<p>The effect of auxin on wheat (<i>Triticum aestivum</i> L.) grain size is contentious. Additionally, the contributions to the IAA pool from de novo synthesis versus hydrolysis of IAA-glucose are unclear. Here, we describe the first comprehensive study of tryptophan aminotransferase and indole-3-pyruvate mono-oxygenase expression from 5 to 20 days after anthesis. A comparison of expression data with measurements of endogenous IAA via combined liquid chromatography–tandem mass spectrometry using heavy isotope labelled internal standards indicates that <i>TaTAR2-B3</i>, <i>TaYUC9-A1</i>, <i>TaYUC9-B</i>, <i>TaYUC9-D1</i>, <i>TaYUC10-A</i> and <i>TaYUC10-D</i> are primarily responsible for IAA production in developing grains. Furthermore, these genes are expressed specifically in developing grains, like those found in rice (<i>Oryza sativa</i> L.) and maize (<i>Zea mays</i> L.). Our results cast doubt on the proposed role of <i>THOUSAND-GRAIN WEIGHT</i> gene, <i>TaTGW6</i>, in promoting larger grain size via negative effects on grain IAA content. Work on this gene overlooked the contribution of IAA biosynthesis from tryptophan. Although IAA synthesis occurs primarily in the endosperm, we show the <i>TaYUC9-1</i> group is also strongly expressed in the embryo. Within the endosperm, <i>TaYUC9-1</i> expression is highest in aleurone and transfer cells, suggesting that IAA has a key role in differentiation of these tissues as has been proposed for other cereals.</p>