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Tuatara: Volume 6, Issue 3, December 1957

Plant Hormones

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Plant Hormones

It is now nearly one hundred years since Julius Sachs advanced the idea of specific organ-forming substances in plants, and Charles Darwin experimented with the ‘Power of Movement in Plants’. Their conclusions gave impetus to much of the subsequent experimentation on hormones. The field has grown with great rapidity and today hormone physiology and biochemistry is one of the most vigorous branches of botany.

This article is a summary of knowledge of the more important hormones and growth factors of plants. The need for brevity has led to omission of many interesting observations, details of which are in the references. Definitions. Order is slowly emerging from the confusing duplication and overlapping of terminology in the field of plant regulators, aided by the increasing knowledge of their chemical nature. The following definitions and/or descriptions of terms are based in part on the report of a special committee on nomenclature of the American Society of Plant Physiologists (Overbeek, et al).

Plant Regulator — a chemical which in small amounts regulates (promotes, inhibits or otherwise modifies) a physiological process. A broad term which includes the hormones (endogenous regulators) as well as exogenous regulators.

Plant Hormone — an endogenous regulator. To be a hormone, a chemical must be produced within the plant, transported from a site of production to a site of action, and be active in small amounts.

Vitamin — an organic chemical required in small amounts from external sources for the normal growth and maintenance of an organism. Green plants (autotrophic organisms) do not require vitamins. Non-green plants and animals (heterotrophic organisms) usually require one or more vitamins.

Auxin — The generic name for the indole hormones. These promote growth by stimulating cell enlargement. A number of other effects of auxin are described in the text. (Indoleacetic acid was called heteroauxin in the older literature. The hypothetical auxin a and auxin b have never been isolated and are now generally considered invalid.)

Antiauxin — (synonyms: auxin inhibitor, auxin competitor, auxin antagonist). A compound which competitively inhibits (in the biochemical sense) the action of auxin.

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Inhibitor — a regulator which inhibits or retards a physiological process. Growth Substance — a term used variously, usually as a synonym for auxin. Growth Factor — usually an exogenous regulator, often equivalent to vitamin or hormone. Frequently used with reference to the culture of micro-organisms, tissues, or organs.

Growth Hormones: Auxin. Darwin's experiments led him to conclude that something is produced in tips of plants which moves downward and stimulates the growth of more basal tissues. In the early part of this century a number of investigators studied the nature of the influence of tips on the growth of basal regions. This work culminated in the simple but conclusive experiments of Went which showed that the growth of the oat coleoptile requires a chemical which is produced in the tip of the coleoptile. The chemical was called auxin. It is widely distributed in plants and appears to be universally required for growth, hence Went's Law of plant growth: ‘Without auxin, no growth.

Indoleacetic acid is now considered to be the principal auxin, but there are several other auxins. All are closely related chemically, and more than one is often present in a tissue. The best known of these are indoleacetaldehyde, indoleacetonitrile, and ethyl indoleacetate.

Auxin has several physiological properties which account for most of its functions. It promotes cell enlargement by increasing the plasticity of the cell wall, thus facilitating the uptake of water. It is transported basipetally (from tip toward base) in organs. This transport is modified by gravity and light: auxin tends to move in the direction of the force of gravity, and away from a source of light. Further, auxin in the plant is inactivated by light. Also, the different organs of plants differ in their sensitivity to auxin; roots are the most sensitive, stems are the least sensitive. Through the interaction of these properties, growth and growth movements of plants are regulated. (The growth movements include phototropism, geotropism (both positive and negative), plagiotropism, thigmotropism, epinasty, and photonasty.)

Auxin has other functions. Its accumulation at the base of a cutting is necessary for the initiation of roots. Auxin from the apical bud inhibits the development of lateral buds. The auxin gradient across a leaf or fruit abscission zone regulates abscission (a relatively high gradient retards abscission, and a low gradient accelerates abscission). These three functions at first glance would appear unrelated, but they may all be expressions of another postulated property of auxin, that is, to mobilise food to regions where auxin is in high concentration. The existence of this property has not yet been established, but there is considerable supporting evidence. If it does exist, it would account for much of what is known concerning the auxin regulation of root inhibition, bud inhibition and abscission. Another function of auxin is the initiation of cell divisions in the cambium. In deciduous trees the cambium is inactive during the winter, in the spring auxin moves down from the expanding buds and initiates cell divisions page 110 in the vascular cambium. (Factors regulating the activity of cork cambium (phellogen) are still unknown.)

