Auxin biosynthesis involves two types of enzymes: the Trp aminotransferases (TAA/TARs) and the flavin monooxygenases (YUCCAs). This two-step pathway is highly conserved throughout the plant kingdom and is essential for almost all of the major developmental processes. Despite their importance, it is unclear how these enzymes are regulated and how their activities are coordinated. Here, we show that TAA1/TARs are regulated by their product indole-3-pyruvic acid (IPyA) (or its mimic KOK2099) via negative feedback regulation in Arabidopsis thaliana. This regulatory system also functions in rice and tomato. This negative feedback regulation appears to be achieved by both the reversibility of Trp aminotransferase activity and the competitive inhibition of TAA1 activity by IPyA. The Km value of IPyA is 0.7 µM, and that of Trp is 43.6 µM; this allows IPyA to be maintained at low levels and prevents unfavorable nonenzymatic indole-3-acetic acid (IAA) formation from IPyA in vivo. Thus, IPyA levels are maintained by the push (by TAA1/TARs) and pull (by YUCCAs) of the two biosynthetic enzymes, in which TAA1 plays a key role in preventing the over- or under-accumulation of IPyA. TAA1 prefer Ala among various amino acid substrates in the reverse reaction of auxin biosynthesis, allowing TAA1 to show specificity for converting Trp and pyruvate to IPyA and Ala, and the reverse reaction.

中文翻译：

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Indole-3-pyruvic acid 调节 TAA1 活性，它在协调生长素生物合成的两个步骤中起着关键作用。

生长素生物合成涉及两种类型的酶：Trp 转氨酶 (TAA/TARs) 和黄素单加氧酶 (YUCCAs)。这种两步途径在整个植物界高度保守，对几乎所有主要发育过程都是必不可少的。尽管它们很重要，但尚不清楚这些酶是如何被调节的以及它们的活动是如何协调的。在这里，我们表明 TAA1/TARs 在拟南芥中通过负反馈调节受其产物吲哚-3-丙酮酸 (IPyA)（或其模拟物 KOK2099）的调节。该调节系统也在水稻和番茄中发挥作用。这种负反馈调节似乎是通过 Trp 氨基转移酶活性的可逆性和 IPyA 对 TAA1 活性的竞争性抑制来实现的。IPyA的Km值为0.7 µM，Trp的Km值为43.6 µM；这允许 IPyA 保持在低水平，并防止 IPyA 在体内形成不利的非酶促吲哚-3-乙酸 (IAA)。因此，IPyA 水平由两种生物合成酶的推动（通过 TAA1/TARs）和拉动（通过 YUCCAs）维持，其中 TAA1 在防止 IPyA 过度积累或积累不足方面发挥关键作用。在植物生长素生物合成的逆反应中，TAA1 在多种氨基酸底物中更偏爱 Ala，这使得 TAA1 表现出将 Trp 和丙酮酸转化为 IPyA 和 Ala 以及逆反应的特异性。其中 TAA1 在防止 IPyA 过度积累或积累不足方面起着关键作用。在植物生长素生物合成的逆反应中，TAA1 在多种氨基酸底物中更偏爱 Ala，这使得 TAA1 表现出将 Trp 和丙酮酸转化为 IPyA 和 Ala 以及逆反应的特异性。其中 TAA1 在防止 IPyA 过度积累或积累不足方面起着关键作用。在植物生长素生物合成的逆反应中，TAA1 在多种氨基酸底物中更偏爱 Ala，这使得 TAA1 表现出将 Trp 和丙酮酸转化为 IPyA 和 Ala 以及逆反应的特异性。