Role of Cytosolic, Tyrosine-Insensitive Prephenate Dehydrogenase inMedicago truncatula

ABSTRACT L-Tyrosine (Tyr) is an aromatic amino acid synthesized de novo in plants and microbes downstream of the shikimate pathway. In plants, Tyr and a Tyr pathway intermediate, 4-hydroxyphenylpyruvate (HPP), are precursors to numerous specialized metabolites, which are crucial for plant and human health. Tyr is synthesized in the plastids by a TyrA family enzyme, arogenate dehydrogenase (ADH/TyrA a ), which is feedback inhibited by Tyr. In addition to ADH enzymes, many legumes possess prephenate dehydrogenases (PDH/TyrA p ), which are insensitive to Tyr and localized to the cytosol. Yet the role of PDH in legumes is currently unknown. This study isolated and characterized Tnt1 -transposon mutants of MtPDH1 ( pdh1 ) in Medicago truncatula to investigate PDH function. The pdh1 mutants lacked PDH transcript, PDH activity, and displayed little aberrant morphological phenotypes under standard growth conditions providing genetic evidence that MtPDH1 is responsible for the PDH activity detected in M. truncatula . Though plant PDH enzymes and activity have been specifically found in legumes, nodule number and nitrogenase activity of pdh1 mutants were not significantly reduced compared to wild-type (Wt) during symbiosis with nitrogen-fixing bacteria. Although Tyr levels were not significantly different between Wt and mutants under standard conditions, when carbon flux was increased by shikimate precursor feeding, mutants accumulated significantly less Tyr than Wt. These data suggest that MtPDH1 is involved in Tyr biosynthesis when the shikimate pathway is stimulated, and possibly linked to unidentified legume-specific specialized metabolism.