Continued research on auxin has made it apparent that auxin physiology is much more complicated than it first seemed. Auxin appears to be present in all living parts of the plant, mature as well as immature. The amounts present are effected by at least three general processes: auxin production, auxin transport, and auxin inactivation. Many of the early investigations did not recognise the existence of these three processes and their results must be re-evaluated. For example, many studies of auxin transport did not take into account the probability of considerable auxin inactivation during the course of transport. Auxin is produced principally in young tissues, but can also be produced by mature tissues. The amino acid tryptophan, a common constituent of proteins, is the precursor of auxin, but the precise chemical steps of its conversion to auxin are not yet settled. The transport of auxin can be through the parenchyma, as it is in the oat coleoptile, but in more mature tissues transport is largely in the phloem. In the coleoptile transport is correlated with the streaming of protoplasm. Auxin inactivation is accomplished by an oxidative enzyme which can function either in the dark or under the influence of light. Mature tissues have relatively high auxin-inactivating capacities. In addition to these general processes other factors, still obscure, also influence the auxin in tissues. The interaction of these processes and factors determines the level of auxin which is available to influence growth and morphogenesis.

Growth Hormones: Gibberellins. The gibberellins produce effects on growth, particularly cell elongation, which are very similar to the effects of auxin, but they function in situations where auxin does not promote elongation. Although physiological and biochemical knowledge of them is still fragmentary, they are growth factors which are probably hormones and hence should be included here. The chemicals derive their name from the fungus Gibberella, from which they can be obtained. Immature seeds are also very rich sources.

One of the most interesting series of experiments with the gibberellins was conducted with a dwarf corn (maize). This particular mutant dwarf had been the subject of an intensive auxin study, and its auxin physiology was found to be completely normal. That is, auxin production, transport and inactivation were identical with those of normal corn, and applications of additional auxin did not affect its growth; the plants never grew more than a few inches tall. However, weekly sprays of gibberellins stimulated the mutant to the normal rate of growth and practically normal appearance. The results of a similar experiment conducted several years earlier, which were at the time puzzling, can now be interpreted as due to gibberellins: an extract from immature bean seeds was applied to a bush variety of beans (Phaseolus); the stems then elongated in the manner characteristic of the tall varieties of beans. In other experiments, gibberellins sprayed on pasture grasses have induced abnormally rapid growth.

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Another effect of gibberellins is in relation to both growth and flowering. Hyocyamus is one of the typical ‘long-day plants’. It grows as a rosette with its leaves clustered about the very short stem until it has been exposed to a period of cold followed by a period of long days. Then the stem rapidly elongates and produces flowers. It has been found that gibberellins can replace the cold treatment; sprays followed by long days stimulate stem elongation with flowering.

Wound Hormone. Following an injury to a plant, the parenchyma cells underlying the injured area are stimulated to divide and form a protective callus. Under the stimulus, cells divide which would otherwise remain intact to the death of the plant. Early experiments showed that if the injured area is washed immediately, cell division is prevented; this suggested that a hormone might be involved. Such a hormone was isolated by Bonner and English. Starting with 100 pounds of string beans they isolated a small amount of a chemical which they called traumatic acid (chemically, decene dicarboxylic acid) which is the wound hormone of beans. However, this compound does not stimulate cell division in other species. So there remain other chemicals yet to be identified as wound hormones.

Root Growth Hormones. Knowledge of root growth hormones has come largely from experiments with the culture of isolated roots. The repeated attempts to culture isolated tissues of plants were successful in 1933 with tomato roots and a culture medium consisting of sucrose, salts, and yeast extract. Yeast extract is a very complex mixture of chemicals and attention was immediately given to determination of the active components. These were soon found to be thiamin and pyridoxin which in small amounts (a few parts per million) could completely replace the yeast extract. Thus tomato roots, which in the field would live only a few months, have been kept growing in culture in a synthetic medium since soon after 1933. Thiamin and pyridoxin were first called growth factors, since their role in the intact plant was not known. However, Bonner showed that they are produced in leaves and transported downward to roots, thus establishing them as hormones.

