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Plant Hormones: Chemical Control of Growth and Reproduction

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  • 1:19 Auxins
  • 2:24 Cytokinins
  • 3:18 Gibberellins
  • 3:58 Ethylene
  • 4:52 Abscisic Acid
  • 5:24 Lesson Summary
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Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

We most often think of hormones as things that control our actions and development. However, even plants have these chemicals to help regulate growth and reproduction.

Plant Hormones

When you hear the word 'hormones,' you may think of typical human hormones, such as testosterone, estrogen or even adrenaline. These hormones in our bodies regulate different physiological activities ranging from the tone of our voices to our height to our fight or flight response. Plants, which seem rather unresponsive most of the time, also have hormones to control physiological activities.

Hormones are chemical signals that coordinate the different parts of an organism. The word 'hormone' comes from a Greek term that means 'to excite.' These chemicals are produced in very small amounts in one area of an organism and are then sent to another part, where a response is triggered. Let's use the human hormone adrenaline to illustrate this. Our adrenal glands are located above our kidneys. They produce a hormone called adrenaline that, when needed, is transported through our bodies to cause an increase in heart rate, dilation of pupils and other responses to make us more alert.

Hormones in plants act in similar ways. They are produced by cells in one area of the plant - such as the leaves, stems or root - and then transported to a different area of the plant in order to produce a response. While the five major plant hormones we will look at sometimes have countless functions, we will focus on the main responses they trigger.

Auxins

Auxins are hormones produced in immature parts of plants that stimulate growth. Auxins were the first plant hormones discovered and have been studied extensively. Auxins are most commonly found in seed embryos, apical meristems and young leaves. The seed embryo has yet to develop, and the cells have not yet become differentiated - that is, the young cells don't know what they'll be when they grow up yet. Similarly, the apical meristem is the location of primary plant growth and contains new cells that have not yet decided what to become.

Auxins stimulate cell differentiation, which means this hormone helps decide if a cell will become ground tissue or vascular tissue or protective tissue. Auxins also help stimulate stem elongation, which is what primary stem growth is all about. Primary stem growth will occur when a high enough amount of auxins target a given area. Auxins also help regulate fruit development. Without auxins, fruits are often too small. Some produce farmers will spray artificial auxins on plants - such as apples or pears - in order to increase the size of the fruits.

Cytokinins

Cytokinins are hormones that are produced in the roots, stimulate growth and have anti-aging effects. Because they're produced in roots, cytokinins must travel up through the plant's xylem in order to reach target areas, such as the stems and leaves. Cytokinins have several responsibilities, including working with auxins to stimulate growth and cell differentiation in both stems and roots. Cytokinins also specifically promote growth and development of chloroplasts, the cell organelle responsible for photosynthesis.

A unique role of cytokinins is to produce anti-aging effects on some plant parts. When you think of anti-aging products, you may think of expensive face creams that advertise giving a younger, brighter look. Cytokinins actually provide a younger, healthier look in plants. Florists commonly use cytokinins to make cut flowers look fresh for longer. By adding this hormone to cut flowers, florists are able to slow down the aging process, providing us with prettier flowers for longer.

Gibberellins

Gibberellins are hormones produced in meristems of stems and roots that help regulate stem elongation. This group of hormones is actually named after a fungus. The fungus caused some rice plants to grow unbelievably tall - even to the point where the rice plants would fall over because the stem was too tall for the roots to support.

We now know that gibberellins help stimulate stem cell elongation in plants. The fungus in the rice plants caused too much production of gibberellins and, therefore, too much stem growth. Some dwarf plant species don't produce enough gibberellins to make the stem elongate - causing the short stature. However, gibberellins can be added to dwarf plants in order to make them grow to normal heights.

Ethylene

Ethylene is a hormone produced in fruits, flowers and aging leaves that promotes fruit ripening. There are a few other effects of ethylene, but these vary depending on the type of plant. Sometimes, ethylene will promote the growth and development of roots, leaves and flowers. However, in other species of plants, ethylene inhibits this growth. No matter the type of plant, ethylene is best known for promoting the ripening of fruit.

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