Gibberellins (GAs) and Gibberellic acid (GA3)#

Gibberellins (GAs) are plant hormones that regulate growth and influence various developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence.

Gibberellin was first recognized in 1926 by a Japanese scientist, Eiichi Kurosawa, studying bakanae, the “foolish seedling” disease in rice. It was first isolated in 1935 by Teijiro Yabuta and Sumuki, from fungal strain (Gibberella fujikuroi) provided by Kurosawa. Yabuta named the isolate as gibberellin.

Interest in gibberellins outside Japan began after World War II. In the United States, the first research was undertaken by a unit at Camp Detrick in Maryland, via studying seedlings of the bean Vicia faba. In the United Kingdom, work on isolating new types of gibberellin was undertaken at Imperial Chemical Industries. Interest in gibberellins spread around the world as the potential for its use on various commercially important plants became more obvious. For example, research that started at the University of California, Davis in the mid-1960s led to its commercial use on Thompson seedless table grapes throughout California by 1962. A known antagonist to gibberellin is paclobutrazol (PBZ), which in turn inhibits growth and induces early fruitset as well as seedset.

Gibberellic acid (also called Gibberellin A3, GA, and GA₃) is a hormone found in plants and fungi. Its chemical formula is C₁₉H₂₂O₆. When purified, it is a white to pale-yellow solid.

However, plants produce low amounts of GA3, therefore this hormone can be produced industrially by microorganisms. Nowadays, it is produced by submerse fermentation, but this process presented a low yield with high production costs and hence higher sale value. One alternative process to reduce costs of the GA3 production is Solid-State Fermentation (SSF) that allows the use of agro-industrial residues. Gibberellic acid is a simple gibberellin, a pentacyclic diterpene acid promoting the growth and elongation of cells. It affects the decomposition of plants and helps plants grow if used in small amounts, but eventually plants develop tolerance to it. GA stimulates the cells of germinating seeds to produce mRNA molecules that code for hydrolytic enzymes. Gibberellic acid is a very potent hormone whose natural occurrence in plants controls their development. Since GA regulates growth, applications of very low concentrations can have a profound effect while too much will have the opposite effect. It is usually used in concentrations between 0.01 and 10 mg/L.

GA was first identified in Japan in 1926, as a metabolic by product of the plant pathogen Gibberella fujikuroi (thus the name), which afflicts rice plants; fujikuroi-infected plants develop bakanae (“foolish seedling”), which causes them to grow so much taller than normal that they die from no longer being sturdy enough to support their own weight.

Gibberellins have a number of effects on plant development. They can stimulate rapid stem and root growth, induce mitotic division in the leaves of some plants, and increase seed germination rate.

Gibberellic acid is sometimes used in laboratory and greenhouse settings to trigger germination in seeds that would otherwise remain dormant. It is also widely used in the grape-growing industry as a hormone to induce the production of larger bundles and bigger grapes, especially Thompson seedless grapes. In the Okanagan and Creston valleys, it is also used as a growth replicator in the cherry industry. It is used on Clementine Mandarin oranges, which may otherwise cross-pollinate with other citrus and grow undesirable seeds. Applied directly on the blossoms as a spray, it allows for Clementines to produce a full crop of fruit without seeds.

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