Gibberellin plant hormones: An Introduction

https://commons.wikimedia.org/wiki/File:Gibberellic_acid.svg#/media/File:Gibberellic_acid.svg

What are gibberellins?

Gibberellins (GAs) are plant hormones that exist in all vascular plants, algae, and many fungi. There are over 136 kinds, all with a similar chemical structure. The different kinds of Gibberellin are differentiated by the acronym ‘GA’ followed by a unique number between 1 and 136. 

The molecular structure of Gibberellic acid, (GA3), one of the 136 kinds of gibberellin found in plants. The grey balls represent carbon atoms, the white balls represent hydrogen, and the red balls represent oxygen. Ben Mills commons.wikimedia.org/wiki/File:Gibberellic-acid-from-xtal-3D-bs-17.png

Most plant functions depend on gibberellins, primarily those related to growth and development. At the molecular level, they regulate synthesis of cell membranes, RNA and protein synthesis, cell wall synthesis, and cell wall loosening. At the organ level, they regulate growth and development, are responsible for breaking seed dormancy, speed up grain germination, inducing flowering, and promote internode (stem) growth. It is thought that gibberellins promote growth by increasing plasticity of the cell wall and the hydrolysis of starch to sugar which reduces the water potential in the cell, inviting more water inside and causing elongation. They can also inhibit growth.

A barley seedling that is GA-deficient mutant (left) compared to a normal barley seedling (right) CSIRO      

           http://www.scienceimage.csiro.au/image/7758

Many different forms of GA work together in the plant to produce specific effects. For example, 24 GAs have been identified in apples. Often, they are activated by environmental changes in temperature and light intensity. GA₃ for example appears to stop deciduous species growing when sunlight decreases. In spinach, longer days trigger a five-fold decrease in GA₁₉ and an increase in GA₂₉, with GA₁₇ and GA₄₄  remaining the same. The interaction of these four GAs makes shoots elongate.

The dwarf pea plant on the right has been treated with GA, and has longer stems with greater distance between nodes. http://botanicalgarden.berkeley.edu/wp-content/uploads/2021/01/IGYA-gibberellins_HF1-555x414.png

The effects of Gibberellin vary with type and the plant species in which they are found. GA₁, GA₃, GA₄, and GA₇ are the most active – the biological functions of these gibberellin types are clearly observable. Other GAs are not biologically active. These could be by-products of metabolic processes and have no known function or are simply responsible for transforming one active GA into another. The effects of GAs also vary depending on their interactions with other types of plant hormones, such as ABA, auxin, ethylene, brassinosteroids, and cytokinin.

Where are gibberellins produced?

GAs are produced in parts of the plants where new cells form: immature tissues in maturing seeds and fruits, young shoots, roots, or young leaf petioles. After GAs are synthesized, they are transported to other parts of the plant as required.

How were gibberellins discovered?

Gibberellins first came to the attention of Japanese scientists in the form of a well-known agricultural problem. For at least two hundred years, rice farmers had noticed that some of their rice seedlings would grow with elongated stems. These would produce reduced grain yields. The early research in this field was dominated by Japanese scientists.

A 1898 scientific paper demonstrated that the rice crop symptoms were associated with the fungus Fusarium moniliforme(Gibberella fujikuroi).In 1926, a paper suggested it secreted a substance that caused the elongation. In 1938, Terijiro Yabuta, an organic chemist, managed to isolate the stem-elongating component of the fungal substance. The component was named gibberellin A. It stimulated growth when applied to the roots of rice seedlings. What was once a troublesome fungal infection responsible for reduced yields turned out to contain a compound that could be agriculturally useful.

Interest in this growth inducing property fuelled further research. In the 1950s, it became clear that plants produce their own GAs independently of this particular fungal infection.UK and US scientists now took up the research program. In the UK, gibberellic acid was isolated. Today, it is called GA₃.

By 1956 it became clear that all higher plants featured different kinds of gibberellins, and the search was on to identify all kinds.  

How are gibberellins used?

Gibberellins have been synthesized commercially for use in agriculture and fruit production and as a way of producing dwarf varieties, particularly of wheat and rice. This was an important driver of the Green Revolution because the dwarfing was accompanied by a mutation that improved grain productivity.  

The best known and most commercially used gibberellin is GA_3,otherwise known as gibberellic acid. One major commercial application of gibberellic acid is in beer manufacture. Its ability to stimulate the release of sugars from starch is used to speed up barley grain malting. It can also make plants flower out of season. GA₄ and GA₇ have been used to shape McIntosh apples.

Plants that grow in a rosette habit such as the foxglove are particularly sensitive to GA Jcart1534 commons.wikimedia.org/wiki/File:Digitalis_purpurea_-_Foxglove.jpg

Growth-inhibiting GAs have been used to reduce the height of pot plants or accelerate flower bud production in some woody plants like azaleas. They have been used to lengthen flower stalks in Gerbera. Other GAs are also applied to induce flowering in plants that grow in rosette form such as henbane, Hyoscyamus niger, foxglove, and digitalis purpurea. Here, the GA encourages the flowering stem to elongate, a process termed ‘bolting’. However, while a particular GA might induce a particular effect in one species, it may not in another.