Anthocyanins and betalains in evolution: Why mutual exclusion remains the best-supported model.

Anthocyanins and betalains are hydrophilic plant pigments with numerous physiological and ecological functions. The biosynthesis routes of anthocyanins and betalains differ, with anthocyanins being synthesized from phenylalanine via the general phenylpropanoid pathway, whereas betalains are derived from tyrosine. Although the precursors phenylalanine and tyrosine are present in all plants, there is no known plant in which both these pigments are co-accumulated. Most plants synthesize anthocyanins, while certain families in the order Caryophyllales produce betalains. There is an apparent mutual exclusion of these two plant pigments. Over the past five decades, evidence has accumulated supporting this theory of mutual exclusion of the two pigments. However, recently published reports claim the presence of anthocyanins in well-known betalain-pigmented plants without providing the necessary evidence. Here, we critically evaluate the origins of such claims, show how methodological limitations and analytical misinterpretations have led to unsupported reports of anthocyanin-betalain co-occurrence, and provide recommendations to ensure robust evidence standards in future studies of plant pigment biosynthesis.