Let us make an in-depth study of the mechanism of gibberellin action in higher plants. After reading this article you will learn about A. Promotion of Stem Elongation and B. Mobilization of Endosperm Food Reserves.
Gibberellins (GAs) are very active molecules even at very low concentrations. Responses of gibberellins in stem elongation of lettuce/rice seedlings at concs. of 10-10 g (0.1 ng) or even lower have been obtained. Obviously, efficient mechanisms in responding cells must be present for amplification of hormonal signal at very low concentrations.
Out of so many physiological effects of gibberellins, the mechanism of two of the effects:
(i) Stem elongation and
(ii) Mobilization of reserve food in the endosperm are extensively studied and better known.
In both those cases, the sequences of events are similar and envisaged on the following lines:
(i) Binding of hormone to a receptor
(ii) Activation of one or more signal transduction pathways.
(iii) Transcription of primary and secondary response genes leading to the physiological response.
However, some of the earlier events are common to all GA responses and it is believed that GA acts by de-repressing the negatively regulated genes of GA response.
A. Promotion of Stem Elongation:
As described earlier, examples of most pronounced effects of GAs on stem elongation are dwarf and rosette plants. Deep-water rice (Oryza sativa) is another important example of elongation of internodes so that its foliage may remain above water in the field.
Gibberellins Stimulate both Cell Division and Cell Elongation:
Stem elongation in plants as a result of gibberellin treatment involves both cell division and cell elongation. In rosette long day plants, GA treatment causes marked increase in mitosis in the sub-apical regions of apical meristems.
Internodes of tall pea plants have more cells than in dwarf ones and their cells are longer in size too. In deep-water rice, stimulation of internodes elongation is partly due to increased cell divisions in the intercalary meristems and partly due to elongation of cells of the latter who have divided with cell elongation preceding the cell divisions.
Gibberellins Increase Cell Wall Extensibility without Acidification:
GAs cause an increase in both mechanical extensibility of cell walls and stress relaxation (loosening) of the walls of living cells of stem.
The minimum lag time for GA-induced growth is far greater than auxins. In deep-water rice, it is 40 minutes while in pea stems it is between 2-3 hours. Obviously, the mechanism of GA-induced growth is different from that of auxin-induced growth.
Unlike auxins, (i) GA does not appear to act by increasing osmotic uptake of water and (ii) in GA-induced growth, there is no stimulation of proton (H+) extrusion to acidify the cell walls. However, since GAs are not found in plant tissues in complete absence of auxins, GA- induced growth may depend on auxin-induced wall acidification. Therefore, growth responses of applied gibberellins and auxins are auditory.
In recent years, a close correlation has been observed in GA induced growth and activity of the enzyme xyloglucan transglycosylase (XET) in many plant tissues.
This enzyme hydrolyses xyloglucans of the cell walls internally and causes molecular rearrangement in the cell wall matrix which could promote extension of cell wall. XET may also facilitate penetration of proteins called expansions into the cell wall causing cell wall loosening and thus increasing mechanical extensibility of cell walls. The activity of the enzyme XET in wall loosening event is specific to gibberellins and is not associated with auxin-induced growth.
GA Stimulated Cell Divisions Are Regulated Between G2 and M Phases of Cell Cycle:
The mechanism of GA stimulated cell division has been extensively studied in intercalary meristems of young internodes of deep-water rice. In this case, the GA – stimulated cell divisions are believed to be regulated between G2 and M phases of cell cycle.
The transitions between different phases of cell cycles are known to be regulated by cyclin- dependent protein kinases (CDKs). GA stimulates cell divisions by increasing the expression of two genes (CDC2) that encode CDKs and M cyclins which are required for entry into mitosis.
