Giberelin

Giberelin, also referred to as gibberellin, is a class of diterpenoid plant hormones that regulate various aspects of growth and development in plants. The family includes many compounds, among them GA1, GA3 and GA4, with GA1 and GA3 being among the most active in many species. Giberelins influence processes such as seed germination, stem elongation, flowering time and fruit set. They were named after the fungus Gibberella fujikuroi, whose infected rice plants exhibited exaggerated growth, a condition known as bakanae disease.

Most giberelins are synthesized in young tissues, especially germinating seeds, shoot tips and developing leaves. The biosynthetic pathway begins in plastids with the formation of ent-kaurene, followed by oxidation steps involving enzymes such as GA20-oxidase and GA3-oxidase to form bioactive gibberellins. The level and distribution of giberelins are regulated by environmental cues and by the plant's own metabolism, and inactive forms can serve as storage or transport forms.

Mechanistically, giberelins promote growth by binding to the GA receptor GID1. This GA-GID1 complex interacts with DELLA proteins, growth repressors, marking them for degradation via the ubiquitin-proteasome system. The removal of DELLA repression leads to expression of GA-responsive genes and physiological responses, including enhanced seed germination through activation of enzymes such as α-amylase in cereal aleurone layers.

Agriculturally, exogenous giberelins or plants with altered GA biosynthesis are used to promote seed germination, increase stem elongation, adjust flowering time, and influence fruit development in a variety of crops. Usage is regulated in many countries as a plant growth regulator, with recommendations on concentration, timing and crop compatibility.