39.5.3: Gibberellins

Learning Objectives

• Identify the locations of synthesis, transport, and actions of gibberellins.
• Interpret and predict the outcome of an experiment demonstrating the action of gibberellins.
• Describe the commercial applications of gibberellins.

During the 1930s Japanese scientists isolated a growth-promoting substance from cultures of a fungus that parasitizes rice plants. They called it gibberellin. Gibberellins (GAs) are a group of about 125 closely related plant hormones synthesized in the root and stem apical meristems, young leaves, and seed embryos. They are likely transported through the vascular tissue. One of the most active gibberellins - and one found as a natural hormone in the plants themselves - is gibberellic acid (GA; figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Chemical structure of the plant hormone gibberellic acid. Image by Minutemen (public domain).

Actions of Gibberellins

Several aspects of plant growth involve GAs, including stimulating shoot elongation, seed germination, and fruit and flower maturation. Other effects of GAs include gender expression (also see Ethylene) and the delay of senescence in leaves and fruit. Synthesis of gibberellins also helps grapevines climb up toward the light by causing meristems that would have developed into flowers to develop into tendrils instead.

Shoot elongation in this case results from both cell division and cell elongation. When applied in low concentrations to a bush or "dwarf" bean, the stem begins to grow rapidly. The length of the internodes becomes so great that the plant becomes indistinguishable from climbing or "pole" beans. GA seems to overcome the genetic limitations in many dwarf varieties.

Gibberellins break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. The seeds of some plant species rely on the imbibition (intake) of water to initiate germination. Intake of water activates gibberellins, which then signals to transcribe the gene encoding amylase, an enzyme that breaks down starches stored in the seed into simple sugars (note these final steps are identical to what occurs in phytochrome-regulated germination). Gibberellins also stimulate cell elongation of young roots during germination. When water is absent, germination in this pathway is blocked by a hormone called abscisic acid, which inhibits the activity of gibberellins. Thus gibberellins and abscisic acid act in opposition in regulating the the germination response.

Many plant species first produce a basal rosette of leaves. When daylength increase or the weather becomes cold, they bolt, producing a long stalk. Eventually, flowers and then fruits develop on this stalk. Gibberellins are responsible for inducing bolting (figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Arabidopsis thaliana plant as basal rosette (left). The plant on the right has bolted and started to flower. Gibberellins induce bolting. Left image by Quentin Groom (public domain), and right image by Frost Museum (CC-BY).

Mechanism of Gibberellin Action

Like auxins, gibberellins generally increase the rate of cell division and longitudinal cell expansion. Gibberellins also exert their effects by altering gene transcription through a mechanism similar to auxin in that a pathway is turned on by inhibiting the inhibitor of that pathway (a double-negative is a positive). First, Gibberellin enters the cell and binds to a soluble receptor protein called GID1 ("gibberellin-insensitive dwarf mutant 1") which now can bind to a complex of proteins (SCF) responsible for attaching ubiquitin to one or another of several DELLA proteins. This triggers the destruction of the DELLA proteins by proteasomes. DELLA proteins normally bind gibberellin-dependent transcription factors, a prominent one is designated PIF3/4, preventing them from binding to the DNA of control sequences of genes that are turned on by gibberellin (also see Shade Avoidance and Etiolation).

The dwarf varieties of rice and wheat carry mutations related to GAs. In the case of rice, the mutation interfere with the synthesis of their gibberellins. The wheat mutation reduces is in the gene coding for a DELLA protein and reduce the plant's ability to respond to its own gibberellins. Dwarf varieties of sorghum and more recently maize (corn) also exist, but in these cases, the mutation interferes with auxin transport, not gibberellin activity.

Commercial Applications of Gibberellins

Gibberellin application assists with seedless grape production. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure \(\PageIndex{3}\)). Like auxins, GAs can be used commercially to induce fruit development in a variety of species.

Figure \(\PageIndex{3}\): In grapes, application of gibberellic acid increases the size of fruit and loosens clustering. (credit: Bob Nichols, USDA)

Gibberellins have a few other commercial applications. They can be applied to artificially induce bolting and flowering, such that plants produce seeds earlier. Addition of gibberellic acid to the winter buds of peach trees helps break dormancy. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

Attributions

Curated and authored by Melissa Ha from the following sources:

• 30.6 Plant Sensory Systems and Responses from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 16.5E Gibberellins from Biology by John. W. Kimball (licensed CC-BY)
• Plant Hormones and Sensory Systems by Biology 1520 Introduction to Organismal Biology (licensed CC BY-NC-SA)