Diversification of Tyrosine Biosynthetic Pathways in Plants: Non‐Plastidic, Tyrosine‐Insensitive Prephenate Dehydrogenases in Legumes

Abstract

Tyrosine (Tyr) is an essential aromatic amino acid synthesized de novo by plants and microbes. Besides being a protein building block, in plants Tyr and a Tyr-pathway intermediate 4-hydroxyphenylpyruvate (HPP) serve as key precursors of a diverse array of natural products, such as cyanogenic glycosides, betalains, tocopherols (vitamin E), plastoquinone, and isoquinoline alkaloids. Tyr biosynthesis can occur via two intermediates, arogenate and HPP. Arogenate dehydrogenase (ADH) and prephenate dehydrogenase (PDH) are key enzymes for the arogenate and HPP pathways, respectively, which are generally feedback regulated by Tyr. Most plants only have plastidic ADH activity and likely synthesize Tyr using the arogenate route within the plastids, whereas both ADH and PDH activity was detected in some legume species. However, the genes encoding enzymes responsible for the plant PDH activity are unknown. Here we report identification and biochemical characterization of PDH enzymes from soybean and Medicago. Comparative genomics and phylogenetic analyses together with recombinant enzyme characterization identified a legume-specific monophyletic clade that contains PDH enzymes. Kinetic analyses revealed that soybean and Medicago PDHs specifically use prephenate but not arogenate substrate. Unlike previously-characterized plant ADHs, both legume PDHs were insensitive to Tyr regulation and localized to the cytosol. Our study provides molecular evidence for the diversification of the primary metabolic Tyr pathway, likely to support the tremendous chemodiversity that exists downstream of HPP and Tyr in different plant species. This work was supported by the UW-Madison startup fund and NSF Grant IOS-1354971 to H.A.M.