ABSCISIC ACID (ABA): FUNCTIONS AND EFFECTS - BIOLOGY - 2024

The abscisic acid (ABA) is one of the main hormones in plants. This molecule participates in a series of essential physiological processes, such as seed germination and tolerance to environmental stress.

Historically, abscisic acid used to be associated with the abscission process of leaves and fruits (hence its name). However, today it is accepted that the ABA does not participate directly in this process. In fact, many of the traditional functions attributed to hormones have been challenged by current technologies.

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In plant tissues, the lack of water leads to the loss of turgor of the plant structures. This phenomenon stimulates the synthesis of ABA, triggering responses of the adaptive type, such as the closing of the stomata and the modification of the expression pattern of genes.

ABA has also been isolated from fungi, bacteria and some metazoans - including humans, although a specific function of the molecule has not been determined in these lineages.

Historical perspective

From the first discoveries of substances that had the ability to act as "plant hormones", it began to be suspected that there must be a growth inhibitory molecule.

In 1949, this molecule was isolated. Thanks to the study of dormant buds, it was possible to determine that they contained significant amounts of a potentially inhibitory substance.

This was in charge of blocking the action of auxin (a plant hormone known mainly for its participation in growth) in oat coleoptiles.

Due to its inhibitory properties, this substance was initially called dormines. Subsequently, some researchers identified substances capable of increasing the abscission process in leaves, and also in fruits. One of these dormines was chemically identified and named "abscisin" - by its action during abscission.

The following investigations were able to corroborate that the so-called dormines and abscisins were chemically the same substance, and it was renamed "abscisic acid".

characteristics

Abscisic acid, abbreviated as ABA, is a plant hormone involved in a series of physiological reactions, such as responses to periods of environmental stress, embryo maturation, cell division and elongation, in seed germination, among others.

This hormone is found in all plants. It can also be found in some very specific species of fungi, in bacteria and in some metazoans - from cnidarians to humans.

It is synthesized inside plant plastids. This anabolic pathway has as its precursor the molecule called isopentenyl pyrophosphate.

It is generally obtained from the lower parts of the fruits, specifically in the lower region of the ovary. The abscisic acid increases in concentration when the fall of the fruits approaches.

If abscisic acid is applied experimentally to a portion of the vegetative buds, the foliar primordia become cataphiles and the bud becomes a wintering structure.

The physiological responses of plants are complex and various hormones are involved. For example, giberillins and cytokinins appear to have contrasting effects to abscisic acid.

Structure

Structurally, the abscisic acid molecule has 15 carbons and its formula is C ₁₅ H ₂₀ O ₄, where carbon 1 'has optical activity.

It is a weak acid with a pKa close to 4.8. Although there are several chemical isomers of this molecule, the active form is S - (+) - ABA, with the 2- cis -4-trans side chain. The R form has shown activity only in some tests.

Mechanism of action

ABA is characterized by having a very complex mechanism of action, which has not been fully disclosed.

It has not yet been possible to identify an ABA receptor - like those found for other hormones, such as auxins or gibberellins. However, some membrane proteins seem to be involved in hormone signaling, such as GCR1, RPK1, among others.

Furthermore, a significant number of second messengers involved in the transmission of the hormone signal are known.

Finally, several signaling pathways have been identified, such as the PYR / PYL / RCAR receptors, 2C phosphatases and SnRK2 kinases.

Functions and effects on plants

Abscisic acid has been linked to a wide range of essential plant processes. Among its main functions we can mention the development and germination of the seed.

It is also involved in responses to extreme environmental conditions, such as cold, drought, and regions with high salt concentrations. We will describe the most relevant below:

Water stress

Emphasis has been placed on the participation of this hormone in the presence of water stress, where the increase of the hormone and the change in the pattern of gene expression is essential in the response of the plant.

When the drought affects the plant, it can be evidenced because the leaves begin to wither. At this point, the abscisic acid travels to the leaves and accumulates in them, causing the stomata to close. These are valve-like structures that mediate gas exchange in plants.

Abscisic acid acts on calcium: a molecule capable of acting as a second messenger. This causes the increase in the opening of the potassium ion channels located outside the plasma membrane of the cells that make up the stomata, called guardian cells.

Thus, a significant loss of water occurs. This osmotic phenomenon generates a loss in the turgor of the plant, making it look weak and flaccid. It is suggested that this system works as a warning alarm to the drought process.

In addition to stomatal closure, this process also involves a series of responses that remodel gene expression, affecting more than 100 genes.

Seed dormancy

Seed dormancy is an adaptive phenomenon that allows plants to resist unfavorable environmental conditions, be it light, water, temperature, among others. By not germinating in these stages, the growth of the plant is ensured in times when the environment is more benevolent.

Preventing a seed from germinating in the middle of autumn or in the middle of summer (if it does so at this time the chances of survival are very low) requires a complex physiological mechanism.

Historically, this hormone has been considered to play a crucial role in stopping germination in periods detrimental to growth and development. It has been found that abscisic acid levels can increase up to 100 times during the seed maturation process.

These high levels of said plant hormone inhibit the germination process, and in turn, induce the formation of a group of proteins that help resistance to extreme water shortage.

Seed germination: removal of abscisic acid

In order for the seed to germinate and complete its life cycle, the abscisic acid must be removed or inactivated. There are several ways to accomplish this purpose.

In deserts, for example, abscisic acid is removed through periods of rain. Other seeds need light or temperature stimuli to inactivate the hormone.

The germination event is driven by the hormonal balance between abscisic acid and giberillins (another widely known plant hormone). According to which substance predominates in the vegetable, germination occurs or not.

Abscission events

Today there is evidence that supports the idea that abscisic acid does not participate in the dormancy of the bud, and ironic as it may seem, neither in the abscission of the leaves - a process from which it derives its name.

It is currently known that this hormone does not directly control the abscission phenomenon. The high presence of acid reflects its role in promoting senescence and the response to stress, events that precede abscission.

Stunted growth

Abscisic acid acts as an antagonist (that is, it performs the opposite functions) of the growth hormones: auxins, citicinins, giberillins, and brassinosteroids.

Often times, this antagonistic relationship includes a multiple relationship between abscisic acid and various hormones. In this way, a physiological result is orchestrated in the plant.

Although this hormone has been considered a growth inhibitor, there is still no concrete evidence that can fully support this hypothesis.

Young tissues are known to present significant amounts of abscisic acids and mutants deficient in this hormone are dwarfs: mainly due to their ability to reduce perspiration and due to the exaggerated production of ethylene.

Cardiac rhtyms

It has been determined that there are daily fluctuations in the amount of abscisic acid in plants. Therefore, it is presumed that the hormone can act as a signal molecule, allowing the plant to anticipate fluctuations in light, temperature and amount of water.

Potential uses

As we mentioned, the synthesis route of abscisic acid is highly related to hydric stress.

For this reason, this route and the entire circuit involved in the regulation of gene expression and the enzymes that participate in these reactions, constitute a potential target to generate, through genetic engineering, variants that successfully tolerate high concentrations of salt and periods of water shortages.

References

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