Other experiments showed that niacin is a root growth factor, and is presumably also a root growth hormone. In various combinations thiamin, pyridoxin or niacin will support the indefinite growth of isolated roots of many species. For a few species other factors are required such as the amino acids glycine, lysine and arginine.

Although the roots of many plants will grow rapidly (at rates at least equal to the rates of roots on intact plants) and indefinitely in synthetic culture media, important problems still remain unsolved. One is the culture of isolated roots of monocotyledonous plants. In spite of numerous attempts, these have never been established in culture. Another is the development of the cambium, which has not been induced in roots of established page 112 cultures. Further, branching of cultured roots is often abnormal. Thus the knowledge of root growth physiology is far from complete and much work lies ahead.

Experiments with root cultures brought to light an important interrelationship of vitamins and hormones. The chemicals thiamin, pyridoxin and niacin are vitamins, necessary in the diet of animals and other heterotrophs for normal growth and maintenance. In the green plant these same chemicals function in the physiological role of hormones. And within the cells of organisms they each function as a part of a vital enzyme. Thus the same chemical may function in any of three physiological roles: vitamin, hormone, enzyme.

Leaf Growth Hormone: Phyllocaline. In a search for hormones other than auxin Went performed an extensive series of grafting experiments. He worked with varieties of garden peas which differed markedly in their growth habits. The results showed, for example, that leaves of different varities differed in their ability to stimulate root growth. Similar differences among roots and buds were observed. Went postulated that these differences in growth were the result of differences in production of special hormones by the varieties. One of these postulated hormones was called phyllocaline. It is produced in cotyledons and mature leaves, and stimulates the growth of young leaves. This hormone was isolated and identified as adenine. Another property of adenine was later discovered; tissue cultures of plant callus ordinarily grow indefinitely as an undifferentiated, or at best, slightly differentiated mass of cells. In the culture medium adenine stimulates the differentiation of leafy buds.

Adenine too has multiple physiological roles: It is a vitamin (B8)

for some organisms and within cells functions as a part of several enzymes and of the energy-storing phosphate compounds.

Flowering Hormone: Florigen. Flowering is influenced by many factors including mineral and carbohydrate nutrition, temperature, photoperiod, and a postulated hormone, florigen. This hormone is produced in leaves (under particular conditions) and is transported to buds where it brings about the conversion of a vegetative stem apex to a reproductive stem apex (flower bud). Numerous experiments indicate its existence, but attempts to isolate florigen have not yet been successful. For further discussion of flowering see the recent article by Sussex.

Reproductive Hormones. In the lower plants a number of hormones influencing reproductive processes have been described, as well as nutritional factors which can be called reproductive vitamins.

One of the best known examples of reproductive hormones is in a heterothallic species of a water mould, Achlya, where Raper in extensive experiments found four hormones: page 113
AFemale vegetative plantProduction of antheridal hyphae
BAntheridial hyphaeProduction of oogonial initials
COogonial initials(a) Attraction of antheridial hyphae (b) Delimitation of antheridia
DAntheridia(a) Delimitation of oogonia (b) Differentiation of oospheres
The function of hormones in one or another phase of reproduction has been demonstrated for:
Green Algae:Acetabularia Chlamydomonas CladophoraDasycladus Dunaliella HydrodyctionMonostroma Protosiphon
Brown Algae:EctocarpusFucus
Golden-brown Algae:Botrydium
Fungi:Achlya Bombardia DictyuchusMucor Neurospora PhycomycesPilobus
The following reproductive vitamins have been identified; they must be provided with culture medium for the indicated developments:
HistamineZygospores of Phycomyces
Thiamin BiotinPerithecia of Melanospora
InositolOogania of Achlya
HypoxanthineZygospores of Phycomyces
Glutaric Acid RiboflavinConjugation of yeast

Growth Factors. Experiments have demonstrated growth factor requirements for many plant parts. Many, possibly all, of these growth factors are plant hormones, but present knowledge is too fragmentary in most cases to permit positive statements.

