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. Additionally, many legumes possess prephenate dehydrogenases (PDH/TyrA_p), which are insensitive to Tyr and localized to the cytosol. Yet the role of PDH enzymes 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 and 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 with 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.

中文翻译：

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细胞质、酪氨酸不敏感的预苯酸脱氢酶在蒺藜苜蓿中的作用。


l-酪氨酸 (Tyr) 是植物和微生物中莽草酸途径下游从头合成的芳香族氨基酸。在植物中，Tyr 和 Tyr 途径中间体 4-羟基苯基丙酮酸 (HPP) 是许多特殊代谢物的前体，对植物和人类健康至关重要。 Tyr 在质体中由 TyrA 家族酶、芳香酸脱氢酶 (ADH/TyrA _a ) 合成，该酶受到 Tyr 的反馈抑制。此外，许多豆类具有预苯酸脱氢酶 (PDH/TyrA _p )，其对 Tyr 不敏感并且定位于细胞质。然而，PDH 酶在豆类中的作用目前尚不清楚。本研究分离并表征了蒺藜苜蓿中MtPDH1 ( pdh1 ) 的Tnt1转座子突变体，以研究 PDH 功能。 pdh1突变体缺乏PDH转录本和PDH活性，并且在标准生长条件下表现出很少的异常形态表型，这提供了MtPDH1负责在M. truncatula中检测到的PDH活性的遗传证据。虽然植物PDH酶和活性在豆科植物中特异发现，但在与固氮细菌共生期间，与野生型（Wt）相比， pdh1突变体的根瘤数量和固氮酶活性并未显着降低。尽管在标准条件下，Wt 和突变体之间的 Tyr 水平没有显着差异，但当通过莽草酸前体喂养增加碳通量时，突变体积累的 Tyr 明显低于 Wt。 这些数据表明，当莽草酸途径受到刺激时，MtPDH1 参与 Tyr 生物合成，并且可能与未识别的豆类特异性专门代谢有关。