Gibberellin

Gibberellins are a family of diterpenoid plant hormones that regulate many aspects of growth and development. They include several natural compounds, such as GA1, GA3, GA4 and GA7, and numerous related gibberellins with varying biological activity. These hormones are produced mainly in developing seeds, young leaves and shoots, and are translocated through the plant to act at distant sites or locally.

The name derives from the fungus Gibberella fujikuroi, which overproduces gibberellins and causes the bakanae or foolish-seedling disease in rice. In higher plants, gibberellins influence germination, stem elongation, leaf expansion, flowering, and fruit development, contributing to the timing and vigor of growth.

Biosynthesis begins with geranylgeranyl diphosphate and proceeds via ent-kaurene to ent-kaurenoic acid, ultimately forming GA12 and other common gibberellins. GA20-oxidases and GA3-oxidases convert GA12 into bioactive GAs such as GA1, GA3 and GA4, while GA2-oxidases deactivate active GAs. Some steps occur in plastids or the endoplasmic reticulum, with tissue-specific regulation controlling final GA activity.

Signaling involves the GA receptor GID1, which binds GA and enables interaction with DELLA repressor proteins. The GA-GID1-DELLA complex is recognized by an SCF E3 ubiquitin ligase (SLY1/GID2), leading to DELLA degradation via the 26S proteasome and activation of GA-responsive growth programs.

Physiological roles and applications: GA signaling promotes seed germination by stimulating α-amylase production in the aleurone layer, enhances stem elongation, and modulates flowering and fruit set. Agriculturally, gibberellins, particularly GA3, are used to regulate plant height, break seed dormancy, and influence fruit development and malting in cereals.