Pollen germination and tube growth factors. Pollen of some species will germinate and grow well in artificial media; pollen of others will grow poorly or not at all. Stigmatic exudates are usually very stimulatory and presumably provide hormones required by the pollen. Chemicals which have been found to promote germination or tube growth of various species include: boric acid, manganous sulphate, ascorbic acid, aminobenzoic page 114 acid, indoleacetic acid, inositol, lactoflavin, guanine, pyridoxin, thiamin.

Growth factors of tissue and organ cultures. Since the successful establishment of root cultures, other organs and several types of tissues have been successfully cultured including embryos, shoots, and callus. Often successful culture has required the use of complex mixtures such as malt extract, young seed extracts, or coconut milk. The latter is a potent source of important growth factors; its use has enabled the culture of very small embryos, but the active chemicals in coconut milk have not been identified. Growth factors which have been identified include: ascorbic acid, adenine, biotin, indoleacetic acid, niacin, pantothenic acid, thiamin. It is of interest to note that each of these is already known to have functions as a vitamin and/or hormone.

Growth Inhibiting Hormones. The discussion to this point has dealt with hormones and other factors which in the main promote growth and development. (A few of these, such as auxin, will under some conditions inhibit or retard growth.) In addition, there is now an increasing list of chemicals whose principal function appears to be the inhibition of growth. Since these chemicals are endogenous, often act at very low concentrations, and move from a site of production to a site of action, they should be considered hormones. Only seed germination inhibitors will be mentioned here; knowledge of others is very fragmentary.

Germination inhibitors act variously: (a) to prevent premature seed germination; (b) to extend the period of germination by permitting only a fraction of the seeds to germinate at any one time; and (c) to suppress germination of competing species while permitting germination of a favoured species. Evenari has described over 120 inhibitors; these are produced in fruit pulp, fruit coats, endosperm, seed coats, embryos, leaves, bulbs, and roots. Identified inhibitors include: hydrocyanic acid, ammonia, ethylene, mustard oils, aldehydes, alkaloids, essential oils, lactones, organic acids. It is of interest that an inhibitor can sometimes stimulate germination. Inhibition or stimulation may result from different concentrations, but sometimes one follows the other from the same concentration.

In a few decades the subject of plant hormones has expanded to a broad and amazingly complex field of plant physiology, at least equal in complexity to the field of animal hormones. This research received much of its initial impetus from Sachs' postulate that plant morphogenesus is regulated by specific organ-forming chemicals. Indeed, there is now much evidence on the effects of specific chemicals (or groups of chemicals). However, the impression should not remain that morphogenesis is regulated solely by such chemicals (that is, by hormones or vitamins). Temperature, light, water, mineral nutrients, foods, and other factors are also important in the development of plants and at times one or more of these factors may have a decisive influence on growth, acting either directly or through intermediate effects on plant hormones.

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For further reading: the Annual Reviews of Plant Physiology contain articles listing and discussing much of the literature, and forthcoming volumes of the Encyclopedia of Plant Physiology will contain numerous valuable articles. The following are selected references:-


Audus, L. J., 1953.—‘Plant Growth Substances.’ Hill, London.

Darwin, C. R., 1880.—‘Power of Movement in Plants.’ London.

Evenari, M., 1949.—Germination inhibitors. Bot. Rev. 15(3): 153-194.

Leopold, A. C., 1955.—‘Auxins and Plant Growth.’ Univ. California Press, Berkeley, U.S.A.

Overbeek, J. van, et al, 1954.—Nomenclature of chemical plant regulators. Plant Physiol. 29: 307-308.

Raper, J. R., 1952.—Chemical regulation of sexual processes in the thallophytes. Bot. Rev. 18(7): 447-545.

Sussex, I. M., 1956.—The causes of flowering. Tuatara 6(1): 1-12.

Tukey, H. B. (ed.), 1954.—‘Plant Regulators in Agriculture.’ Wiley, New York.

Went, F. W., and Thimann, K. V., 1937.—‘Phytohormones.’ Macmillan, New York.

* Professor of Botany (on sabbatical leave), University of California, Los Angeles.