growth regulators

Chapter 12 Plant growth regulators



AUXINS 93

GIBBERELLINS 94

CELL DIVISION HORMONES: CYTOKININS 95

GROWTH INHIBITORS 96

ETHYLENE 97

The growth and development of plants is regulated by a number of chemical substances which together exert a complex interaction to meet the needs of the plant. Five groups of plant hormones are well established; they are the auxins, gibberellins, cytokinins, abscisic acid and its derivatives, and ethylene. These substances are of wide distribution and may, in fact, occur in all higher plants. They are specific in their action, are active in very low concentrations, and regulate cell enlargement, cell division, cell differentiation, organogenesis, senescence and dormancy. Their action is probably sequential. Other hormones concerned with flower formation and reproduction, but as yet uncharacterized, have also been envisaged. The essential role of these substances is illustrated by cell and tissue cultures; without the addition of suitable hormones no development or cell division occurs.


The effects of these very active substances on the production of secondary metabolites, particularly with a view to producing plants containing an enhanced proportion of active constituent, are of interest to pharmacognosists. In such studies the manner in which the results are recorded is all-important, particularly as the treatment may also influence the size of the test plant compared with the controls. For commercial purposes yield per hectare is an obvious criterion, whereas for biosynthetic studies yield per plant or per cent fresh weight may be of more significance. For final drug evaluation per cent dry weight is the most likely requirement.


In spite of the early enthusiasm for research on drug enhancement by the use of hormones applied to medicinal field crops, very little in the way of useful practical application emerged; the results were, however, of interest and selected examples of this older work continue to be retained in this chapter.



AUXINS


These growth-promoting substances were first studied in 1931 by Dutch workers who isolated two growth-regulating acids (auxin-a and auxin-b, obtained from human urine and cereal products, respectively). They subsequently noted that these had similar properties to indole-3-acetic acid (IAA), the compound now considered to be the major auxin of plants, and found particularly in actively growing tissues. Several similar acids, potential precursors of indoleacetic acid, have also been reported as natural products; they include indoleacetaldehyde, indoleacetonitrile and indolepyruvic acid. These compounds and IAA are all derived, in the plants, from tryptophan.


Typical effects of auxins are cell elongation giving an increase in stem length, inhibition of root growth, adventitious root production and fruit-setting in the absence of pollination. A number of widely used synthetic auxins include indole-3-butyric acid, naphthalene-1-acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D).



image

In the plant, oxidative degradation of IAA to give a number of products is controlled by IAA oxidase. Some substances such as the orthodiphenols (e.g. caffeic and chlorogenic acids and quercetin) inhibit the action of the enzyme and, hence, stimulate growth themselves. Conversely, monophenols such as p-coumaric acid promote the action of IAA oxidase and so inhibit growth. IAA may also be conjugated in the plant with aspartic acid, glutamic acid, glycine, sugars and cyclitols; such bound forms may represent a detoxication mechanism or are inactive storage forms of the hormone.


The main practical uses of auxins are: (1) in low concentrations to accelerate the rooting of woody and herbaceous cuttings; and (2) in higher concentrations to act as selective herbicides or weed-killers. Placed for 24 h in a 1:500 000 solution of NAA, cuttings will subsequently develop roots. This includes cuttings from trees such as holly, which were formerly very difficult to propagate in this way and had to be raised from seed or by grafting. Similarly, indole-3-butyric acid was successful with Cinchona cuttings, saving some 2 or 3 years compared with growth from seed. Similar results have been obtained with cuttings of Carica, Coffea, Pinus and other species. In biogenetic studies, use has been made of auxins to induce root formation on isolated leaves such as those of Nicotiana and Datura species. Auxins used in suitable concentration (usually stronger than when used for rooting cuttings) selectively destroy some species of plant and leave others more or less unaffected. They have, therefore, a very important role as selective weed-killers in horticulture and agriculture. Thus 2,4-D is particularly toxic to dicotyledonous plants while, in suitable concentration, having little effect on monocotyledons. It can, therefore, be used to destroy such dicotyledonous weeds as dandelion and plantain from grass lawns. (N.B. Certain carbamate and urea derivatives have an opposite effect and can be used to destroy grass without serious injury to dicotyledonous crops.)


There have been several reports on the effects of auxins on the formation of secondary metabolites on medicinal plants.


