Anthocyanins and betalains: evolution of the mutually exclusive pathways

The curious fact that the anthocyanidin and the betalain pathways, each leading to both yellow and red pigments, have never been found in the same plant is examined from a genetic and evolutionary point of view. The betalain products, alkaloids called betacyanins and betaxanthins, are found only in one group of Angiosperms, the Caryophyllales, while anthocyanins are widely distributed. However, their distribution within the plant and both their vegetative and reproductive functions are essentially identical. Whereas the functions in pollination and seed dispersal of both pigments are well known, their vegetative functions are ill-defined. The structural and many of the regulatory genes of the anthocyanin pathway are now well known, but our knowledge of the betalain pathway is severely restricted. Future needs in physiological and molecular genetic studies of the evolutionary mechanism of the mutual exclusion of these two pathways, as well as the vegetative functions of the two groups, are emphasized in this review.