Gibberellins are named after the fungus Gibberella fujikuroi, which they have been identified for the first time (1926). From the chemical point of view, they are tetracycle diterpenoids. The symbol used for gibberellin notation is GA, equipped with a numeric index starting with 1 (GA1, GA2, GA3, …).
There have been identified over 70 types of gibberellins, gibberellic acid 3 (GA3) is considered more active. Gibberellins are found in the free state and bound with glycosides.
Biosynthesis
Gibberellin’s predecessor is kaurene. In the chemical structure of both of them, there is a common backbone—gibban, to which certain side groups are attached that determine their specificity. Thus each plant species has its own set of gibberellins.
Gibberellins precursors ent-kaurene and ent-gibberellin are synthesized in young leaves, in germinated seeds, apical buds, stem apices, etc. under the action of ent-kaurensynthase, an enzyme encoded by a nuclear gene (Le) and localized in plastids. In young leaves, light stimulates gibberellins.
The passive transport of gibberellins happens with the phloemic and xylemic flow (5–20 mm/h). Some authors claim that they migrate like organic metabolites and are accumulated in areas of growth.
Mechanism of action.
The mechanism of gibberellin action is interpreted as an intervention at the level of genes, inducing the de novo synthesis of α-amylase and protease.
In cotton, the Ltp3 gene encoding the LTP3 protein has a maximum concentration during fiber elongation and maturation and occurs under the action of plant hormones. GA3 induces the expression of the mitochondrial gene analogous to orfH522 (Fig. ) and the synthesis of the 16kD protein (Fig.) associated with cytoplasmic male sterility in sunflower.
Exogenous treatment with GA3 does not alter the transcriptional profile in the cytoplasmic male sterility line (S+), but determines the appearance of a discrete fragment of 321 bp specific to the nucleotide sequence analyzed in the fertile line (F+).
The PCR product (317 pb) obtained with specific primers for the actin gene sequence, used as a positive control for reverse transcription reactions showed that cDNA synthesis and subsequent PCR amplifi-cations occurred in all studied genotypes.
So the absence of amplification products of 321 bp in the fertile line and its appearance after application of gibberellins GA3 tells about the induced expression in the fertile line of a gene similar to orfH522 associated with cytoplasmic male sterility (CMS) in sunflower.
Biological significance
The physiological effects of gibberellins on plants are multilateral. Among the most important functions can be mentioned:
- stimulates stem elongation in dwarf plants, so many dwarfism genes are gibberellin deficiency genes;
- accelerates flowering in long-day plants;
- stimulates caryopsis germination in cereals, and stimulates fruit growth;
- determines changes in the photoperiod;
- intervenes in ceasing bud dormancy. Brings seeds out of the dormant state and influences their germination by intensifying the formation of ribosomes and nucleic acids, but also by permeating membranes
- in the endosperm gibberellins are participating in endoplasmic reticulum development, cell wall degradation, and synthesis of a large number of hydrolytic enzymes that catabolize seed reserves and the formed metabolites ensure embryo and seedling development.
- among the enzymes induced by gibberellins are α-amylase, some proteases, acid phosphatase, β-gluconase, α-glucosidase and ribonuclease;
- determines the sex switch in plants, causes parthenocarpy;
- intensifies transpiration, photosynthesis and respiration;
- shows synergism with auxins, due to its action on auxinoxidases;
- by stimulating cell division, gibberellins control mitotic activity, activate enzymes responsible for phospholipid biosynthesis.
Practical applications
Based on these properties, gibberellins have wide applications in practice—they are used to stimulate tomato fruit formation, to stop dormancy in tubers, buds, seeds, etc.
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