Plant Growth Regulators

Machi F. Dilworth, associate program manager, Competitive Research Grants Office, Office of Grants and Program Systems.

Plant growth regulators are hormone-like chemicals that occur naturally in plants, and play a central role in their growth and development. Five major classes of plant growth regulators have been identified as plant hormones, but scientists believe that there are more waiting to be discovered.

The five growth hormones include auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Collectively, they regulate many facets of plant growth and development including seed germination, root growth, stem elongation, leaf expansion, flowering, seed development, fruit ripening, and dropping of leaves and fruits.

Each of these processes is directly relevant to agriculture; what is harvested for food, feed, or fibers is the result of one of these developmental events. For example, cereal grains are the seed, potatoes are the highly specialized stem, spinach is the leaf, and cotton comes from the highly developed cell walls of the ovule, a part of the maternal reproductive structure. The more we understand these processes, the better chance we will have of developing a strategy for crop improvement. With the advent of biotechnological techniques, it is now distinctly possible to manipulate the natural regulatory mechanisms to produce improved crop varieties with desirable genetic traits.

Physiological and Biochemical Regulation

Earlier research has identified a general role for each of the five classes of plant hormones as follows; auxin, a cell elongation hormone; gibberellin, a regulator of tissue and plant parts; cytokinin, a cell differentiation hormone; abscisic acid, a senescence regulator; and ethylene, a fruit-ripening hormone. Further research has uncovered more detailed information about their physiology and biochemistry. How these hormones regulate plant growth and development is extremely complex, yet the genetic makeup of each plant determines its precise developmental program. Moreover, the environmental factors interact with plant hormones in the regulatory process.

Interaction of Growth Hormones

The five classes of growth hormones can act independently in regulating many developmental events, but evidence is accumulating that they interact with each other as well. One of the clearest examples of this interaction is the auxin control of the biosynthesis of ethylene. Indole acetic acid, an auxin, has been demonstrated to stimulate the enzyme that catalyzes the last step of the ethylene biosynthesis. The interaction of auxin and cytokinin in determining shoot and root formation from callus tissue (a mass of undifferentiated plant cells) has been a well-known phenomenon since the 1960's. Abscisic acid has inhibited the induction of a starch-hydrolizing enzyme by gibberellin in the seed of barley. The gibberellin-to-auxin ratio has been shown to determine the number and length of cotton fibers.

Interaction Between Growth Regulators and Environmental Factors

Examples of the interaction between the plant growth regulators and environmental factors are equally abundant, suggesting even more complicated mechanisms of hormonal regulation. Environmental factors such as light, moisture, and temperature are important signals to which the plants react in executing their developmental programs. Light, for example, plays a crucial role in plant morphogenesis including seed germination, shoot growth, and flowering.

Some of the effects of environmental signals seem to work through the same processes as do plant hormones while others do not. The effect of light on seed germination can be replaced by gibberellin, for example, but the effect of light on the initiation of flowering cannot be replaced by a known plant growth regulator.

These observations are based on biochemical and physiological studies that have been carried out by many investigators. The diversity of the phenomena involving plant growth regulators has puzzled many researchers as to the fundamental mechanism of hormone action in regulating plant growth and development. It has led to a hypothesis that the plant hormones do not act directly in the cell, but through some secondary messengers such as a calcium ion or a small carbohydrate molecule. So far, the hypothesis has not been proved or disproved. It also should be recognized that not all plant growth regulators need to work through the same mechanism.

Plant Growth Regulators and Gene Expression

Most of the recent advances in plant growth hormone research are being made in the studies on the effect of Plant hormones on gene expression. The new molecular biology and various immunological techniques have allowed new approaches to the old problem of how the plant growth regulators regulate developmental processes.

It appears that all five plant growth regulators influence the expression of genes at the transcriptional (DNA to messenger RNA) level, the translational (messenger RNA to protein) level, and the posttranslational modification of the proteins.

Auxin. The first suggestion was made in the 1950's that auxin-regulated cell elongation may be mediated by auxin-controlled gene expression. It is only in the past few years, however, that conclusive evidence shows that auxin induces specific gene products, both messenger RNA's and proteins, in elongating shoots. The manner by which an auxin interacts with the gene and the nature of the gene products are being investigated by several laboratories.