Gibberellins: Metabolism, Assay and Responses | Plant Hormones | Botany

Here is an elaborated discussion on gibberellins, highlighting:- 1. Introduction to Gibberellins 2. Gibberellin Metabolism 3. Natural Occurrence 4. Assay 5. Responses 6. Agricultural Uses.

Introduction to Gibberellins:

Before the discovery of gibberellins (GAs), Japanese farmers had long been aware of the presence in their fields of uncommonly tall rice seedlings that rarely flowered or lived to maturity. They concluded that these plants were diseased and named the disease bakanae (foolish seedling disease), since the plants that looked so promising early in the season did not fruit.

In 1926, the fungus Gibberella fujikuroi (Fusarium moniliforme) was isolated and identi­fied as the causative pathogen. From this fungus Japanese workers extracted a cell-free extract of GA-like substances that produced the same height increase as in diseased plants.

Following the discoveries of the Japanese, a flurry of research activity in growth regulator research in general, and with GAs in particular, emerged in the 1950s in Great Britain and the United States, especially after the observation that GAs were also present in higher plants.

GAs are diterpenoids, which places them in the same chemical family as chlorophyll and carotene. The basic chemical moieties of the GAs are the gibbane skeleton and the free carboxyl group. The various forms of mosses, ferns, and/or vascular plants and identified as GA-like substances. The GAs has a common chemical structure but ex­hibit a broad spectrum of biological activity. The GA-like substances were less specific chemically than GAs and had a narrower range of activity.

All plant organs contain GAs at varying levels, but the richest sources and probable sites of synthesis were found to be the fruits, seeds, buds, young leaves, and root tips. Seeds are especially rich in GAs; immature seeds are rich in GAs, but these occur in bound forms as seeds mature.

Plant species and type and age of tissue vary in GA kind and concentra­tion. In general, intercalary meristems have lower than normal levels and respond to GAs from exogenous sources. For example, young stems of genetic dwarfs, certain other intercalary meristems, and seeds of some species are responsive to exogenous GA, probably due to suboptimal endogenous levels.

Gibberellin Metabolism:

Biosynthesis of GAs occurred principally in immature fruits and seeds, buds, leaves, and roots. Although GAs is known to inhibit root growth, roots are a source of GAs to other organs. Generally seeds are the single richest source, as evidenced by the rapid growth of the fruit that surrounds them.

Three chemical metabolites were commonly involved in GA biosynthesis:

1. Mevalonic acid acts as a precursor to the formation of isoprene, the basic moiety in the 19- and 20-carbon gibbane skeletons.

2. Kaurene is formed from isoprene.

3. GA is formed from kaurene, the major GA precursor.

The breakdown of GAs in plant tissues from either endogenous or exoge­nous sources is less well understood. It appears evident that bound and free forms are readily reversible.

Seeds are high in bound form, but soaking and chilling seeds released GAs in free form. Cold exposure (vernalization) of soaked seeds and stratification of dormant buds increases GAs in free form, the result being induction of flowering and breaking of dormancy, respectively.

In breaking dormancy, GAs can substi­tute for red light. Internode and leaf growth evidently has a GA-light interac­tion. These findings appear to illustrate the rate of interconversion between free and bound forms and the interaction with light via the phytochrome pigment receptor.

GA activity can be inhibited chemically, presumably if the receptor sites are blocked by molecules structurally similar to GAs. Abscisic acid (ABA), an inhibitor of GAs, prevented GA₃ reversal of dwarfism.

It GA differ primarily by substitution of hydroxy, methyl, or ethyl groups onto the gibbane skeleton and by the presence of the lactone ring, which is produced by the condensation of carbon 20 to carbon 19 in the gibbane structure (Fig. 7.8).

The lactone ring present, for example, in GA₃, GA₄, and GA, is responsi­ble for the greater biological activity of these analogs, compared with GA₁₂ and GA₁₃ and similar analogs that do not have the lactone ring.

Different GAs are designated by a letter-number code (GA₁, GA₂, GA₃, …, GA₅₂). The number of distinctly different GAs was reported to be 52. Gibberellic acid (GA₃), the first identified, is the best known and most widely researched. It was first crystallized from the fungus Gibberella fujikuroi. Interestingly, GA₃ has the broadest range of biological activity. Commercial sources of GA₃ are obtained from fungal cultures, al­though GA₃ and most other GAs are widely distributed among higher plants.

Natural Occurrence of Gibberellins:

A large number of GAs with requisite chemical structure and biological activity occurs naturally, and many have been isolated from bacteria, fungi, is chemically similar to GA. Although not structurally similar, ethylene may also inhibit GA activity.

A number of synthetic chemicals from exogenous sources, termed growth retardants, effectively blocked GA₃ activity. Syn­thetic growth retardants such as AMO-1618, CCC, SADH or daminozide, Phosfon-D, and morphactins act as anti-GAs.

OAs is assumed to be translocated symplastically, but their presence in both phloem and xylem under certain conditions suggests both symplastic and apoplastic transport. Phloem transport rate was ob­served to be similar to that of carbohydrate, about 5 cm. h^-1. While auxin movement is polar and basipetal, GA moved freely basipetally and acropetally.

