Farmers in Asia were well aware of a disease of rice plants called bakanae (foolish seedling) disease in Japan. Infected plants would grow excessively taller than the rest of the rice in the paddy, fall over, and be unharvestable. This disease was found to be caused by a fungus known as Gibberella fujikuroi. Work on this physiology problem in Japan occurred just before the wars in the first half of the 20th century. Scientists in the 1950s rediscovered this work and extracted a range of chemicals that elicit the growth response in rice seedlings. These are now known as gibberellins. Since that time, close to 100 slightly different gibberellins have been identified chemically.
All of the gibberellins are based on a kaurene carbon skeleton. During the synthesis of gibberellins, the central 6-member ring is reduced to 5 carbons to make the basic gibberellin. The carbons are numbered 1 through 20.
You should notice that there are two different gibberellin classes...one class with 19 carbon atoms and one with 20 carbon atoms. Those gibberellins with the C19 structure (note the lactone bridge between carbons 19 and 10) can be very active in growth stimulation. Some of the best-known gibberellins are those below which are active in growth stimulation, etc.
Most higher plants analyzed use GA1 as the active endogenous GA. Cabbage family members and confiers often use GA4 instead. GA3 is of fungal origin (remember Gibberella fujikuroi), but it can substitute for GA1 quite easily as it differs by just a double bond. Because GA3 can be produced in bulk fungal culture, it is commercially important. Below is an example of an inactive gibberellin. Gibberellins with hydroxylation in the beta-configuration at carbon 2 are all inactive.
Whether a particular gibberellin (GA) is active or inactive was determined by simple bioassays, for example dwarf rice (cultivar 'Tan ginbozu') seedlings could have a microdrop of some gibberellin solution placed between the coleoptile and first leaf. This was absorbed and the growth of the second leaf sheath would indicate how active the GA was.
In spite of the many different specific gibberellins found in plants, most are inactive forms that serve as biosynthesis precursors or breakdown products of the active GAs.
Terpenoids are compounds made from assembly of isoprene units. Isoprenes are flexible five-carbon units:
These can be joined end to end, and folded back to make the heterocyclic systems of gibberellins. The synthesis pathway near the end of that process is outlined below. In this first stage of the path, cyclase enzymes close the isoprene units into a polycyclic system. These enzymes occur in the proplastids and are blocked by AMO 1618, cycocel, and Phosphon D.
In the second stage of the synthesis, associated with the endoplasmmic reticulum, the basic kaurene is contracted into a true gibberellin...GA12. These steps are catalyzed by monooxygenases and are blocked by paclobutrazol, tetcyclacis, and uniconazole.
The chemicals mentioned above as blocking the formation of gibberellin synthesis early in the pathway are quite effective dwarfing agents. One of the dwarfing agents you have experience with is B-Nine. This was the agent that turned the genetically tall pea into a phenotypic dwarf.
The next part of the pathway is operated by an enzyme known as GA 20-oxidase in the cytosol. This enzyme coverts GA12, an inactive GA, into a range of intermediate GAs.
GA 20-oxidase ends its role when GA20 has been produced.
The GA20 is then converted by GA 3-oxidase into GA1 which is the native active gibberellin found in many higher plants. At the bottom of this figure you also see that the pool of active GA1 and its precursor GA20 can be depeleted by the action of GA 2-oxidase. This enzyme converts those GAs into inactive forms.
Of note for your laboratory exercise is that in 1997 it was demonstrated that in peas...work that started with Mendel's study of inheritance of tall and dwarf characteristics...dwarf peas have a defective 3β-hydroxylase which converts GA20 to GA1. This conversion makes an inactive GA into an active GA. This explains why the dwarf plants are dwarf...they lack appreciable active GA. Spraying such dwarf peas with gibberellic acid (GA3 an active GA) can result in pea plants of normal height.
An interesting note is that there are many different versions of the le (dwarf) gene in peas. These have differing effectiveness in the 3β-hydroxylation, resulting in differing levels of GA1 in the tissue, correlating with the height of the plants! This relationship is shown below.
