PLANT GROWTH RETARDANTS AND AGRICULTURE

Great economic benefits would flow from it for Pakistan

By Dr S.M. ALAM & A. SHEREEN
NIA, Tando Jam
Mar 11 - 17, 2002

Growth retardants or inhibitors are those chemicals, which exert inhibitory effects on the general growth and development of plants. The growth behaviour and yield formatlon of crop plants are governed by their genetic potential, climatic conditions, the supply of nutrients and many other growth factors. Specific control of these processes by the application of plant bioregulators of natural of synthetic origin is increasingly emerging in recent years. Because of their specific properties in regulating shoot growth, the plant growth retardants have become the most widely used group of bioregulators in agricultural and horticultural practice. When applied in appropriate concentrations growth retardants modify plant architecture in a typical fashion. Internode elongation and thus plant height are reduced without affecting the number of internodes and leaves concomitantly, the green colour of the foliage is intensified and leaf thickness and epicuticular wax may increase. In contrast to the shoot, the growth retardants maintain or slightly enhance root formation. Therefore, the root-shoot ratio is clearly changed in favour of the root.

The morphological effects of growth retardants are accompanied by alteractions in the developmental and physiological behaviour of treated plants, the most striking changes include reduction of water consumption, retardation of senescence, and improved resistance to environmental stresses. As a result of this bio-regulation, the economically most important use of growth retardants is in improving lodging resistance and yield formation, particularly in cereals. Among the growth retardants of the first generation, which have found large scale application in agriculture for decades, the ethylene-releasing ethephon and particularly, compounds of the onium-type such as chlormequat chloride and mepiquat chloride are the most prominent representatives. In addition to these conventional growth retardants, extremely effective compounds have been discovered and developed in recent years. These include substances with a nitrogen-containing heterocycle, such as pyrimidine (e.g. in ancymidol), 4-pyridine (in inabenfide), triazole (e.g. in uniconazole, paclobutrazol, triapenthenol) and norbornanodiazetine (in tetcyclacis) and compounds with a cyclohexanetrione (acylcyclohexanedione) structure as the chemical feature. The latter group with prohexadione and calcium cimectacarb represents the most recent and thus the third generation of growth retardants known. In model experiments the conventional growth retardants are clearly surpassed by the new types of compounds in activity of reducing shoot growth. Each derivative of the three chemical classes (except the ethylene- releasing ethephon) shares the common action of directly inhibiting gibberellin biosynthesis, but at distinct enzymatic steps.

The onium-type compounds, with a positively charged moiety, appear to interact with the cyclization of geranylgeranylpyrophosphate to ent-kaurene, catalyzed by ent-kaurene synthase. The reactions of the next stage in biosynthesis, leading form ent-kaurene to ent-kaurenoic acid, are the targets of the N- heterocyclics. Finally, the cyclohexanetriones are known to interfere with certain steps beyond aldehyde which directly lead to the biologically active gibberellins, particularly to GA1. These are all hydroxylation reactions catalyzed by soluble 2-oxoglutarate-dependent dioxygenases. They appear to be inhibited by the compounds competitively with respect to their co-factor.

At present, most knowledge of the mode of action underlying the morphological and physiological effects of growth retardants has been compiled for the class of chemicals with a nitrogen-bearing heterocycle. All these compounds have a structural element in common: the lone pair of electrons on the hybridized nitrogen atom in the heterocycle. This pair of electrons is found on the periphery of the molecules, enabling it to interact with plant cytochrome dependent monooxygenases. It bonds to the protoheme iron of cytochrome, preventing the oxygen required for the catalytic reaction from binding. Thus, the enzyme is inactivated. Many oxidative reactions in different metabolic pathways are catalyzed by such microsomal enzymes. However, the action of this type of growth retardants seems to be confined to methyl hydroxylases, particularly in the terpenoid pathway. This includes the biosynthesis of gibberellins, abscisic acid (ABA), cytokinins and sterols. In a latter section this topic will be discussed in more detail. Emphasis should first be laid on the effect of the retardants on growth. The sites of action of growth retardants are the subapical and intercalary meristems located e.g. at the base of internodes and leaf sheaths. In these areas cells are produced in a zone of meristematic activity with cell division and only slight cell elongation. Newly formed cells entering the growth zone outside the meristem undergo considerable cell elongation without cell division. When applied to stems of chrysanthemum, growth retardants are able to reduce the meristematic zone with its cell division activity. Histological studies on various shoot sections of sunflower, soybean and maize seedlings treated with tetcyclacis indicated that the type of effect on longitudinal growth depends on the concentration applied. Thus, the shortening occurring at low retardant concentration is primarily caused by an inhibition of cell elongation. However, at higher concentrations the stunting of the respective shoot sections is increasingly due to a reduced rate of cell division. In conclusion, cell elongation principally occurring in the growth zones outside the meristems is the more sensitive process to growth retardants as compared to cell division.

