<Li> ent - Kaurene </Li> <P> The bioactive GAs are GA1, GA3, GA4, and GA7 . There are three common structural traits between these GAs: 1) a hydroxyl group on C - 3β, 2) a carboxyl group on C - 6, and 3) a lactone between C - 4 and C - 10 . The 3β - hydroxyl group can be exchanged for other functional groups at C - 2 and / or C - 3 positions . GA5 and GA6 are examples of bioactive GAs that do not have a hydroxyl group on C - 3β . The presence of GA1 in various plant species suggests that it is a common bioactive GA . </P> <P> Gibberellins are involved in the natural process of breaking dormancy and other aspects of germination . Before the photosynthetic apparatus develops sufficiently in the early stages of germination, the stored energy reserves of starch nourish the seedling . Usually in germination, the breakdown of starch to glucose in the endosperm begins shortly after the seed is exposed to water . Gibberellins in the seed embryo are believed to signal starch hydrolysis through inducing the synthesis of the enzyme α - amylase in the aleurone cells . In the model for gibberellin - induced production of α - amylase, it is demonstrated that gibberellins (denoted by GA) produced in the scutellum diffuse to the aleurone cells, where they stimulate the secretion α - amylase . α - Amylase then hydrolyses starch, which is abundant in many seeds, into glucose that can be used in cellular respiration to produce energy for the seed embryo . Studies of this process have indicated gibberellins cause higher levels of transcription of the gene coding for the α - amylase enzyme, to stimulate the synthesis of α - amylase . </P> <P> Gibberellins are produced in greater mass when the plant is exposed to cold temperatures . They stimulate cell elongation, breaking and budding, seedless fruits, and seed germination . They do the last by breaking the seed's dormancy and acting as a chemical messenger . Its hormone binds to a receptor, and Ca activates the protein calmodulin, and the complex binds to DNA, producing an enzyme to stimulate growth in the embryo . </P>

Gibberellins are naturally produced in which part of a plant