Genome-wide identification and analysis of Medicago truncatula Small auxin upregulated RNA (SAUR) gene family uncover their roles in nodule formation

Small auxin-up RNAs (SAURs) gene family, one of the three major early auxin-responsive gene families, plays a critical role in environmental responses and developmental regulation. A comprehensive identification of SAUR family genes were performed basing on the latest updated Medicago truncatula reference genome sequence. Our data showed that 133 SAUR genes were located on all eight M. truncatula chromosomes unevenly. Phylogenetic analysis indicated that the SAUR proteins from M. truncatula and Glycine max were divided into five major groups. For detail study, 12 MtSAUR genes were randomly selected from different subgroups. Three classical motifs (Motif I-III) were identified in our selected SAUR proteins and most of them were intronless genes. The qRT-PCR data showed that 12 selected MtSAUR genes could be expressed at least in one of the M. truncatula organs tested. Interestingly, MtSAUR13 was showed a specific expression in the root and root meristem. MtSAUR18, MtSAUR20, MtSAUR57 and MtSAUR91 showed highest expression levels in the flower. We also found that there was a close relationship between AuxRE number and auxin responsive expression. Expression profiles based on microarray data have provided insights into the possible function in nodule formation under different symbiotic bacterial infections. This study provides a fundamental basis for understanding the involvement of MtSAUR genes at the initiation phase of nodule formation.