Plant Grow Regulators

The Pgrs (Plants grow regulators) are molecules that influence the development of plants and are generally active already at very low concentrations. We know natural (phytohormones) and synthetic regulators.

The substances recognized as phytohormones include Auxin, Cytokine, Abscisic acid, polyamine ethylene, nitric oxide etc ...

Are there any differences between animal hormones and plant hormones? For example, animal hormones are synthesized by specific organs or tissues and by definition act in areas other than those in which they are produced.

This characteristic is not necessarily shared by phytohormones: some exert their action exactly in the place where they are synthesized.

Although each phytohormone has its own specific functionality, their combination produces different reactions in plants.

Let's analyze the main ones


Being able to alter the elasticity of the cell wall, these phytohormones cause elongation of the cells themselves. The main auxin is indole-acetic acid, although there are also others, some of which have recently been discovered (indole-butyric acid). The movement of these phytohormones can be basepetal (which develop from the top towards the roots) and vice versa.

Some of the effects:

Elimination of the apex dominance, which helps the grower to obtain more leading apexes without one dominating the others. How it happens? Simple, auxin suppresses the growth of secondary inflorescences in favor of main "buds".

Rhizogenesis, being used for cutting to help the clone produce new roots.

Phototropism, which enables the plant to recognize those parts that receive less light, allowing them to elongate and allowing the grower to use different techniques.

Finally, the presence of auxin in the flowers favors the production of seeds, which is important for the work of breeders.


These phytohormones are partially responsible for cell division and elongation of stems and other tissues. They were discovered by Japanese researchers studying a disease in rice. The disease has recently caused the germinated seedlings to acquire a yellowish color and the stem stretches excessively, leading to the death of the plant. The researchers found that these symptoms were caused by the Gibberella fujikuroi fungus. This fungus produces a large amount of these phytohormones which are introduced into the host plant.

Since then, various types of gibberelins have been discovered and isolated. These were given successive numbers as they were discovered; GA1, GA2, GA3, etc. GA3 is gibberellic acid.

Gibberelines are mainly synthesized in the meristematic organs and in the development of tissues.

Functions of gibberellins

Germination of seeds.

In the seeds, some of the gibberelines join the glycosides and become inactive in this form. During germination, enzymes destroy this combination and the gibberelines are unlocked and reactivated. This stimulation of germination was demonstrated in a series of experiments which showed how the application of gibbereline accelerated the germination of lettuce seeds. It has also been shown that exposure to light accelerated the germination of lettuce seeds. Subsequent studies have shown that light accelerates the transformation of gibberelines from the inactive to the active form.

Sexual expression

In species with unisexual flowers - that is, separate male and female flowers, or on the same plant (monoecious) or on different individuals (dioecious) - the gibbberelines seem to have a regulating effect on sexual expression. For example, the application of gibberelins in female asparagus plants produces the appearance of male and hermaphrodite flowers. On the contrary, the application of gibberellin in corn plants produces the appearance of female flowers in the tassels (male inflorescences).

Influence during the juvenile period.

Young plants are different from adult plants - for example, the development of fruit trees must mature for several years after the seeds germinate before they are able to produce flowers and fruit. In some cases they also have different characteristics when they are adults (for example, the presence of thorns or leaves with different shapes). The Gibberelines play an important role in the transition from the juvenile period to the adult period. In some plants, such as ivy, the exogenous application of gibbereline produces the expression of branches with youthful characteristics.

As with auxins, gibberelines stimulate fruit development in some species.

Flowering induction.

Some plants require long days or cold periods to bloom, but the application of gibbereline induces flowering regardless of the photoperiod or temperature.


The discovery of these phytohormones is mainly due to in vitro cultivation studies. The first observation was that "coconut milk" (the endosperm of the fruit) promoted the growth of several tissues grown in vitro. The first isolated and identified natural cytokinin was named zeatin, as it was isolated from corn seeds. (Zea mays) The main function of cytokinins is to produce cell division and delay senescence. As mentioned above, cytokinins in combination with auxins lead to the formation of undifferentiated cell masses called corns. They also stimulate the development of the lateral apices when applied exogenously, breaking the apical domain.


Ethylene is a simple hydrocarbon which is a gas under normal conditions. The effects of ethylene on plants were discovered when the streets were lit with carbide lamps. The combustion process led to the emission of ethylene and the leaves of the trees near these lamps showed yellowing.

The main function of ethylene is the ripening of fruits and the senescence of leaves and flowers. In species with climacteric fruit, ripening is induced by an increase in this hormone. It is also responsible for the color change of some non-climacteric fruits (that is, the ripening of which is not affected by ethylene), as occurs in citrus fruits. Ethylene is used to ripen fruit that has been harvested prematurely. It is applied by burning in closed chambers or with ethephon, a product that decomposes into ethylene when it is hydrolyzed in the plant.

Another function attributed to both ethylene and gibberelines is the regulation of sexual expression in dioecious plants. The application of ethylene in asparagus induces the appearance of female flowers in male plants.

Ethylene plays an important role together with jasmonic acid in stimulating the production of substances that protect the plant from biotic and abiotic stress.

    Abscisic acid

As the name suggests, this hormone is directly involved in the abscission and senescence of leaves, flowers and fruits.

As in the case of ethylene, this phytohormone is capable of giving the plant greater resistance in case of stress, preventing dehydration.

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