Mechanism of Gibberellins Action
Mechanism of Gibberellins Action
Gibberellins seem to affect plant growth and development in variety of ways. In many cases, it has been found that the GA stimulation of plant growth in dwarf seedlings is associated white increase in auxins content. Further, GA suppresses the activity of Indole acetic acid oxidase, the enzyme responsible for IAA degradation. Based on these observations, it was suggested that GA affects plant processes through auxins. However, several effects of gibberellins are independent of auxins, the auxins can not replace gibberellins. For example, the application of GA to barley aleurone layers causes a rapid increase in amylase activity, while auxins do not have such effect.
It appears that most of the physiological effects of GA are mediated via increased enzyme synthesis and membrane permeability. These aspects are described as follows:-
[I] Enzyme induction–
Several plant proteases, ribonuclease, RuBP carboxylase, ß 1,3-glucanase and nitrate reductase etc. are known to be induced by gibberellings, J.E. Varner, G.R. Chandra and M. J. Chrispeels in 1960’s demonstrated that GA caused de novo synthesis of hydrolytic enzyme during seed germination. GA seems to increase both transcription as well as translation, as the increase in enzyme activity is inhibited by actinomycin D (an inhibitor of transcription) and cycloheximide and fluorophenylalanine (inhibitors of translation). During GA treatment, more of radioactive phenylalanine was incorporated into proteins, indicating an increased translation. There is also an increased incorporation of ribonucleotides into RNA during GA treatment.
In barley aleurone layers. GA specially enhance the translatable m-RNA for α amylase. The increase in RNA seems to be the result of increased RNA polymerase activity as has been demonstrated in pea stems and wounded potato tubers. The hormone may also unveil some sectors of the chromatin DNA, making it available for RNA synthesis.
However, gibberellins are known to inhibit the activity of some enzyme such as invertase and peroxidase in sugar cane.
[II] Membrane permeability-
Some effects of gibberellins seem to be mediated via increased permeability of the cellular and organellar membranes. Gibberellins for example, not only increase the synthesis of α-amylase but also activate its release from barley aleurone layers. In the presence of exogenous GA, a major portion of enzyme activity is detected in incubation medium which increase with the incubation time. H.W. Evins (1971) detected changes in the aleurone cell membranes within 1 to 2 h of GA treatment. The permeability of some synthetic membranes has also been shown to be affected by GA treatment. One synthetic membranal identity is the liposomes, which are prepared from lecithin, cholesterol and diacetyl phosphate. Permeability of liposomes to glucose increased during GA treatment. In this respect, GA is similar to many steroid hormones. of animal origin, which also change the membrane permeability.
Gibberellins may affect the permeability of plant membranes either by affecting the synthesis of molecules involved in membranes or by changing their organization. The enzymes choline transferase and phosphatyl choline glyceride transferase increase by 2 to 4 folds after 2 to 12 h GA treatment (Johnson and Kende,
1971). These enzymes are involved in the synthesis of lecithin, a major phospholipid membrane component. Further, GA3 treatment increases the incorporation of 14C-Choline in partially purified endoplasmic reticulum preparations.
[III] Gibberellin interaction with DNA-
Gibberellins and possibly auxins and cytokinins as well, have been reported to interact with DNA and change its physical properties. GA3, IAA and kinetin change the melting points of DNA. GA,, has been shown to bind with adenine-thymine rich segments of DNA in vitro condition (Kessler and Snir, 1969). However, it is not known whether such interactions of GA with DNA are necessary for various physiological and biochemical responses.
[IV] Analogy with Animal Hormones-
Structural similarities of GA with steroid hormones (of animal origin) have prompted suggestions that GA may have a mode of action that resembles that of steroid hormones of animals. Accordingly, the hormone may bind to a specific site (receptor) in the membrane of the target cell (the cell where the hormone is to elicit response). There is a structural change in receptor protein as a result of this binding, which then moves on to the nucleus, binds to the chromatin DNA and activates transcription.
Using another analogy with animal hormone system, some scientists have demonstrated that cyclic AMP (a messenger molecule in animal hormone action) can stimulates α a-amylase formation in barley aleurone layers in a manner similar to GA. The effects of cyclic AMP effects are.
However, the presence of a specific hormone receptor and involvement of cyclic AMP as a second messenger molecule do not appear to be universal phenomena.
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