Let us make an in-depth study of the gibberellins. After reading this article you will learn about 1. Discovery and Chemical Nature of Gibberellins 2. Distribution of Gibberellins in Plant 3. Biosynthesis of Gibberellins in Plants 4. Gibberellins Transport in Plant and 5. Deactivation of Gibberellins.
Discovery and Chemical Nature of Gibberellins:
The discovery of gibberellins is quite fascinating and dates back to about the same period when auxins were discovered, but it was only after 1950 that they came into prominence. A young Japanese scientist Kurosawa had been trying to find out why the rice seedlings infected by the fungus Gibberella fujikuroi (asexual stage Fushrium monoliforme) grew taller and turned very thin and pale.
These are the symptoms of ‘Backanae disease’ (meaning foolish) which is known to Japanese for over a century. In 1926, he succeeded in obtaining a filtered extract of this fungus which could cause symptoms of the Backanae disease in healthy rice seedlings. In 1935, Yabuta isolated the active substance which was quite heat stable and gave it the name gibberellin.
Yabuta and Sumiki (1938) isolated gibberellin in crystalline form and identified gibberellin-A and gibberellin-B from their original preparation. In fact, these were the mixtures of different gibberellins which could not be separated at that time due to lack of suitable techniques. Later work showed that gibberellin-A was probably a mixture of 3 biologically active and gibberellin-B a mixture of one biologically active and one inactive gibberellins. The biological activity of gibberellins and their effect on different developmental processes of the plants were also studied by Japanese workers and published.
But, the European and American scientists did not give due importance to this work probably because:
(i) Most of the scientists working on growth hormones were swayed by the impact of Indole-Acetic Acid (auxin) and other synthetic growth substances and
(ii) English translation of the Japanese work were not available to them until after Second World War.
It was only in 1950 when Mitchell at the Biological Warfare Centre in U.S.A. and Stodola (1955) at the U.S. Dept. of Agriculture were engaged along with team of workers to isolate this substance on a commercial basis, that the importance of gibberellins was realised by western scientists. In England, Brian, et al (1955) at the Imperial Chemical Laboratories independently obtained pure sample of a single gibberellin which was named as gibberellic acid. Later on, its structure was established by Cross et al (1961).
That there are different types of gibberellins had already been indicated by the work of Yabuta & Sumiki. Apart from fungal source, the gibberellins were then found to be present in wide variety of higher plants. The first higher plant gibberellin (GA) was isolated from immature bean seeds (Phaseolus coccineus) in 1958 which was later shown by MacMillan (1960) to be identical with GA1 earlier isolated from G. fujikuroi.
It is now known that there are over 125 different gibberellins (GAs) which have been isolated and chemically characterised. However, not more than 15 different GAs have been detected from any single species of higher plant. The status of gibberellins (GAs) as natural plant growth hormones is now fully established. The gibberellins (GAs) are a large family of tetracyclic diterpene acids which have a common entgibberellane skeleton. The latter is itself derived from entkaurene ring structure (Fig. 17.15).
(The substituent that project above the general plane of the ring system are said to be in β-form and those below the plane of ring are said to be in a-form and are represented by full or wedge shaped ( — or —) thick bond lines and dash bonds lines respectively).
Different GAs are named as GA1, GA2, GA3, GA4…. GA125 and so on. The subscript numerals are assigned to gibberellins roughly in order of discovery and are simply a cataloging convenience only. These do not show close chemical similarity or metabolic relationships among themselves. GA3 is called as gibberellic acid which is readily extracted from fungal cultures and is most common commercially available form and is widely used by scientists to show growth promoting properties of gibberellins.
i. Some gibberellins have the full complement of all 20 carbons of gibberellin skeleton. These are called as C20 – GAs e.g., GA12, GA27, GA53 etc. Others have lost one carbon (20th carbon) to metabolism and contain only 19 carbon atoms. These are called as C19 – GAs e.g., GA1, GA3, GA20 etc. (Fig. 17.16).
Besides this, there are other minor variations in basic structure of different GAs such as number and positions of – OH and methyl groups and state of oxidation at C – 20 (in case of C20 – GAs). All GAs have – COOH group at 7th carbon position.
ii. Despite the large array of gibberellins found in plants, only a few of them have been shown to be biologically active. Others are either, (i) inactive forms or (ii) precursors of other biologically active forms of gibberellins.
iii. In general, C19 – GAs appear to be more active biologically than C20 – GAs. In addition, those GAs with 3- β -hydroxylation, 3- β -1, 3 dihydroxylation or 1, 2-unsaturation are generally more active while those with both 3-β-OH and 1, 2-unsaturation such as GA3 exhibit the highest biological activity. GA3 is however, rare in higher plants. GA1 and GA20 (both C19 – GAs) are perhaps the most important biologically active GAs in higher plants. GA1 appears to be the chief biologically active gibberellin regulating stem elongation in higher plants.
Distribution of Gibberellins in Plant:
Gibberellins are found in all parts of higher plant including shoots, roots, leaves, flower, petals anthers and seeds. Gibberellins activity has also been shown in plastids. In general, reproductive parts contain much higher concentrations of gibberellins than the vegetative parts.
