Betalains are a class of red and yellow indole-derived pigments found in plants of the Caryophyllales, where they replace anthocyanin pigments. Betalains also occur in some higher order fungi. They are most often noticeable in the petals of flowers, but may color the fruits, leaves, stems, and roots of plants that contain them. They include powerful antioxidant pigments such as those found in beets.
The name "betalain" comes from the Latin name of the common beet (Beta vulgaris), from which betalains were first extracted. The deep red color of beets, bougainvillea, amaranth, and many cacti results from the presence of betalain pigments. The particular shades of red to purple are distinctive and unlike that of anthocyanin pigments found in most plants.
There are two categories of betalains:
- Betacyanins include the reddish to violet betalain pigments.
- Betaxanthins are those betalain pigments which appear yellow to orange.
It was once thought that betalains were related to anthocyanins, the reddish pigments found in most plants. Both betalains and anthocyanins are water soluble pigments found in the vacuoles of plant cells. However, betalains are structurally and chemically unlike anthocyanins and the two have never been found in the same plant together. For example, betalains contain nitrogen whereas anthocyanins do not.
It is now known that betalains are aromatic indole derivatives synthesized from tyrosine. They are not related chemically to the anthocyanins and are not even flavonoids. Each betalain is a glycoside, and consists of a sugar and a colored portion. Their synthesis is promoted by light.
The most heavily studied betalain is betanin, also called beetroot red after the fact that it may be extracted from red beet roots. Betanin is a glucoside, and hydrolyzes into the sugar glucose and betanidin. It is used as a food coloring agent, and the color is sensitive to pH. Other betalains known to occur in beets are isobetanin, probetanin, and neobetanin.
Taxonomic significance 
Betalain pigments occur only in the Caryophyllales and some Basidiomycota (mushrooms). Where they occur in plants, they sometimes coexist with anthoxanthins (yellow to orange flavonoids), but never occur in plant species with anthocyanins.
Among the flowering plant order Caryophyllales, most members produce betalains and lack anthocyanins. Of all the families in the Caryophyllales, only the Caryophyllaceae (carnation family) and Molluginaceae produce anthocyanins instead of betalains. The limited distribution of betalains among plants is a synapomorphy for the Caryophyllales, though their production has been lost in two families.
Economic uses 
Betanin is commercially used as a natural food dye. It can cause beeturia (red urine) and red feces in some people who are unable to break it down. The interest of the food industry in betalains has grown since they were identified as antioxidants which may protect against oxidation of low-density lipoproteins.
See also 
- Strack D, Vogt T, Schliemann W (February 2003). "Recent advances in betalain research". Phytochemistry 62 (3): 247–69. doi:10.1016/S0031-9422(02)00564-2. PMID 12620337.
- Robinson, Trevor (1963). The Organic Constituents of Higher Plants. Minneapolis: Burgess Publishing. p. 292.
- Salisbury, Frank B.; Cleon W. Ross (1991). Plant Physiology (4th ed.). Belmont, California: Wadsworth Publishing. pp. 325–326. ISBN 0-534-15162-0.
- Kimler, L. M. (1975). "Betanin, the red beet pigment, as an antifungal agent". Botanical Society of America, Abstracts of papers 36.
- Francis, F.J. (1999). Colorants. Egan Press. ISBN 1-891127-00-4.
- Stafford, Helen A. (1994), "Anthocyanins and betalains: evolution of the mutually exclusive pathways", Plant Science 101 (2): 91–98, doi:10.1016/0168-9452(94)90244-5, ISSN 0168-9452, retrieved 20 May 2013
- Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2004). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. p. 465. ISBN 0-7167-1007-2.
- Cronquist, Arthur (1981). An Integrated System of Classification of Flowering Plants. New York: Columbia University Press. pp. 235–9. ISBN 0-231-03880-1.
- Escribano, J.; M. A. Pedreño, F. García-Carmona, R. Muñoz (1998). "Characterization of the antiradical activity of betalains from Beta vulgaris L. roots". Phytochem. Anal. 9 (3): 124–7. doi:10.1002/(SICI)1099-1565(199805/06)9:3<124::AID-PCA401>3.0.CO;2-0.
- Tesoriere, Luisa; Mario Allegra, Daniela Butera, and Maria A. Livrea (October 1, 2004). "Absorption, excretion, and distribution of dietary antioxidant betalains in LDLs: potential health effects of betalains in humans". American Journal of Clinical Nutrition 80 (4): 941–5. PMID 15447903.
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