All hormones work together interdependently as a signaling network to regulate plant defense responses. There are five classic types of hormones—auxins, cytokinins, gibberellins, ethylene and abscisic acid—and new classes are still being discovered and synthesized. In the first installment in a three-part series on plant hormones, we look at auxins.
Auxins are a class of plant hormones involved in the regulation of almost every plant process. Its name is from the Greek word auxein, which means to increase. As an extremely versatile and adaptable chemical species, more and more roles for auxins are being uncovered at an alarming rate. They are found throughout plants, from leaves, roots and shoots, to seeds, fruits and flowers.
Among their many roles, auxins control branching and plant elongation, and contribute to cell division. In recent years, the contribution of auxins to floral formation and development has been uncovered. Auxins mediate the plant properties of gravitropism and phototropism. Gravitropism is the response of a plant to gravity (roots travel downward in the direction of gravitational pull and shoots travel upward in the opposite direction). This effect is displayed when you try to grow a plant upside down. Phototropism is the response of a plant to light. Roots shy away from a light source while shoots bend towards the light. These survival mechanisms enable a plant to place the most chlorophyll pigment molecules in a position to absorb the most light energy possible.
In nutrient-depleted soils, auxins are transported to root tips to stimulate taproot growth so the plant can search for nutrients deeper in the soil. Auxin concentration in the shoots is lowered because the plant's focus is not on growing larger when the required nutrients are not available. In nutrient-rich media, auxins initiate lateral root growth to absorb more available nutrients rather than expand the taproots outside the existing root zone. In the shoots, auxin concentration goes up because the plant has everything it needs to produce healthy meristems.
Four auxins exist in nature and are synthesized by plants. Since their discovery, more auxins have been derived from existing ones and others have been synthesized in the laboratory. Unlike all other plant hormones, auxins have their own unique chemical transport system. They are synthesized in young leaves and shoots from tryptophan, an amino acid. From there, they are transported via the vascular tissues to where they are needed.
The most important, prevalent and understood of the natural auxins is indole-3-acetic acid (IAA). IAA is produced by algae, plants, bacteria and fungi. It thickens the cambium layer of plants by actually enlarging xylem cells. This enables plants to transport more water throughout the vascular system at a faster rate, and also contributes to the structural integrity of the plant. It enhances fruit set development and contributes to leaf senescence—the process responsible for creating the color of autumn foliage. Certain species of beneficial bacteria produce IAA in the root zone, which stimulates roots and root hair formation.
Another agriculturally significant auxin is indole-3-butyric acid (IBA). If you have used a rooting gel to make clones, you have used IBA to initiate adventitious root production. This auxin is used as a rooting hormone for seeds and cuttings, and is present in most rooting gel products on the market. While it is naturally produced by plants, the IBA used in rooting hormone products is usually synthetic. While IBA has other important roles in plant growth and development besides root growth stimulation, the exact mechanisms are still unclear.
All auxins are toxic to plants at high concentrations. To deal with this, plants bond the auxins with other molecules such as amino acids or sugars, thereby deactivating it. These new derivatives are not toxic to the plants, and at this point can be stored away for future use. Chemical companies have used the toxicity of auxins as herbicides.
A good number of herbicides on the market are actually just auxins or auxin derivatives at high concentrations. One auxin-based herbicide, called 2,4-D, is absorbed by dicots while not readily absorbed by monocots. When someone sprays it on their lawn, the herbicide selectively kills the weeds (dicots) while not affecting the grasses (monocots).
Some of these herbicides have had detrimental effects in the environment and to humans. The most extreme case is that of Agent Orange, an auxin-based herbicide used in the Vietnam War to clear brush for improved visibility of opposition troops. Side effects from large-scale use of the chemical include birth defects, lowered birth rates and an increased rate of numerous cancers and diseases.
When used properly, auxin supplementation is beneficial to plants, especially in indoor gardens where the seasons are shorter and plants are unable to synthesize the necessary amounts in a feasible time frame.
Naturally extracted IAA and IBA are found in small amounts in high-quality, cold-pressed sea kelp, and in some higher-end products on the market. To take advantage of the multiple uses and benefits of this dynamic plant hormone, the auxins can be supplied throughout a plant's entire life cycle.