Gibberellins: Hormones in plants

Gibberellins

Gibberellins were discovered in the 1930s by Japanese scientists investigating a disease of rice caused by the fungus Gibberella fujikuroi, that results in tall, seedless plants. Nearly 100 gibberellins have been identified in plants though many do not have biological activity. The most studied, and probably most significant gibberellin is GA3. In common with the other gibberellins, it has a structure based on ent-gibberellane (Fig. 3).

Gibberellin effects

Environmental responses. Many species remain as rosette plants until they have been exposed to either low temperatures (vernalization) or a number of long days. Spinach, for instance, retains a short, squat form until day-length increase, when it begins to grow upwards and flower. Gibberellin levels are low in rosette plants, but increase dramatically in response to the changed environment and initiate the growth response. GA1 is most significant in elongation responses and GA9 in flowering.

Seed germination

In some seeds which show dormancy, gibberellin application will break dormancy. In other seeds, gibberellins are essential in coordinating the processes of germination, increasing in activity upon rehydration of the seed

and initiating the activity of the hydrolases which mobilize seed storage reserves. The role of gibberellins in germinating barley is of economic importance in the malting process, part of beer brewing.

Other effects of gibberellins

Gibberellins are involved in regulating the transition from juvenile to mature growth form in some perennial species such as ivy (Hedera helix); in initiating flowering and promoting fruit formation.

Synthesis

Gibberellins are diterpene acids synthesized by the terpenoid pathway. Terpenoids are compounds built of repeating isoprene units:

The location of the early stages of gibberellin synthesis is the plastid, where isoprene synthesis occurs from glyceraldehyde-3-phosphate and pyruvate. Later stages occur in plastids in meristems and in enzymes of the endoplasmic reticulum (ER) and cytoplasm.

Transport

Highest levels of active gibberellins in plants are found in young rapidly growing tissues like young leaves and buds, and developing seeds and fruits. Transport of gibberellins in the plant occurs predominantly in the phloem and is non-polar.