Plant Hormones Study Guide By Simone Greenberg


elongation, Germination and Fruit Matuartion


Gibberellin is created in the root and stemapical meristems, young leaves, and seed embryos. They stimulate stem and leaf growth by cell elongation and cell division. Although biologists are not 100% sure, evidence suggests that gibberellin activates enzymes that loosen cell walls and allow expansion proteins to enter the cell. Gibberellin works with the plant hormone, auxin, to achieve stem elongation. The hormone, abscisic acid, is an antagonist of gibberellin and instead, slows growth and inhibits germination.


The release of gibberellin by the embryo of a seed signals that the seed is ready to germinate and is no longer in a dormant state. Some plants can be manipulated and are able to break dormancy (even when under unfavorable conditions) if stimulated by gibberellin.

Gibberellin-induced release of enzymes and carbohydrate mobilization during germination of a seed

Fruit Maturation

Gibberellin controls fruit growth and development by inducing the development of seedless fruits from unfertilized pistils (female plant organs), especially in the case of fruits such as apples and pears.

Gibberellin and Gene Expression

Gibberellins gene expression is an inhibitory pathway. As the ligand, they bind to the receptor, GID1. This whole complex then binds to the proteasome activator, SCF (responsible for Ubi to Della protein). This triggers the destruction of the DELLA protein via the proteasome. This relieves the inhibition that the gibberellin provides, and transcription begins. Normally, DELLA proteins bind gibberellin-dependent transcription factors. This stops them from binding to the genes that are turned on by gibberellin.


Gibberellin hormones can be extremely useful in the agricultural industry. For example, seedless grapes are often treated with gibberellin to create larger fruits, as well as longer stems which reduces mildew infection. Additionally, gibberellin can be used to ripen fruits, (such as lemons, oranges, and cherries) at specific times, which helps the market when certain fruits are out of season. Lastly, gibberellin treatment helps break dormancy in “seed potatoes” resulting in uniform crop emergence.


flowering, Stress, and Leaf Shedding

Ethylene is a gas produced by plants in response to stress. Ethylene is detected by transmembrane receptors in the endoplasmic reticulum of the cells. Binding of ethylene to these receptors creates a signaling cascade that leads to the activation of transcription factors and turns gene transcription on. Many effects on plants that are caused by auxin such as stoppage of root elongations can be due to auxin-caused ethylene production.


Ethylene helps with fruit ripening through converting starch and acids to sugars. Today, people store unripe fruit in sealed bags and allow the gas released by mature fruit to help ripen other fruit.


If a plant comes into contact with an obstacle, ethylene gas is released and then triggers a the plant to avoid the obstacle. This is called a triple response. Once the obstacle is avoided (through slowing of stem elongation, thickening on stem or curvature) the ethylene gas ceases and the stem continues normal, vertical growth.

Seedling growth showing a triple response to ethylene.

Ethylene triggers leaf abscission (the loss of leaves) as older leaves produce less auxin and become more sensitive to ethylene. More and more ethylene produces enzymes that digest tough components of the leaf cells such as the cellulose and other components in the cell wall.


Ethylene can be used in agriculture as a natural way to quickly ripen fruit. Farmers can choose to ripen a large amount of fruit easily by sealing fruit off in the presence of a few already ripened fruits. Additionally, through genetic engineering, molecular biologists have created tomatoes that ripen on demand (see picture below). These tomatoes are picked while still unripened and will not ripen unless ethylene gas is added. This will help reduce the amount of spoilage fruits and vegetables, which will be beneficial to both producers and consumers.


Numerous Effects

Brassinosteroids are a a group of 40 different steroids similar to cholesterol and sex hormones in animals. They help with cell elongation and division in stem segments and seeds at very low concentrations. They are very similar to the effects of auxin, and help slow leaf shedding and promote xylem differentiation. Additionally, brassinosteroids inhibit root growth, and formation of the stomata. They enhance germination and gravitropism, and increase ethylene production.

Shows the importance of brassinosteroids. The plant on the write shows interrupted growth without the presence of brassinosteroids.

Gene Expression

Brassinosteroids are effective through binding to cell-surface receptors that cause rapid metabolic changes in the cytosol and regulate the expression of genes.


Brassinosteroids have been found to increase the resistance to freezing. Commercially, this could benefit the agricultural industry by allowing them to potentially grow crops year round, especially in places that have diverse seasons. This would allow all regions of the country to consume local crops that are in season and fresh. Additionally, brassinosteroids have increased the yield of wheat and rice. This would allow more of these grain crops to be sold and would presumably lower the cost of these of crops due to the high yield.

works cited

"Auxin and Ethylene in Leaf Abscission." PowerShow. Crystal Graphics Inc, 2017. Web. 08 Mar. 2017.

Avissar, Yael, Jung Choi, Jean DeSaix, Vladimir Jurukovski, Robert Wise, and Connie Rye. "OpenStax CNX." OpenStax CNX. Rice University, 2017. Web. 08 Mar. 2017.

Kimball, John W. "Brassinosteroids." Brassinosteroids. Saylor Foundation, 12 Aug. 2016. Web. 08 Mar. 2017.

Kimball, John W. "Ethylene." Ethylene. Saylor Foundation, 12 Aug. 2016. Web. 08 Mar. 2017.

Kimball, John W. "Gibberellins." Gibberellins. Saylor Foundation, 12 Aug. 2016. Web. 08 Mar. 2017

Kumar, Srinibas. "Applications of Gibberellins in Agriculture and Horticulture." Biology Discussion. Biology Discussion, 26 Oct. 2015. Web. 08 Mar. 2017.


Title Picture:

Gibberellin Pathway:

Germination of seed:




Triple Response:


Brassinosteroid plants:

Rice and Wheat:

Urry, Lisa A., Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson. "Plant Responses to Internal and External Signals." Campbell Biology: AP Edition. By Jane B. Reece. Tenth ed. Glenview: Pearson Education, 2014. 836-65. Print.

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