Primary Auxin Response Genes GH3s and DAO1 Modulate Stamen Elongation in Arabidopsis thaliana and Solanum lycopersicum

Late stamen development is mainly regulated by auxin, which acts through the Auxin Response Factor 8 (ARF8) to promote stamen elongation in Arabidopsis thaliana. Auxin concentration in the stamens peaks at the beginning of late development and then declines, but the contribution of auxin inactivation to auxin homeostasis in stamen has never been determined. In this study, we show that the expression of the acyl amido synthetases AtGH3.3, AtGH3.5, and AtGH3.6 (AtGH3s), which are involved in the conjugation of indole-3-acetic acid (IAA) with amino acids, is repressed, while the IAA oxidase AtDAO1 is upregulated in Arabidopsis arf8-7 stamens. The gh3s triple mutants exhibit a 24% increased stamen length, while gh3s and dao1 single mutants develop stamens with a reduced length of about 4% and 21%, respectively. We show that AtGH3s transcription is upregulated in dao1 stamens, whereas AtDAO1 expression is reduced in gh3s triple mutant stamens. Consistent with the long stamen phenotype, gh3s flower buds contain increased levels of free IAA and decreased levels of IAA-Asp. We also show that genes orthologous to AtARF8, AtGH3s and AtDAO1 are expressed in tomato stamens during flower development. We show that SlGH3s expression is mainly increased, while that of SlARF8A, SlARF8B and SlDAO2 is decreased in stamens from the parthenocarpic fruit (pat) mutant, which develops stamens of reduced length. Accordingly, free IAA levels decrease while IAA-Asp levels increase in tomato stamens. Collectively, our results demonstrate that GH3s and DAO1 modulate auxin homeostasis and regulate stamen development in both Arabidopsis and tomato.