Seedlings and young plants of Mentha piperita, when treated with derivatives of NAA, gave in the mature plants an increased yield (30–50%) of oil which itself contained 4.5–9.0% more menthol than the controls. The study of the effects of auxins on alkaloid formation has concentrated principally on the tropane alkaloids of Datura species. Morphological changes in the plants were observed (2,4-D, for example, produced abnormal and bizarre forms of D. stramonium; an increase in trichome production, particularly in branched non-glandular forms; smooth fruits as distinct from those with spines; and a proliferation of vascular tissue). Generally, workers found no marked effect on alkaloid production or on the type of alkaloids produced, although a Russian paper records that with thornapple and scopolia tissue cultures a stimulating effect on alkaloid production was obtained with NAA and an inhibiting effect with 2,4-D; similar results were reported for Rauwolfia serpentina tissue cultures with these two hormones. An increased alkaloid production has been reported for submerged cultures of certain ergot strains when treated with various auxins (IAA; NAA; 2,4-D; indole propionic acid; indole butyric acid), whereas unpredictable irregular quantitative and qualitative effects on ergoline alkaloid production were observed with the same hormones in Ipomoea, Rivea and Argyreia (Convolvulaceae) suspension cultures. Experiments carried out in Hungary involving the injection of IAA into poppy capsules 1 and 2 days after flowering produced a relatively elongated capsule form and, in general, a reduced alkaloid content. In studies on anthraquinone production by cell suspension cultures of Morinda citrifolia, Zenk and co-workers have shown that cells grown in the presence of NAA have a substantial anthraquinone production but those with 2,4-D as sole auxin do not. IAA appears to have no beneficial effect on the production of sennosides in Cassia angustifolia.



GIBBERELLINS


This group of plant growth regulators was discovered by Japanese workers in connection with the ‘bakanae’ (foolish seedlings) disease of rice. In this, the affected plants become excessively tall and are unable to support themselves; through a combination of the resulting weakness and parasite damage they eventually die. The causative organism of the disease is Gibberella fugikuroi, and in 1926 Kurosawa found that extracts of the fungus could initiate the disease symptoms when applied to healthy rice plants. Some 10 years later, Yabuta and Hayashi isolated a crystalline sample of the active material which they called ‘gibberellin’. Preoccupation with the auxins by western plant physiologists, the existence of language barriers and the advent of World War II meant that a further 10 years elapsed before the significance of these findings was appreciated outside Japan. In the 1950s groups in Britain, the USA and Japan further investigated these compounds, which were shown to have amazing effects when applied to plants. It soon became apparent that a range of gibberellins was involved, and they are now distinguished as GA1, GA2, GA3, etc. GA3, commonly referred to as gibberellic acid and produced commercially by fungal cultivation, is probably the best-known of the series; its structure was finally determined in 1959. The first good indications that gibberellins actually existed in higher plants came with West and Phinney’s observation in 1956 that the liquid endosperm of the wild cucumber (Echinocytis macrocarpa) was particularly rich in substances possessing gibberellin- like activity and Radley’s report of a substance from pea-shoots behaving like gibberellic acid on paper chromatograms. Finally, in 1958 MacMillan and Suter isolated crystalline GA1 from Phaseolus multiflorus. By 1980, 58 gibberellins were known of which about half were derived from the Gibberella fungus and half from higher plants. GA117 was characterized from fern gametophytes in 1998 (G. Wynne et al., Phytochemistry, 1998, 49, 1837). Of these many GAs most are either dead-end metabolites or are intermediates in the formation of active compounds; only a limited number have hormonal activity per se. It is now considered possible that these substances are present in most, if not all, plants.


Gibberellins are synthesized in leaves and they accumulate in relatively large quantities in the immature seeds and fruits of some plants. The most dramatic effect of gibberellins can be seen by their application to short-node plants—for example, those plants producing rosettes of leaves (Digitalis, Hyoscyamus)—when bolting and flowering is induced; also, dwarf varieties of many plants, when treated with the hormone, grow to the same height as taller varieties. Other important actions of the gibberellins are the initiation of the synthesis of various hydrolytic and proteolytic enzymes upon which seed germination and seedling establishment depend.


The growth effect of gibberellins arises by cell elongation in the subapical meristem region where young internodes are developing. The effects of gibberellins and auxins appear complementary, the full stimulation of elongation by either hormone necessitating an adequate presence of the other.


As with auxins, gibberellins appear also to occur in plants in deactivated forms; thus, β-D-glucopyranosyl esters of GA1, GA4, GA8, GA37 and GA38 are known. As such they may serve a depot function. The glucosyl ester of GA3 has been prepared in several laboratories.


The biogenetic pathways of the gibberellins appear to be similar in both higher plants and Gibberella. They arise at the C20 geranylgeranyl pyrophosphate level of the isoprenoid mevalonic acid pathway (q.v.) with cyclizations giving the C20 tetracyclic diterpernoid entkaurenoic acid, which by a multistep ring contraction furnishes the gibbane ring system as exemplified by the key intermediate GA12-aldehyde. Several pathways diverge from GA12-aldehyde to give the known 90 or so gibberellins. All GAs have either the ent-gibberellane (C20 GAs) or the ent-20-norgibberellane (C19 GAs) (loss of C-20) carbon skeleton. Both types are modified by the position and number of OH groups, oxidation state of C-18 and C-20, lactone formation, presence and position of double bonds on ring A, epoxide formation, the presence of a carboxyl group and hydration of the C16–C17 double bond.



image
< div class='tao-gold-member'>

Related posts:

  1. taxonomic approach to the study of medicinal plants and animal-derived drugs
  2. of natural origin
  3. products
  4. glycosides, glucosinolate compounds, cysteine derivatives and miscellaneous glycosides

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 18, 2016 | Posted by in PHARMACY | Comments Off on growth regulators

Full access? Get Clinical Tree
Get Clinical Tree app for offline access