13.2.1—
Introduction

Amongst plant growth regulators, the auxins have been studied the longest and have involved the greatest amount of research. This class of hormones is known to promote cell enlargement or cell elongation, a process which requires extension of the cell wall. Figure 13.1 shows the chemical structures of the major natural and synthetic auxins.

Figure 13.1
Structure of various auxins.

Commencing in 1940, the majority of studies involving auxins were begun in an attempt to determine the molecular interaction with the plant cell. Most of this research was directed toward the cell wall because it was believed that auxin

action required a change in the cell wall to allow for cellular expansion. Thus, the effects of the hormone were thought either to change the cell wall deposition or to hydrolyse certain cross-linkages in cell walls making them more elastic.

This type of research was carried out for several years without achieving a clear understanding of how auxin might control such processes within the cell wall. In the 1950's a new area of research was begun. Under the direction of F. K. Skoog in Madison, Wisconsin, a report (Silberger & Skoog, 1953) was published which showed that the auxin, indole-3-acetic acid, remarkedly affected the RNA and DNA contents of plants. Auxin increased the content of nucleic acids in tobacco tissue cultured on a sucrose-agar medium. This increase occurred prior to the auxin-induced growth of the tissue at concentrations of IAA which were optimal for cell enlargement. For many years following this discovery much research was devoted to the general mechanism by which auxin increased the synthesis of nucleic acids.

A great share of this research was begun in J. Hanson's laboratory in Urbana, Illinois by West and Key (West et al., 1960) who showed that the synthetic auxin, 2,4-D, (see (Fig. 13.1), produced a wide range of morphological and physiological changes in the hypocotyl of soybean and the mesocotyl of corn.

Within 15–24 hours after 2,4-D treatment, cellular enlargement was noted concomitant with an increase in the size of the nucleus. Accompanying these changes were dramatic increases in RNA content, most of which were due to increased ribosomal-RNA. Chrispeels and Hanson (1962) suggested that auxin acts on the nucleus causing it to revert to a meristematic type of metabolism. The role of the nucleus in such a sequence of events is of obvious importance since the nucleus has been shown to accumulate RNA in response to auxin. Beginning in the 1960's, Key began experiments on auxin regulation of nucleic acid synthesis. He and Ingle (1964) demonstrated that auxin controls the synthesis of nucleic acids other than that of ribosomal-RNA. Their data revealed that auxin causes production of nucleic acid which appears to be of the messenger-RNA type.

Subsequent experiments by O'Brien et al., (1968) showed that treatment of soybean with auxin caused a large increase in chromatin-directed RNA synthesis. It was of interest to note that auxin-induced RNA synthesis produced a type of RNA which was different from the control RNA as judged by molecular size and nearest neighbour analysis. Following this area of research other studies (Hardin & Cherry, 1972; Hardin et al., 1970; Hardin et al., 1972) showed that 2,4-D increased the activity of RNA polymerase. Current research suggests that particular cytoplasmic or membrane bound factors may enhance the activity of RNA polymerase. It is hypothesized (Hardin et al., 1972) that the mechanism of action of auxin is to bind to a receptor molecule located on the plasma membrane. The receptor is then released and moves into the nucleus where it modulates the activity of the RNA polymerase. The increase in RNA polymerase activity leads to increased synthesis of messenger-RNAs, which in turn regulate, or control, the synthesis of specific proteins within the target cells.

A primary challenge is to isolate the factor which binds to, or reacts with, auxin, and determine whether this is the primary action of the hormone. Secondarily, research is needed to determine what series of events this interaction puts into metabolic play. It is likely, once the auxin has bound to the receptor molecule within the target cells, that the changes in nucleic acid synthesis cause an increased activity of the various enzymes associated with the cell wall. Many other activities which have been measured over the many years are probably secondary effects resulting from the primary action of the auxin reacting with its receptor molecule.