Immature seeds are especially rich in gibberellins (10-100 µg per g fresh weight) and are most favorite plants parts for the isolation of gibberellins and their study. In mature seeds, the gibberellins tend to form their derivatives. In plant, the gibberellins may occur in two different forms – free gibberellins and bound gibberellins. Bound gibberellins usually occur as gibberellin-glycosides.
Biosynthesis of Gibberellins in Plants:
The gibberellins which are chemically related to terpenoids (natural rubber, carotenoids & steroids) are thought to be formed by the condensation of a 5-C precursor—an isoprenoid unit called as isopentenyl pyrophosphate (IPP) through a number of intermediates to give rise to gibberellins. The primary precursor for the formation of this isoprenoid unit and synthesis of gibberellins is however, acetate.
(Besides gibberellins, carotenoids, rubber, steroids, Abscisic acid (ABA) and part of cyto-kinins are also derived from 5-C isoprenoid unit.).
Biosynthesis of gibberellins in plants is shown schematically in Fig. 17.18
In plants GAs are biosynthesized in apical tissues and there are three main sites of their biosynthesis, (it) developing seeds and fruits, (ii) young leaves of developing apical buds and elongating shoots and (iii) the apical regions of roots. The pathway of GA biosynthesis (Fig. 17.18) can be divided into three stages each of which is accomplished in a different cellular compartment.
(a) Stage I. Formation of terpenoid precursors and entkaurene in plastids:
GAs are biosynthesized from a 5-C precursor IPP. The IPP may be synthesized either in plastids or cytosol. From IPP, 10-C (GPP), 15-C (FPP) and 20-C (GGPP) precursors of terpenoids are formed by condensation of 5-C units (IPP). After the formation of GGPP, the pathway becomes specific for GAs.
GGPP is converted by two cyclization reactions through copalyI pyrophosphate into entkaurene. These reactions are catalysed by the enzymes cyclases which are located in pro-plastids and not in mature chloroplasts and in-fact constitute the first step that is specific for GAs. This step of GA biosynthesis is inhibited by compounds such as Amo-1618, Phosphon D and CCC.
(In more recent literature, various pyro-phosphorylated compounds such as IPP, DPP, FPP and GGPP etc. involved in terpenoid biosynthesis are referred to as diphosphates instead of pyrophosphates. However, their old abbreviated forms are still retained (e.g.) IPP now means isopentenyl diphosphate).
(b) Stage II. Oxidations to form GA12 and GA53. on ER through GA12 aldehyde:
The entkaurene is transported from plastids to ER (endoplasmic reticulum). Now a methyl group on entkaurene at 19th-carbon position is oxidised to carboxylic group which is followed by contraction of ring B from 6-C to 5-C ring structure to form GA12-aldehyde. GA12– aldehyde is subsequently oxidised to give GA12 which is precursor to all other GAs in plants. Hydroxylation of GA12 at C -13 results in the formation of GA53.
The enzymes catalysing the above oxidation reactions are mono-oxygenases which are located on ER and utilize cytochrome P450 in these reactions. Activity of these enzymes is inhibited by paclobutrazol and other inhibitors before GA12– aldehyde (Fig. 17.18).
(c) Stage III. Formation of all other GAs from GA12 or GA53 in cytosol:
All other steps in the biosynthesis of GAs from GA12 or GA53 are carried out in cytosol by soluble enzymes called dioxygenases. These enzymes require molecular O2 and 2-oxoglutarate as cosubstrates and use ferrous iron (Fe++) and ascorbic acid as cofactors. Activity of these enzymes is inhibited by cyclohexanetriones.
Environmental factors such as temperature and photoperiod are known to affect biosynthesis of gibberellins.
Gibberellins Transport in Plant:
Gibberellins have been found from both phloem and xylem exudates from a variety of plants. Unlike auxins, the transport of gibberellins in plants is non-polar. It is believed that gibberellins are trans located through phloem according to a flow pattern which is similar to those of carbohydrates and other organic solutes.
However, gibberellin transport may also occur in xylem due to its lateral movement between the two vascular tissues i.e., xylem & phloem. The gibberellins are not trans located in plant as free molecules but probably in their bound form as gibberellin-glycosides. The movement of gibberellins from scutellum to the cells of the aleurone layer in the germinating cereal seeds is well established.
Deactivation of Gibberellins:
There may be several mechanisms for deactivation of gibberellins to regulate level of biologically active GAs in plants:
(i) The introduction of a 2-β-hydroxyl group into a GA markedly reduces its biological activity. Conversion of active GA1 into inactive GA8 is one such example. Another example is conversion of GA20 to GA29 (in pea seeds at particular stage of development).
(ii) Conversion of free GAs into their bound forms such as gibberellin-glycosides or gibberellin-glycosyl ethers or esters also inactivates GAs especially in mature seeds.
(iii) Gibberethione (formerly known as pharbitic acid) formed in seeds of Pharbitis nil from GA3 by the oxidation of its 3-P-OH group followed by addition of mercaptopyruvic acid or L-cysteine to the resulting a, P -unsaturated ketone, has no GA activity and is considered as a GA deactivation product.