Some Repressors of GA Response Have Been Identified:
Studies on GA-induced signal transduction pathways involved in stem elongation have led to the identification of three main types of transcriptional factors that act as repressors of GA response. These are,
(i) GAI (Gibberellin insensitive dwarfs mutants)
(ii) RGA (Gibberellin deficiency reversion mutants)
(iii) SPY (Spindly or slender mutants)
(GAI and RGA both have a conserved region at the amino terminal of the protein which is known as DELLA, the latter being involved in GA-response. Both GAI and RGA are therefore, also called as DELLA Repressors. DELLA is sequence of amino acids denoted by their code letters. (See Appendix 8 for details).
The above mentioned GA response mutants have successfully been obtained in Arabidopsis and also in some other plant species. Gibberellin appears to induce its effect on stem elongation by de-repressing negatively regulated genes Le., by deactivating or degrading the repressors of GA response so that GA induced genes are transcribed and stem elongation or growth occurs. A schematic representation of the interaction of GA and various repressor genes of GA signal transduction chain that regulate stem elongation is given in Fig. 17.19.
According to this scheme,
(i) The transcriptional factors GAI and RGA act as inhibitors or repressors of transcription of those genes that leads to growth with the result that growth is suppressed.
(ii) SPY is also a negative regulator or repressor which acts upstream of GAI and RGA in GA-signal transduction chain and plays inhibitory role directly or by enhancing the effects of GAI and RGA.
(iii) In presence of GA, these repressors are deactivated or degraded so that transcription of genes occur that leads to stem elongation (growth).
It is however, not clear whether GA nullifies RGA and GAI independently or through SPY or both.
B. Mobilization of Endosperm Food Reserves:
As mentioned earlier, the GAs cause de novo synthesis of a -amylase in cells of aleurone layer of germinating cereal grains. (The GAs synthesized by coleoptile and scutellum of the embryo are released into the starchy endosperm from where they diffuse into the cells of aleurone layer). GAs also bring about secretion of a -amylase and other hydrolytic enzymes from the cells of aleurone layer into the starchy endosperm where complex carbohydrates are hydrolyzed into simple sugars which are then trans located to growing embryo to provide energy source.
Although the mechanism of induction of a-amylase synthesis in cells of aleurone layer by GA and its secretion from cells of aleurone layer into the endosperm, have been extensively studied and known in fairly good detail, but they are yet far from being completely elucidated. The sequence of events may be summarised as follows: (Fig. 17.20).
(i) The gibberellin (chiefly GA,) combines with a receptor on the outer surface of plasma-membrane of aleurone layer cell
(ii) The GA-receptor complex interacts with a heterotrimeric G protein (also situated on the surface of plasma membrane) and initiates two separate signal transduction pathways;
(a) a calcium (Ca2+) independent signal transduction pathway which involves cyclic GMP (cGMP) as signaling intermediate (secondary messenger) leading to the expression of a-amylase gene and (b) a calcium (Ca2+) dependent signal transduction pathway which involves calcium, calcium binding protein calmodulin and a protein kinase as signalling intermediates (secondary messengers) leading to the stimulation of secretion of a-amylase and other hydrolytic enzymes from cells of aleurone layer into the endosperm for starch degradation.
(The primary messenger is the hormone GA itself).
Some details of calcium independent signal transduction pathway are as follow:
(iii) A GA signalling intermediate is activated.
(iv) The activated GA signalling intermediate goes into the nucleus and binds with DELLA repressor proteins (parts of GAI and RGA) and also SPY (not shown in the figure).
(v) DELLA repressors and SPY are degraded or inactivated.
(vi) Due to inactivation or degradation of these repressors, GA-MYB & other genes are switched on. GA-MYB m-RNA goes into the cytosol for translation and a GA-MYB protein (a transcription factor) is synthesized.
(vii) GA-MYB protein enters into nucleus and binds with promotor genes for a-amylase and other hydrolytic enzymes.
(viii) α-amylase gene and other genes that encode other hydrolytic enzymes are transcribed
(ix) α-amylase m-RNA moves out from nucleus to rough ER in cytoplasm for translation process and α-amylase protein (enzyme) is synthesized.
(x) α-amylase proteins (enzymes) are secreted via Golgi-bodies.