Flavonoids are phenolic compounds composed of fifteen carbons that are found in land plants, including bryophytes (hornworts, liverworts, mosses) and vascular plants (ferns, gymnosperms, and angiosperms). There are five major types of flavonoids: anthocyanins, flavones, flavonols, isoflavonoids, and proanthocyanidins. They are synthesized in the cytoplasm and subsequently accumulated in small vacuoles that fuse with the central vacuole in both the epidermis and cortex. The original function of flavonoids in plant cells is thought to be defensive, providing protection against insect, fungal, and viral attacks and consumption by invertebrate and vertebrate herbivores. Over evolutionary time, their functions became diverse. Anthocyanins in floral and vegetative tissues range in colors from yellow to blue. Those found in petals and pollen grains of the flower attract pollinator insects and birds that visit for food. Visibly colorless flavonoids in the epidermal cells, such as flavones and flavonols, serve as ultraviolet shields for the underlying cells. Proanthocyanidins accumulate in vacuoles of cortical cells, seed coats, and secondary tissues such as bark, where they form mixtures of brown-black reddish pigments called condensed tannins in the walls. Isoflavonoids and flavones are secreted into the surrounding soil layers and function as signals in the interaction between plant roots and nitrogen-fixing bacteria to form nodules.

See Also

Anthocyanins; Pigments; Vacuoles.

Bibliography

Bohm, B. A. Intoduction to Flavonoids. India: Harwood Academy, 1998.

Dixon, R. A., and C. L. Steele. "Flavonoids and Isoflavonoids—A Gold Mine for Metabolic Engineering." Trends in Plant Science 4 (1999): 394-400.

Structure of flavonoids.

Stafford, H. A. "Flavonoid Evolution: An Enzymic Approach." Plant Physiology 96 (1991): 680-85.

——. "Teosinte to Maize—Some Aspects of Missing Biochemical and Physiological Data Concerning Regulation of Flavonoid Pathways." Phytochemistry 49 (1998): 285-93.