Abstract

Diverse natural products are synthesized in plants by specialized metabolic enzymes, which are often lineage-specific and derived from gene duplication followed by functional divergence. However, little is known about the contribution of primary metabolism to the evolution of specialized metabolic pathways. Betalain pigments, uniquely found in the plant order Caryophyllales, are synthesized from the aromatic amino acid L-tyrosine (Tyr) and replaced the otherwise phenylalanine-derived anthocyanins. Using biochemical, molecular and phylogenetic analyses, we uncovered a coordinated evolution of Tyr and betalain biosynthetic pathways in Caryophyllales. We found that Beta vulgaris, which accumulates high levels of betalains and Tyr, synthesizes Tyr via plastidic arogenate dehydrogenases (TyrAa/ADH) encoded by two ADH genes (BvADHα and BvADHβ). Unlike BvADHβ and other plant ADHs that are strongly inhibited by Tyr, BvADHα and other ADH orthologs in Caryophyllales exhibited relaxed sensitivity to Tyr. In addition, we found that the de-regulated ADH α arose during the evolution of betalain pigmentation in the Caryophyllales and later experienced relaxed selection and gene loss in lineages that reverted from betalain to anthocyanin pigmentation, such as Caryophyllaceae. Finally, using phylogeny and targeted metabolic profile we showed that additional Tyr-derived compounds are present in Caryophyllales plants having ADH α. These results suggest that relaxation of Tyr pathway regulation increased Tyr production and contributed not only to the evolution of betalain pigmentation, but also other Tyr-derived metabolites. The work highlights the significance of upstream primary metabolic regulation for the diversification of specialized plant metabolism.