Plant Hormones and Adventitious Roots
Hormones are chemicals produced by one part of a plant and transported to another part of the plant where they cause a reaction to occur. Hormones are stored within the plant until they are needed. They are transported via phloem and other tissues. Hormones are released in small concentrations. They are released to all cells but only specific cells will react because they have special receptors for each hormone and they perform specific functions.
Auxin was the first plant hormone to be discovered in 1926. It was named Indoleacetic acid (IAA). There are a number of different auxins. It is not known how many there are naturally occurring. Natural auxins are synthesized in the shoot apical meristem, new leaves, and fruit. From these locations it is transported to the root tips.
There are synthetic compounds developed by scientists that mimic auxins and other hormones. They are called plant-growth substances. An example of a synthetic auxin is called 2, 4-Dichlorophenoxy acetic acid (2, 4-D). It causes disruption in normal plant growth and development. It is commonly used as an herbicide on broad-leaved plants in lawns.
Another group of plant hormones are called cytokinins. These hormones stimulate cell division in a plant. They exist naturally and artificially. Currently, only two are known to occur naturally, zeatin and isopentenyl adenine. The first cytokinin discovered was named kinetin. Cytokinins can have several responses in plants depending on the location and concentration. They can prevent leaves from aging and promote fruit and embryo development. In spring plants produce large amounts of cytokinins in the roots. The hormone is then transported up the plant to the dormant buds where they begin to open up and become active.
Gibberellins are another group of hormones. There are 62 known to exist. The most abundant is GA3 (Giberellic acid). Gibberellins are responsible for converting juvenile plants to adult and also involved in flowering.
Abscisic acid is a hormone that helps plants deal with stressful conditions like heat and cold. Ethylene is the only gaseous hormone. It is responsible for the ripening of fruit.
Adventitious roots help increase absorption and transport function of roots. Auxins are responsible for adventitious root development. Shortly after germination the radical in some plants dies. Lateral roots are formed producing an extensive root structure. From here adventitious roots are produced. Adventitious roots form from leaves and stem cuttings as well as a practice of asexual reproduction. The roots develop at the nodes, the part of the stem where the leaves are attached. Roots also can develop from leaf cuttings. The roots will form at the basal position of the leaf and stem. IAA promotes root development. Auxin concentration gets very high before the roots emerge at the location where they are going to grow.
Biology of Plants, 6th ed. Raven, Peter H., Everert, Ray F., Eichhorn, Susan E. Worth Publishing 1999.
Botany: An Introduction to Plant Biology. Mauseth, James D. Jones and Bartlett Publishers. Sudbury, Massachusetts 1998.
Plant Physiology 4th ed. Salisbury, Frank B., Ross, Cleon W. Wadsworth, Inc Belmont, California 1992.