JARS v64n2 - The Word: Hormone

The Word: Hormone
Bruce Palmer
Cutten, California

Spring is here and our rhododendrons are beginning to bloom and produce new growth. What causes this activity every year so predictably? The answer is hormones; HORMONE is the word for this issue of the Journal. The word derives from the Greek "hormaein" , to excite. It's a confusing word because we tend to associate hormones with sex and because hormones in vertebrate animals are very different from those in plants. A hormone is defined as a secretion that controls a specific activity in a cell. Hormones are produced in very small amounts but have profound effects. In vertebrate animals, cells in specialized tissues usually secrete hormones and each hormone controls specific cell activities in other tissues. The thyroid gland on the front of the human neck, for example, secretes thyroid hormones to control metabolism in cells throughout the body. The level of thyroid hormones sent out by the thyroid gland is in turn controlled by the Thyroid Stimulating Hormone secreted by the pituitary gland in the brain. We analyze thyroid gland activity by testing for levels of TSH in the blood.

In plants the nature of hormones is quite different. Typically they are not secreted in special tissues and are not quite as restricted in their actions as animal hormones. Often they are referred to as Plant Growth Regulators rather than hormones. There are five major classes of plant hormones: auxins, cytokinins, gibberellins, abscissic acid (ABA) and ethylene. These plant hormones control plant growth and development by affecting the division, elongation and differentiation of cells, often in response to environmental stimuli such as day length and temperature changes.

The first plant hormone to be discovered was auxin (Greek, "auxien," to increase). Charles Darwin studied the bending of plant stems in response to light, which led to the discovery that a substance secreted by plant cells could be extracted into agar and used to cause bending and stem elongation in other plants. This effect of auxin acts by causing individual cells to elongate. We now know that there are a number of different auxins and that they have many effects, some of which we apply commercially. Indoleacetic acid and some other auxins are applied to stem ends to produce roots on cuttings. The same auxin will inhibit root growth if applied to roots. Auxins applied to some flowers will cause the fruits to be seedless. Applied to berry plants, auxins will prevent fruit drop. 2,4-D is an artificial auxin that causes weeds to grow so rapidly that they die.

Cytokinins (Greek, "kytos," a hollow vessel, and "kinesis," motion) were the next group of plant hormones to be discovered. Whereas auxins cause plant growth primarily by causing elongation of cells, cytokinins promote cell division, but few practical commercial applications have been developed for them. Experiments have been carried out to cause plants to spread rather than grow upward and to stop aging in leaves, but the results haven't been refined or widely applied.

Gibberellins (named from the fungus species that causes rice to elongate and become unharvestable) are plant hormones that have several effects. They break dormancy and cause the germination of seeds. This action is applied in the brewing industry to cause barley seeds to produce maltose at an even rate, making our favorite single-malt scotches more dependable. Seed growers are able to induce such crops as cabbages to seed in one year instead of two with applications of gibberellins, and they are used in the table grape industry to produce fuller fruits in seedless varieties.

Other plant growth regulators are used commercially, such as abscissic acid (ABA) and ethylene. ABA is chiefly concerned with leaf fall and fruit fall, while ethylene's effects are wider ranging and affect growth, senescence, fruit ripening and stress responses, as well as other activities. The best known commercial application of ethylene is its use in fruit containers to hasten ripening during transit to market. Any general botany book, such as Raven et al (1986) or sites on line, such as www.plant-hormones.info, can be consulted for those wanting more information. The three hormones mentioned above are the ones most often mentioned and used. They work together and separately and in response to day length and temperature to regulate the growth of plants.

Commercial growers have learned to control hormones, day length, temperature, water and nutrients to produce beautiful plants for the trade. Florists' azaleas such as the Kurume group and its more recent successors are produced for sale in closed, controlled "greenhouses" by using specific combinations of temperature, day length and synthetic growth regulators in specific orders and at specific times. (Hartman et. al. 1988).

As always with things in science, we don't yet have all the answers about plant hormones. The plant growth regulator of current interest is florigen. In the late 1800s, it was proposed that some controlling agent was responsible for flowering time. Early in the twentieth century the name florigen was proposed and the florigen hypothesis formulated. Until very recently no substance other than the known hormones was isolated that could be shown to control flowering, but in the mid 1990s it was demonstrated that two genes probably produce RNA molecules that either cause flowering directly or through their proteins. These were declared to be florigen and the discovery was widely publicized as a major breakthrough in botany and horticulture. The florigen hypothesis has yet to be accepted widely enough to be considered a theory, but substances that appear to be florigen are being patented (Zeevaart, 2006). Meanwhile the commercial plant trade is doing fine without an additional hormone.

In summary, then, if you are giving or receiving a blooming "hothouse" azalea on Mother's Day, it is instructive to realize that a variety of artificially controlled factors, including hormones, are responsible for its shape and beauty. The control of the factors is rather complex and has a long experimental history, but they are the same ones that work naturally in our gardens to produce the Rhododendron trusses and new growth we prize so highly.

References
Hartman, Hudson T., Anton M. Kofranek, Vincent E. Rubatzky & William J. Flocker. 1988. Plant Science, Growth, Development and Utilization of Cultivated Plants . Englewood Cliffs, New Jersey: Prentice-Hall. 674 pp.
Raven, Peter H., Ray F. Evert & Susan E. Eichorn. 1986. The Biology of Plants, Fourth Edition . New York: Worth Publishers. 775 pp.
University of Bristol, UK. 2010. www.planthormones.info/
Zeevart, Jan A.D. 2006. "Florigen Coming of Age after 70 Years". The Plant Cell . 18 (8), pp. 1783-1789.

Bruce Palmer is a member of the Eureka ARS Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii system for 25 years.