Gibberellins

Gibberellins are a group of plant hormones that regulate various growth and developmental processes in plants. Gibberellins play a key role in promoting stem elongation, especially in the internodes, which are the segments between leaves or branches. They are also involved in seed germination, flowering, and fruit development.

There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA1 , GA2 , GA3 and so on. However, Gibberellic acid (GA3 ) was one of the first gibberellins to be discovered and remains the most intensively studied form.

1.0Discovery of Gibberellin

In the late 1800s and early 1900s, Japanese rice farmers faced a crop disease called bakanae, ("foolish seedling”) linked to the fungus Gibberella fujikuroi. In 1926, E. Kurosawa observed similar symptoms in healthy rice treated with substances from this fungus. By 1938, Japanese researchers isolated and named the active material gibberellin. After World War II, Western scientists like Cross and Stodola isolated gibberellic acid from fungi. Simultaneously, Japanese researchers identified three gibberellins—GA1, GA2, and GA3—with GA3 being identical to gibberellic acid. It was discovered that gibberellins were present not only in fungi but also in higher plants. The first higher-plant gibberellin, identical to gibberellin GA1, was isolated from runner bean seeds. Since then, gibberellins have been found to be widespread in higher plants.

2.0Biosynthesis of Gibberellin

Gibberellins are believed to form through the condensation of a 5-carbon precursor called isopentenyl pyrophosphate (IPP). This precursor, derived from acetyl CoA or mevalonic acid, undergoes a series of intermediates to produce gibberellins. In plants, gibberellins are primarily synthesized in apical tissues, with three main sites of biosynthesis: developing seeds and fruits, young leaves in growing buds and shoots, and the apical regions of roots. This synthesis plays a crucial role in regulating various aspects of plant growth and development.

3.0Physiological Effects

All GAs are acidic. They produce a wide range of physiological responses in the plants.

• Seed Germination: 

Certain light-sensitive seeds like lettuce and tobacco exhibit poor germination in the dark. Exposure to light or red light initiates vigorous germination. Gibberellic acid treatment in the dark can overcome the light requirement.

• Dormancy of Buds: In temperate regions, buds formed in autumn remain dormant until spring due to cold. Gibberellin treatment can break this dormancy, facilitating sprouting in potatoes after harvest.
• Root Growth: Gibberellins generally have minimal impact on root growth. In some plants, higher concentrations may inhibit root growth, and they can inhibit root initiation in isolated cuttings.
• Delay senescence: Gibberellins also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period.
• Elongate and improve fruit shape: Gibberellins cause fruits like apples to elongate and improve their shape.
• Speed up the malting process: GA3 is used to speed up the malting process in the brewing industry.
• Increase the yield  of sugarcane: Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crops with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre. 
• Hastens the maturity period: Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
• Elongation of Internodes: Gibberellins cause significant internode elongation, overcoming genetic dwarfism in plants like dwarf pea and maize. Application of external gibberellin compensates for deficient endogenous gibberellins or counteracts the effects of natural inhibitors.

• Bolting and Flowering: Herbaceous plants exhibit a rosette-habit in early growth. Gibberellins also promote bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit. 

• Parthenocarpy: Gibberellins stimulate pollen germination and fruit growth, inducing parthenocarpy. In cases where auxins fail, gibberellins successfully produce seedless and fleshy tomatoes and large-sized grapes.
• Light Inhibited Stem Growth: Light-grown plants have shorter, thicker stems, contrasting with etiolated, taller stems in dark-grown plants. Gibberellin treatment of light-grown plants stimulates stem growth, turning them dark brown.
• De novo Synthesis of α-Amylase: Gibberellins induce de novo synthesis of the enzyme α-amylase in the aleurone layer during germination. This enzyme breaks down starch into simple sugars, providing an energy source for the growing embryo.