The buckwheat plant contains high levels of rutin (flavonol 3-O-rutinoside) in many organs, including its seeds, cotyledon, leaves, stem, and flowers. The enzymes that catalyze the decomposition and synthesis of rutin in buckwheat are unique in terms of having relatively low K_m values, indicating that buckwheat developed rutinosidase and glycosyl transferase enzymes specifically suited for rutin metabolism. In Tartary buckwheat seeds, high levels of rutin content and rutinosidase activity cause strong bitterness, which may effectively protect the seeds from being eaten by animals. The stress responses observed in buckwheat leaves suggests that rutin and rutinosidase are involved in enhancing the defense system against environmental stresses, including UV light, low temperature, and desiccation.