Gibberellins
Introduction
Gibberellins (GAs) are a large family of plant hormones that play a crucial role in various growth and developmental processes. These include seed germination, stem elongation, leaf expansion, and flowering. Gibberellins are named after the fungus Gibberella fujikuroi, from which they were first isolated.
History and Discovery
The discovery of gibberellins dates back to the early 20th century when Japanese scientists were investigating a disease in rice plants known as the "foolish seedling" disease. The disease, caused by the fungus Gibberella fujikuroi, led to excessive stem elongation and reduced fertility in rice plants. The active compound causing these symptoms was isolated from the fungus and named gibberellin.
Structure and Classification
Gibberellins are diterpenoid acids and are classified into two groups based on their structure: the bioactive gibberellins and the inactive, or precursor, gibberellins. The bioactive gibberellins are those that are capable of stimulating plant growth, while the inactive gibberellins serve as precursors in the biosynthesis of the bioactive forms.
Biosynthesis and Metabolism
Gibberellins are synthesized in plants through the terpenoid pathway. The biosynthesis involves several enzymatic steps, including the conversion of geranylgeranyl diphosphate to ent-kaurene, which is then oxidized to form the gibberellin skeleton. The final steps involve a series of modifications to the gibberellin skeleton to produce the bioactive forms.
The metabolism of gibberellins involves their conversion to inactive forms through a process known as deactivation. This is achieved through hydroxylation, a reaction catalyzed by the enzyme gibberellin 2-oxidase.
Functions
Gibberellins have a wide range of functions in plants. They are involved in promoting cell elongation and division, stimulating seed germination, promoting flowering, and regulating the transition from juvenile to adult phases in plants.
Role in Agriculture
In agriculture, gibberellins are used to promote plant growth, increase fruit size, and improve crop yield. They are also used to break seed dormancy and promote germination in certain crops. However, excessive use of gibberellins can lead to undesirable effects such as excessive stem elongation and reduced fertility.