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 ubiquitous phenylalanine-derived anthocyanins. This study combined biochemical, molecular and phylogenetic analyses, and uncovered coordinated evolution of Tyr and betalain biosynthetic pathways in Caryophyllales. We found that Beta vulgaris, which produces high concentrations of betalains, 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 exhibited relaxed sensitivity to Tyr. Also, Tyr-insensitive BvADHα orthologs arose during the evolution of betalain pigmentation in the core Caryophyllales and later experienced relaxed selection and gene loss in lineages that reverted from betalain to anthocyanin pigmentation, such as Caryophyllaceae. These results suggest that relaxation of Tyr pathway regulation increased Tyr production and contributed to the evolution of betalain pigmentation, highlighting the significance of upstream primary metabolic regulation for the diversification of specialized plant metabolism.
Bibliographical noteFunding Information:
We thank S. Swanson for the help with confocal microscopy, S. R. Franco for writing assistance, A. Dewanjee for SoADH proteins expression, I. Goldman for the B. vulgaris seeds, G. J. Hatlestad and A. M. Lloyd for sharing the RNAseq data of table beet W357B, and to the Cambridge University Botanic Garden for access to living collections. This work is supported by the National Science Foundation Graduate Research Fellowship (grant no. DGE-1256259) to S.L.N. and the Agriculture and Food Research Initiative competitive grant (2015-67013-22955) from the USDA National Institute of Food and Agriculture to H.A.M.
- arogenate dehydrogenase (ADH/TyrA)
- metabolic pathway evolution
- tyrosine biosynthesis