Гиббереллины: регуляция роста и развития растений

Class of plant hormones

Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. a revolution that is credited to have saved over a billion lives worldwide.

The bioactive GAs are GA1, GA3, GA4, and GA7. There are three common structural traits between these GAs: hydroxyl group on C 3β, a carboxyl group on C 6, and a lactone between C 4 and C 10.

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. In this pathway, bioactive GA is produced from trans geranylgeranyl diphosphate (GGDP). Multigene families encode the 2ODDs that catalyze the formation of GA12 to bioactive GA4. Environmental stimuli regulate AtGA3ox1 and AtGA3ox2 activity during seed germination. In Arabidopsis, GA20ox overexpression leads to an increase in GA concentration.

Most bioactive GAs are located in actively growing organs on plants. During flower development, the tapetum of anthers is believed to be a primary site of GA biosynthesis.

Arabidopsis, a plant, and Gibberella fujikuroi, a fungus, possess different GA pathways and enzymes. The function of CPS and KS in plants is performed by a single enzyme, CPS/KS, in fungi. In fungi, the GA biosynthesis genes are found on one chromosome, but in plants, they are found randomly on multiple chromosomes. Plants produce low amount of GA3, therefore the GA3 is produced for industrial purposes by microorganisms. Industrially the gibberellic acid can be produced by submerged fermentation, but this process presents low yield with high production costs and hence higher sale value, nevertheless other alternative process to reduce costs of the GA3 production is solid state fermentation (SSF) that allows the use of agro industrial residues.

Several mechanisms for inactivating GAs have been identified. 2β hydroxylation deactivates GA, and is catalyzed by GA2 oxidases (GA2oxs). Cytochrome P450 mono oxygenase, encoded by elongated uppermost internode (eui), converts GAs into 16α,17 epoxides. Rice eui mutants amass bioactive GAs at high levels, which suggests cytochrome P450 mono oxygenase is a main enzyme responsible for deactivation GA in rice. In a gamt1 and gamt2 mutant, concentrations of GA is developing seeds is increased. Levels of AtGA20ox1 and AtGA3ox1 expression are increased in a GA deficient environment, and decreased after the addition of bioactive GAs, Conversely, expression of AtGA2ox1 and AtGA2ox2, GA deactivation genes, is increased with addition of GA. Removal of IAA by removal of the apical bud, the auxin source, reduces the concentration of GA1, and reintroduction of IAA reverses these effects to increase the concentration of GA1. Auxin increases GA 3 oxidation and decreases GA 2 oxidation in barley. Auxin also regulates GA biosynthesis during fruit development in peas. These discoveries in different plant species suggest the auxin regulation of GA metabolism may be a universal mechanism. Ethylene decreases the concentration of bioactive GAs.

Recent evidence suggests fluctuations in GA concentration influence light regulated seed germination, photomorphogenesis during de etiolation, and photoperiod regulation of stem elongation and flowering.

Bioactive GAs and abscisic acid levels have an inverse relationship and regulate seed development and germination. Levels of FUS3, an Arabidopsis transcription factor, are upregulated by ABA and downregulated by GA, which suggests that there is a regulation loop that establishes the balance of GA and ABA.

In the early 1990s, there were several lines of evidence that suggested the existence of a GA receptor in oat seeds that was located at the plasma membrane. However, despite intensive research, to date, no membrane bound GA receptor has been isolated. This, along with the discovery of a soluble receptor, GA insensitive dwarf 1 (GID1) has led many to doubt that a membrane bound receptor exists. GID1 was first identified in rice and in Arabidopsis there are three orthologs of GID1, AtGID1a, b, and c. GA binding to GID1 causes changes in GID1 structure, causing a 'lid' on GID1 to cover the GA binding pocket. The movement of this lid results in the exposure of a surface which enables the binding of GID1 to DELLA proteins. DELLA proteins are characterized by the presence of a DELLA motif (aspartate glutamate leucine leucine alanine or D E L L A in the single letter amino acid code). F box proteins catalyse the addition of ubiquitin to their targets. It was later found that DELLAs repress a large number of other transcription factors, among which are positive regulators of auxin, brassinosteroid and ethylene signalling. DELLAs can repress transcription factors either by stopping their binding to DNA or by promoting their degradation. Microtubules are also required for the trafficking of membrane vesicles. Membrane vesicle trafficking is needed for the correct positioning of several hormone transporters. One of the most well characterized hormone transporters are PIN proteins, which are responsible for the movement of the hormone auxin between cells. In the absence of GA, DELLA proteins reduce the levels of microtubules and thereby inhibit membrane vesicle trafficking. This reduces the level of PIN proteins at the cell membrane, and the level of auxin in the cell. GA reverses this process and allows for PIN protein trafficking to the cell membrane to enhance the level of auxin in the cell.