BIOL 266 M2 - Gibberellins

Second of the five plant hormone

Gibberellins. A class of plant hormones named after a rice fungal pathogen, Giberella. They are responsible for tall and spindly rice seedlings. All plants produce gibberellins.

GA#

GA meaning gibberellic acid. There are now more than 125 GAs, but most of them are intermediates (as the name describes, just bridging) or breakdown products. Only four GAs are physiologically active. This is GA1, GA3, GA4, and GA7.

What do Gibberellins do?

Promote plant stem elongation.

What type of mutants are there for Gibberellins?

The first mutant has defective enzmes. These cause the plants to produce lower levels of active gibberellins. These are considered dwarf phenotypes.

The second mutant has defective gibberelin receptors. Althought they produce normal levels of gibberellins, they are unable to respond to gibberellins. These are also considered dwarf phenotypes.

How do GAs function?

They regulate gene transcription. Gibberelin molecules bind to specific receptors. When binded, these degrate DELLA proteins (proteins that repress gene transcription. Without these DELLA proteins, transcription happens that are involved in growth and development of the plant.

Green Revolution

A coordinated and conventional plant breeding program that focused on increasing yields of imported crops like rice, wheat, maize/corn. The product of the Green Revolution produced semi-dwarf plants that had high yields of rice, wheat, and maize/corn compared to their dwarf counterparts. These semi-dwarf plants, although having shorter stems were stronger and less susceptible to falling over. The mutation in the DELLA protein caused these dwarf mutations to occur.

Gibberellic molecules will still bind to the receptors but DELLA proteins won’t get degraded. This mean these mutants are GA-Unresponsive, causing less stem growth.

What else is GA involved in?

Seeds require a cold period (seeds are exposed to cold temperatures to promote germination). GA’s can be a substitute for this germination period. They also promote flowering which allows plants to mature and seed quicker.

GA applications

Agricultural and horticultural applications. GA drenches allow the flower to elongate and spread the flowers out, causing production of larger fruit (fruit can take up more space). GA antagonists produce stockier, more rigid ornamental plants.