13.1—
Introduction

The genesis of hormone research in plants occurred with the discoveries of Charles Darwin and his son Francis, who published, almost a century ago, their book entitled The Power of Movement in Plants'. Their experiments dealing with phototropism revealed that an 'influence' produced in the tip of a canary grass coleoptile was translocated to the basal portion where it caused the organ to grow towards the light. Almost fifty years later Went isolated a substance with just these properties which was produced in the tip of coleoptiles and transported to the basal portion of the organ. This substance was named auxin. Ten years after Went's (1934) discovery, the structure of this material was identified as indoleacetic acid (IAA).

The nature of auxin effects on plant growth and development was studied in many laboratories in the late 1930's and 1940's. However, it was not until World War II that the selective action of indoleacetic acid served as a basis for developing new chemicals for the control of growth in specific plants. At that time phenoxyacetic acids were discovered and their derivatives were carefully studied under the cover of military secrecy, both in the United States and England. The results of those studies were first published in scientific journals in 1945 and 1947. Today the most widespread uses of growth regulators are as herbicides.

Plant hormones may be divided into five general groups. Auxins include the native indoleacetic acid, as well as the synthetic phenoxyacetic acids, notably 2,4-D (2,4-dichlorophenoxyacetic acid). The second group is the gibberellins which are steroid-like compounds comprising over forty different structures. The third group, the cytokinins, are all N ⁶ substituted adenosine compounds. The fourth class is represented by a single substance, abscisic acid, which is also derived from isoprenoid units as are the gibberellins. Ethylene, the simplest of all the plant hormones, is a gas and therefore is easily spread from organs of production to organs of sensitivity.

It is interesting that of these five classes of hormones, two are generated from mevalonic acid. Ethylene is produced by the metabolism of methionine and indoleacetic acid is produced by the removal of carbon and nitrogen from tryptophane. Cytokinins, in terms of synthesis, are probably the most complex hormones. They are possibly produced from the breakdown of transfer RNA (tRNA). In the intact tRNA, the adenosine moiety of the RNA is modified by the addition of an isoprenoid group; subsequently, the tRNA is probably degraded, yielding isopentenyl adenosine, (IPA) the endogenous cytokinin.

One might reasonably imagine that an understanding of the physiological role and biochemical mechanisms of action of the plant hormones would have been elucidated quite some time ago. This of course, is in relation to what is known about the many and complex hormones in animals. Unfortunately, however, the plant hormones are not as well understood. Probably the main obstacle to the elucidation of the action of plant hormones has been the fact that they have overlapping and complementary effects on the actions of each other.

During the years between 1960 and 1970 substantial progress appeared to be made on the biochemical mechanisms of action of plant hormones. Much of this research dealt with the effect of the hormones on nucleic acid metabolism, protein synthesis and the synthesis of specific enzymes, since it was felt that the mechanism of hormone action would involve the control of the production of messenger-RNA (mRNA) and the de novo synthesis of enzymes. In the late 1960's the general nature of the research was radically changed because many investigators had found that the plant hormones mediated an effect on cell growth within a few minutes. This information implied a very rapid primary response mechanism of hormone action; consequently, these effects could not be mediated by changing the rate or type of synthesis of nucleic acids or of specific proteins. Many years of research have since been devoted to studies dealing with short-time growth responses to plant hormones. It now seems most likely that plant hormones regulate plant growth through both short-term and long-term growth controls.

It is fairly well understood that animal hormones first interact with the animal cell by binding to, or reacting with, a receptor site of some type within the target cell. Plant physiologists and biochemists are now beginning to consider that the action of many of the plant hormones similarly involves their binding to receptor agents which in turn amplify the action of the hormone and thereby bring about specific changes in nucleic acid synthesis, protein synthesis, enzyme activity and possibly other physiological responses, such as changes in permeability of membranes.

This chapter will summarize the historical evidence which has led to the current level of understanding of the action of plant hormones. Emphasis is placed on the reactive sites within target cells and how the interaction of the hormones with their specific receptors may regulate the growth and development of plant cells.

When a hormone is applied to a responsive plant system it brings about a specific change which results eventually in a measurable biochemical or physiological effect. Two distinct aspects of the measured effect are involved: the specific change in metabolism and the series of steps which lead to the physiological effect. Usually, the molecular interaction of the hormone at its site of action is referred to as the mechanism of action. The subsequent sequence of reactions leading to the physiological effect is referred to as the mode of action. Therefore, by definition each hormone has its own distinctive mechanism of

action even though the manifestation of the hormone mechanism may depend upon prior action by other factors. Thus, it is possible for the hormonal mechanisms in one plant system to lead to a series of physiological responses which may be completely different from those in a second system. This difference in mode of action can be brought about because the second system has more or less of another hormone, has other biochemically rate-limiting components or possesses structural and cytological differences.