Gibberellin Assay:

The minute concentration of GAs in plant tissues makes identification and quantification difficult, and until recently, quantification was restricted to bioassay. Recent advances in chromatography (gas and liquid column and thin layer chromatography) are effective in separation. Nuclear magnetic resonance and mass- and fluorospectrometry are used to assay GAs and other growth substances physiochemically.

Weaver (1972) has listed the more successful bioassay tests:

1. Barley aleurone- Sterile, embryoless seeds are treated with GAs, releas­ing a-amylase and converting starch to sugar, which can be quantified. This test is simple and rapid.

2. Dwarf pea- Genetic dwarf pea plants are treated with GAs and grown under red light for observation of height change, compared with controls.

3. Other expansion growth tests based on the principle of increased internode elongation from GAs.

Other assays included the lettuce and cucumber hypocotyl test and the dwarf rice test.

Responses to Gibberellins:

A wide range of responses of numerous woody and herbaceous plants to GAs have been reported. GA acts synergistically with auxins, cytokinins, and probably with the other hormones, in what might be called a systems approach, or synergism.

For example, apical dominance, cambium growth, geotropism, abscission, and parthenocarpy are attributed to auxin activity, but GAs also influence or are essential for these responses. GA₃ was highly effective in increasing fruit-set, even in apple and pear, which responded poorly to auxins. Parthenocarpy can be induced in stone fruits that fail to respond to IAA.

The best known GA response is the stimulation of internode growth. Dwarf plants of maize, pea, and bush bean became normal after treatment with GAs. The requirement of a period of cold to induce flowering in certain biennials (e.g., beet and cabbage) was replaced by treatment with GA₃.

Release of a-amylase and resultant starch hydrolysis and germination require GAs:

Flowering has not been linked to a specific hormone, but GAs was shown to be active in flowering and maintaining the indeterminate growth habit (nonflowering) in a photoperiod-sensitive pea cultivar under long days. A graft-diffusible GA₉ metabolite was isolated that delayed flowering. Long days, which promoted flowering in all cultivars, resulted in a 10-fold drop in the GA₉ metabolite, evidence of direct action of a growth hormone in flowering.

These findings are significant in providing a plausible explanation for the cause of indeterminate and determinate growth habits. Indeterminate flowering plants, such as northern latitude soybean cultivars, produce flowers and fruits from axillary buds in response to photoperiod but maintain a vegetative terminal bud. All buds flower nearly simultaneously on determinate types. The control mechanism may be the GA level in buds, which could repress flowering of the terminal bud of indeterminate types or all buds under long days.

GA responses can be generally summarized as follows:

1. Whole-plant, genetic dwarfs elongate stem internodes to normal plant height if treated with GAs, whereas excised parts generally do not respond.

2. Most plant species and cultivars have sufficient endogenous levels of GAs and do not respond to exogenous sources. Genetic dwarfs, especially single-gene dwarfs, responded to GA₃ as an exogenous source.

3. Positive responses to GAs occur over wide concentration ranges, in contrast to auxin responses over only a narrow concentration range. Thus even high GA levels are not toxic and elicit no positive or negative responses, except on sensitive dwarf plants, whereas high concentrations of auxins are effective herbicides.

The GAs vary greatly in biological activity. GA₇ and GA₃ had the widest range of activity, although GA₄, GA₇, and GA₉ were more active than GA₃ in cucumber hypocotyl elongation. Dwarf maize responded to GA₄, while dwarf beans did not, but both responded to GA₇. GA₄ is many times more active on test plants than GA₈. A common characteristic of GA₄, GA₇, and GA₉ is the absence of the hydroxyl radical at carbon 7. It is likely all GAs is synergistic with auxin.

Agricultural Uses of Gibberellins:

In the 1950s expectations for GAs to improve crop production were high. Control of flowering and enhancement of growth and productivity were visu­alized. A great many researchers around the world initiated research on GAs, investigating their effect on growth habit and yield parameters of a host of economic species. Germination and emergence were enhanced in certain geno­types, but the effects on wheat were negative. Yield of dry matter generally was not affected despite an increase in height.

The discovery that GA₃ can produce sterile male plants created interest in GAs in the hybrid seed industry. GA₃ caused a high degree of male sterility in maize, but results were inconsistent, being highly dependent on dosage and time of application. Use of GAs to produce male sterility has not become a common practice, due to inconsistent response.

The use of GA₃ on ‘Thompson Seedless’ grape is a success story. GA₃ treatment at 200 ppm at the calypta (floral bracts) fall produced larger grapes with improved table quality. GAs is also used in the malting industry to promote a-amylase activity and the resultant starch hydrolysis in embryo less barley seeds.

Despite these uses, in general the high expectations for GAs remain largely unfulfilled, primarily due to the fact that modern crop cultivars have been selected for desired growth and reproductive habits that indirectly assure adequate endogenous GA levels and therefore no need for exogenous sources.