA levels, in the plant, can be determined by stressful environmental conditions, which determines ABA’s role in the ability to adapt to adverse environmental changes.
• In addition to the participation of ABA in adaptation to water stress (stomatal closure), other roles are in recess of seeds (inhibition of germination) and buds. Possibilities of using ABA as a growth regulator are being developed.
• Due to their roles in adapting to adverse environmental conditions, basic knowledge about ABA can give rise to biotech applications of great potential.