Significance
The plant hormone auxin (indole-3-acetic acid [IAA]) plays a central role in regulating plant growth and development, but the regulation of IAA biosynthesis is poorly understood. Here, we identified a key push and pull feedback mechanism, which governs auxin biosynthesis through the tryptophan aminotransferase TAA1. We demonstrate that novel inhibitors of TAA1 function in both monocots and eudicots and present a catalytic mechanism that explains both its substrate specificity and its inhibition.

Abstract
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.