The auxin conjugate hydrolase family of Medicago truncatula and their expression during the interaction with two symbionts

We have characterized the regulation of auxin conjugate hydrolysis in Medicago truncatula during development and interaction with two symbionts. In Medicago truncatula five putative auxin amidohydrolase genes (MtIAR31, MtIAR32, MtIAR33, MtIAR34, MtIAR36) were identified, homologous to the AtIAR3 gene from Arabidopsis thaliana. The MtIAR32 transcript is the most abundant transcript in whole seedlings after germination, whereas MtIAR33 is the least abundant. MtIAR32 is also the most abundant transcript in adult roots, stems, and basal leaves. MtIAR31 is most highly expressed in flowers and MtIAR36 in terminal leaves. Sinorhizobium meliloti-infected seedlings primarily upregulated MtIAR33 and -34 transcripts. Glomus intraradices-infected seedlings upregulated MtIAR33 and -34. MtIAR31, -32, -33, and -34 have hydrolytic activity against IAA-aspartate and IBA-alanine. MtIAR33, -34, and -36 hydrolyze the ester bonds of IAA-glucose. MtIAR36 solely possesses activity against IAA-glycine, -alanine, and -isoleucine. IBA was increased in Glomus intraradices-inoculated roots of Medicago compared to controls, whereas IAA was not. Our results suggest an intricate control system that regulates free and conjugated auxins in Medicago.