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Part 6: Phytochemistry—Plant Defense

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Plants have a sophisticated means of responding to attacks by herbivores and disease-causing organisms. When being attacked by an herbivore, a system called the Induced Resistance (IR) pathway is activated. This pathway causes the secretion of compounds that either make the plant seem less appetizing (unappealing odor, a bitter taste, etc.) or poison the predator causing illness or death.

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Hypersensitive Response

Another resistance pathway is sparked by fungal, viral, or bacterial infections. When a disease-causing organism binds to plant cells, a localized response called the Hypersensitive Response (HR) is induced. The plant closes off the infected location causing rapid cell death in the area surrounding the infection and preventing the infection from spreading throughout the plant. This pathway causes discolored blotches to appear on the plant’s leaves or flowers where cell death has occurred. When one area of the plant is terminally infected and the HR response is activated, the plant will then produce salicylic acid (a compound similar to the active ingredient in aspirin) and nitric oxide (NO). Together, these compounds spark a systemic response releasing protective and restorative compounds throughout the plant. These compounds work to kill the microbes, heal wounds, and repair damaged tissues.

Important families of these protective compounds are essential oils. Most of the research performed today is designed to find the cleansing properties of essential oils. 

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