To Grow or not To Grow
As gardeners, we take plant growth for granted. After all, that's what we expect when we plant a seed, bulb, tuber, or small plant in our gardens. This normal process relies on various biochemicals that act to promote or inhibit biological processes in the plant. These compounds are known as plant growth regulators, or more commonly, plant hormones. Here I will introduce you to the three most prominent groups, and to their functions in the plant.
This first group is the first of the plant hormones to have been discovered. The thumbnail picture above shows a diagram of the chemical structure of indole-acetic acid, or IAA, the first naturally-occurring auxin to be discovered. In fact, the word "auxin" is derived from the Greek, auxein, meaning "to grow". Auxins figure prominently in the responses of plants to light and in root growth by stimulating cell elongation. When your windowsill plant leans towards the light, it is the effect of auxin that is causing that lean to occur. When technicians in a plant tissue culture lab wish to stimulate root production in a tissue sample growing in vitro (that is, "in glass", or in the test tube), they place it in a growing medium dominated by auxin. When produced by the terminal, or apical, bud on a plant, auxin can inhibit the growth of the lateral buds. This is what produces apical dominance, or the situation where the tip bud is stronger and grows faster than all the other growing points on the plant. Auxins produced by developing seeds influence and promote the development of fruit tissue. For example, if all developing seeds are removed from a young strawberry fruit, the fruit never matures. However, if such a fruit is treated with auxin, it goes on to develop and ripen normally. Because auxin stimulates root production, we make use of it by applying rooting compounds to cuttings in order to get them to root faster and better. These rooting compounds contain natural and/or synthetic auxin compounds.
These compounds promote cell division, which is also known as cytokinesis. Because of this, the first cytokinin discovered was named kinetin. Interestingly, kinetin is not known to be present in plants, although a cytokinin with a similar chemical structure, known as zeatin, is found commonly in plants. Zeatin was so named because it was first found in Zea mays, or corn.
Kinetin was isolated from herring sperm (of all places!) in 1955, and the compound now called zeatin was isolated from corn in 1961. Since then, over 200 natural and synthetic cytokinins are now known.
The primary functions of cytokinins are the stimulation of cell division and the promotion of bud development. Plant tissue culture labs use cytokinins to stimulate shoot production in test tube tissue masses. Auxins and cytokinins work together; just a change in the ratio of one to the other is all that is necessary to determine whether roots or shoots will develop in a plant tissue sample. Micropropagation, or plant tissue culture, labs utilize this information to formulate various media for the growth of little shoots and roots in the test tube. Leaf expansion resulting from the enlargement of cells is also a function of cytokinins.
This group of compounds was discovered as a result of a search for the cause for a plant disease. Infected rice seedlings elongated 'foolishly" and produced poorly developed grain, or none at all. A study of this problem in 1926 ultimately resulted in the discovery that a bioactive compound produced by the infecting fungus was responsible for the effect seen in the seedlings. The fungus was Gibberella and the name given to the original extract isolated was "gibberellin".
Subsequent studies showed that this extract was not a single compound, but a group of related compounds. These have since been found to occur naturally in plants, and now at least 126 different gibberellins are known. The most familiar is known as gibberellic acid, or GA3. The "3" came because in the early days, new gibberellins were numbered according to the order in which they were discovered. Gibberellin A3 was the third one, and was the same as gibberellic acid.
Gibberellins act by stimulating cell division and elongation. When applied to a plant, they will cause the plant to grow taller than it would if it had not been treated. These compounds can also break dormancy in some plant seeds requiring cold treatment or light to induce germination, and can delay the death or senescence of leaves and citrus fruits.
Image credit: Public Domain image