Horseradish, (Armoracia rusticana), hardy perennial plant of the mustard family (Brassicaceae) known for its hotly pungent fleshy root, which is made into a condiment or table relish. Native to Mediterranean lands, horseradish is now grown throughout the temperate zones and is a troublesome weed in many cool, moist areas. The root is traditionally considered medicinal and is commonly used as a substitute for true wasabi (Eutrema japonicum) in Japanese cuisine. The intact horseradish root has hardly any aroma. When cut or grated enzymes from the now-broken plant cells break down sinigrin (a glucosinolate) to produce allyl isothiocyanate (mustard oil), which irritates the mucous membranes of the sinuses and eyes.