Science Fair Project Encyclopedia
Plant hormones (or plant growth regulators, or PGRs) are internally secreted chemicals in plants that are used for regulating their growth. According to a standard definition, they:
- are signal molecules produced at specific locations;
- occur in low concentrations;
- cause altered processes in target cells at other locations.
It is accepted that there are five major classes of plant hormones with perhaps a few more in the future. The five major hormone classes are auxins, cytokinins(CK’s), ethylene, gibberellins (GA’s), and abscisic acid (ABA). Recently it has been suggested that brassinosteroids (BA’s), jasmonates (JA’s), salicylates (SA’s), and polyamines are major classes.
Plant Hormone Theories
There are few theories about plant hormones but a lot of experimental findings. A simple straight forward theory is to classify the five "classic" plant hormones, Auxin, Cytokinins (CKs), Gibberellins (GAs), Ethylene, and Abscisic Acid (ABA), plus two of the "new" plant hormones, Brassinosteroids (BAs), and Salicylic Acid (SA) into three groups. These are Growth Hormones, Stress Hormones and Shock Hormones.
The assumption of the theory is that plants are interested in growing larger during the vegetative period of their life and this growth requires both good environmental conditions and an amount of the four basic nutrient groups that exceeds that needed to keep the plant at its current size. Basic nutrients required for growth are sugar, gases (carbon dioxide and oxygen), water, and minerals.
- Growth Hormones - The theory sees growth hormones being made mostly in young and meristematic cells, and much less in mature cells. The theory holds that the growth hormones are made when an excess of nutrients exists beyond that needed for survival, and thus a signal that growth can begin is warranted. Growth hormones (Auxins and Cytokinins) stimulate metabolism, cause growth, and balance growth.
- Auxin simply would be the signal that more than enough shoot derived nutrients (sugar and gases) exist in the cell, all systems are go shoot-wise and root oriented growth can begin. General growth (think cell division) can also begin when complimented with the Cytokinin signal.
- Cytokinin would be the signal that more than enough root derived material nutrients exists than needed for survival, so shoot oriented growth can begin.
- Stress Hormones, in contrast, can be seen to be made mostly in mature cells that are faced with a scarcity of nutrients and to a much lesser extent in young and meristematic cells faced with the same level of scarcity. Stress hormones (GA's, Ethylene and maybe BA's) complement growth hormones. They are triggered by a deficit of nutrients, when there are not enough nutrients to allow growth. They allow a plant to survive this period and redistribute resources in order to resume growth.
- BA/GA - The theory holds that GA and BA are part of the same hormone signal pathway and that these two chemicals signal that there is less than enough shoot derived nutrients (sugar and gases) existing than needed for survival. In this case root growth must be stopped, roots even cut back and resources distributed to the shoot for additional shoot growth.
- Ethylene in contrast would be the signal that not enough minerals and water exist are being harvested, and shoot growth must stop, leaves be pruned back and resources shifted to the root for new root growth and greater mineral and water harvesting.
- Shock Hormones may be made by all cells in equal amounts faced with the same conditions. Shock hormones are the ABAs and SAs.
- ABA allows a plant to properly respond to rapidly developing stress situations.
- SA allows the plant to return to normal conditions after the stress has passed. --Socrtwo 06:23, 5 Mar 2005 (UTC)
- The Top Plant Hormone (Non-Theory) Site on The Web
- Another Good Site
- PowerPoint Presentation That's A Good Simple Introduction
- A Start at a General Plant Hormone Theory (Material in Theory Section Is Based on This Link)
- Sidwell Friends Plant Hormone Table - The Inspiration of the Plant Hormone Tables in Wikipedia and in the Article Above
- Hormonal Regulation of Gene Expression and Development - Detailed Intro Including Genetic Information
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