As particularly emphasized by studies with genetically defined mutants, gibberellins and auxins are the groups of phytohormones that promote longitudinal shoot and leaf growth, mainly by affecting cell elongation. As mentioned above, growth retardants with N-bearing heterocycles interfere with gibberellin biosynthesis by selectively inhibiting the oxidative steps from ent-kaurene to ent- kaurenoic acid. These reactions are catalyzed by the cytochrome dependent kaurene oxygenase, which is affected by the retardant in the same concentration range as the elongation process itself. Thus, the reduction in cell elongation caused by N-heterocyclics appears to be closely linked to their influence on gibberellin biosynthesis.

Heterotrophically cultivated cell suspensions offer an appropriate model system for meristematic tissue since growth is governed in both cases mainly by cell division activity. This was supported by an experiment comparing the effect of various growth retardants on cell division activity in suspension cultures with their influence on shoot growth of intact plants. At the high concentration of the same relative efficiency of the retardants was obtained for rice, maize, soybean, and sunflower in both systems. However, as illustrated by the effects of ancymidol, tetcyclacis, and paclobutrazol in cell suspensions, sterols play a more important role in cell division than gibberellins. It was shown with tetcyclacis that before cell division activity of rice cells ceases their terpenoid synthesis, membrane permeability, and protein, RNA and DNA synthesis are reduced in a chronological sequence. Concomitantly, the phytosterol content in the cells, particularly stigmasterol, decreases. The addition of cholesterol but not applied gibberellins fully restores normal cell division activity. Sterol production in intact plants is also influenced by N-heterocyclic retardants. However, their relative potency often depends on the plant species and material analyzed, the type and particularly in the case of chiral triazoles the stereoisomer used, and on the retardant concentration which usually must be higher than that necessary for blocking gibberellin biosynthesis. However, in contrast to the phytohormonal gibberellins, sterols mainly function as membrane components. A strong reductrion in sterol production certainly leads to cell damage, whereas a slight change in sterol metabolism creating sterols with membrane-altering properties might rather result in the observed cytostatic effects especially in meristematic cells.

As enzyme targets of growth retardants in sterol biosynthesis, cytochrome dependent obtusifoliot-14 methyl demethylase and sterol desaturase, which also appears to be a cytochrome have been proposed. The latter enzyme atalyuses the formation of double bond in stigmasterol. Evidence for inhibition of this enzyme is given by a decline in the stigmasterol/sitosterol ratio after treatment with paclobutrazol or tetcyclacis. Hence, there are clear indications that the inhibition of cell division caused by higher retardant concentrations both in heterotrophic cell suspensions and in subapical meristems is mediated by a change in sterol synthesis and, thus, modified membrane properties. With the new types of growth retardants more insights have been obtained into the relative importance of gibberellin and sterol production for cell elongation and cell division. There is no doubt that the retardant-caused morpho-regulation with its alterations in shoot growth and root-shoot ratio by itself can influence the physiological behaviour of a plant. Nevertheless, intensive studies in recent year have revealed further effects of N-heterocycli on the plant's hormone status, including influences on abscisic acid, cytokinins, and ethylene. N-heterocyclic retardant appears to cause a biphasic response of endogenous ABA levels. Shortly after treatment, ABA levels are transiently increased as observed in cell suspensions and detached leaves and young plants.

Experiments with detached leaves of Xanthium strumarium and cell suspensions of oilseed rape suggested an inhibition of the conversion of ABA to interactive phaseic acid (PA) by tetcyclacis. This presumably cytochrome dependent enzymatic step isinvolved in the major pathway of ABA catabolis, which is initiated by methyl hydroxylation of ABA. The induced accumulation of endogenous ABA in suspensions of oilseed rape cells was closely correlated with an enhanced potassium and water content of the cells. Moreover, in leaves it was accompanied by an increase in stomatal resistance and a reduced transpiration rate. Together with reported rises in proline and other amino acids, these water-conserving effects of N-heterocyclic retardants can explain improved adaptation of treated plants to drought, low temperature, and other environmental stresses.

The senescence-retarding potency of growth retardants can be further explained by their inhibition of ethylene biosynthesis, which is a common effect of the new types of compounds. As shown in sunflower cell suspensions and in leaf discs as well as in intact plants of several species, blocking is most likely at the level of the conversion of 1-amino-cyclopropane-l-carboxylic acid (ACC) to ethylene. The effect can not be overcome by adding GA3, and as evident from sunflower cell suspensions, is not functionally related to growth. Hence, this might be seen as a direct effect on the ethylene-forming enzyme or, perhaps more likely, as an indirect influence via a retardant-caused change in the membrane properties. In this context, some evidence exists for interrelations between a decreased ethylene production and enhanced levels of cytokinins and polyamines. A possible explanation of this effect may be the capacity of ethylene to acelerate cytokinin degradation. In the case of polyamines, the inhibition of ethylene production by retardants could lead to an enhance flux of S-adenosylmethionine (SAM), a common precursor of both pathways, particularly into spermidine and spermine. Besides cytokinins, spermine and spermidine have also been found to exibit antisenescence properties.

Conclusion

Apart from their agricultural importance, plant growth retardants also offer special benefit for physiological research. Their chemical interference with phytohormone metabolism and interrelations gives further opportunities for evaluating casual connections to physiological and development events in the plant system.