The synthesis pathway for GAs is shown below with the names of genes that catalyze the steps. You will see that the le (dwarf) allele is defective in the gene for GA 3-oxidase, preventing GA1 synthesis. Peter Davies and co-workers discovered that this mutant, initially studied by Gregor Mendel, has a point mutation that results in a single amino acid change (alanine changed to threonine) resulting in less than 5% normal conversion of GA20 to GA1.
Another dwarf pea (na) has a defect called nana defective in the synthesis genes of the P450 monooxygenase type. These plants have a mature height of 1 centimeter! They cannot make ANY gibberellin of any kind. This makes sense since it blocks the path before the synthesis of GA12.
At the opposite extreme, an ultratall mutant called slender produces no GA at all (na na le le) but has high auxin (crys crys) and looks much like an etiolated (remember your dark grown plants?) plant but with expanded green leaves! Wild!
Also recall that the gibberellin synthesis pathway can be blocked with various compounds mentioned above. The compounds which do this are shown below.
Again, in your lab exercise you used B-Nine to block GA biosynthesis and thereby convert a tall genotype into a dwarf phenotype.
There are also correlations between growth and GA content. In the graphs below you can see the progress of the biosynthesis pathway after pollination and as seeds develop in immature fruits.
More on how GA works to produce growth later...for now let us review a very important diagram:
As we observed in our examination of auxin biology, gibberellic acid pool size is regulated homeostatically.
The relationships between synthesis and degradation (deactivation) are shown below. As you can see, as spinach is induced to flower, the GA concentration increases over time, but the level of active GA1 is maintained at about 500% of initial. The synthesis pathway is active however, accumulating precursor GA20 and also turning over the GA1 into inactive GA8.
As with many of the hormones, there are various sensitivities in various tissues that change over time. Because of this, GA elicits pleiotropic responses. Your book lists several of these.
GA stimulates growth of dwarf varieties, and dwarfing agents that block GA biosynthesis can be used to control height of tall forms. Virtually all Easter lilies, poinsettias, and chrysanthemums that you buy at a florist have been treated with "dwarfing agents" (GA biosynthesis inhibitors).
GA stimulates the development of flowers or pine cones, increases floral size, but also makes dicot flowers male-only in certain "plastic" hermaphroditic species. In monocots, gibberellins can induce female expression in the tassels (usually male flowers). In GA defective mutant corn, the ears produce anthers:
GA stimulates the expansion of fruits. Probably the most famous example is in grapes. The seedless green and red table grapes you enjoy lack ovules to produce GAs, so the growers must spray (or dip) the senescing flower clusters with GA to get the fruits to enlarge. So every table grape you buy at the store is thus treated with GA at least 3 times in the growing season. Fortunately GA is not known to cause and human problem in studies.
GA stimulates seed germination in species that are kept dormant by Abscisic Acid (remember your project on this idea?). One organism has been extensively studied by plant physiologists in this regard: Barley (Hordeum vulgare). This seed is germinated (malted) to produce beer. The sprouting process involves in part the conversion of starch in the endosperm of the seed into maltose (a disaccharide). This maltose is converted to alcohol and carbon dioxide by fermentation of the seed extract by yeast (fungi) during beer making.
Gibberellins are known to stimulate the de-novo synthesis (think central dogma) of α-amylase in the aleurone cells that surround the starchy endosperm. This response is summarized graphically below.
This and much more has been condensed into a diagram depicting the total response.
Seed germination in lettuce follows a similar mechanism, however it appears that gibberellins are rather less-important. The main signal stimulating gene expression of amylase and other germination-initiating enzymes is light. Thus the photoactivation is achieved by phytochrome in its Pfr form.
As we leave this topic, let me take a parting shot at most textbooks. I think most have shamefully made attributions and statements that are misleading and false in some sections. I believe the book has hidden or ignored several lines of evidence and introduced errors by these omissions. The work of Richard Pharis and Peter Kaufman, in particular, should have been elucidated and included. I understand that some researchers are more socially accepted than others, that the "American Plant Physiology Club" often excludes Canadians, and so on...but to hide or exclude their work in a general textbook is not acceptable. That is particularly true when that work so clearly addresses mechanisms that have not been found elsewhere, but which has been verified independently.