Plant Hormones
Plant hormones are widely used in commericial agriculture. Determining how plant hormones are synthesized provides information necessary to understanding their complete function.
What are plant hormones? Plant hormones are a group of naturally occurring, organic compounds which influence physiological processes at low concentrations (Davies 1995).
Importance of plant hormones: Plant hormones are key players in regulating plant growth and development. They control the rate of growth and differentiation of the individual organ systems as they develop to produce the form that we recognize as a mature plant (Davies 1995).
Discovery of plant hormones: The concept of the existence of hormones occurring in plants was first published by Charles Darwin, in The Power of Movement of Plants (1881). Darwin, studying phototropism in plants, noticed that tips of plants when exposed to light would curve toward the light. This observation led Darwin to believe that a chemical messenger transports a signal from the coleoptile (tip) to the rest of the plant (Figure 1) (Moore et al 1998).
Figure 1: Darwin's study of phototropism.
In 1913, Peter Boysen-Jensen continued Darwin's experimental work, finding that if he removed the coleoptile, plant growth stopped. Boysen-Jenson replaced the coleoptile with a piece of agar and then placed the coleoptile on top of the agar block. The plant again was able to curve toward the light. This experiment proved that a signal must be transported from the coleoptile to the rest of the plant, as Darwin had originally surmised (Figure 2) (Moore et al 1998).
Figure 2: Boysen-Jensen showed a chemical messenger was being transported to the rest of the plant.
In 1918, Arpad Paal set forth to discover the chemical signal. He removed the tips of dark grown coleoptiles and placed them on one side of the cut surface. He observed that the coleoptiles then curved away from the side on which the tips were placed, even though the plants were still in the dark. This result suggested that a substance produced in the coleoptile is transported downward and this substance stimulates growth of the plant (Figure 3) (Moore et al 1998).
Figure 3: Paal discovered that the chemical signal induced cell growth.
Finally in 1926, Frits Went proved that a chemical signal is transported from the coleoptile to the rest of the plant. He removed coleoptiles from Avena sativa and placed them on agar blocks. The agar blocks were then placed on one side of the cut surface and stored in the dark. Sure enough the coleoptiles started growing away from the side with the agar block. This proved definitively that a chemical substance is produced in the coleoptile and then transported to the rest of the plant to elicit a specific cellular growth response. Thus, the research of the four scientists established the existence of the first plant hormone. Went named this plant hormone auxin (from the Greek word auxein meaning "to grow") (Figure 4) (Moore et al 1998).
Figure 4: Went discoverd the signal that was transported from the tip to the rest of the plant and named the signal auxin
Isolation of the most biologically active auxin, IAA. In 1933, Kogl et al. reported the isolation of auxin A and B. The structures of these molecules were quite complex. Kogl reported the molecular weight of auxin A to be 328 not so far away from Went's prediction, presented in his thesis, of 376. Two years later a different substance, indole 3 acetic acid (IAA), was isolated by Kogl that had increased auxin bioactivity. When IAA was used in the Avena curvature test, increased curvature was noted. IAA had previously been isolated, but not known to function as an auxin (Figure 5) (Jacobs 1979).
Figure 5: A 3D structure of Indole 3 Acetic Acid (IAA)
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