Identification and Analysis of Medicago truncatula Auxin Transporter Gene Families Uncover their Roles in Responses to Sinorhizobium meliloti Infection.

Auxin transport plays a pivotal role in the interaction between legume species and nitrogen-fixing bacteria to form symbioses. Auxin influx carriers auxin resistant 1/like aux 1 (AUX/LAX), efflux carriers pin-formed (PIN) and efflux/conditional P-glycoprotein (PGP/ABCB) are three major protein families participating in auxin polar transport. We used the latest Medicago truncatula genome sequence to characterize and analyze the M. truncatula LAX (MtLAX), M. truncatula PIN (MtPIN) and M. truncatula ABCB (MtABCB) families. Transient expression experiments indicated that three representative auxin transporters (MtLAX3, MtPIN7 and MtABCB1) showed cell plasma membrane localizations. The expression of most MtLAX, MtPIN and MtABCB genes was up-regulated in the roots and was down-regulated in the shoots by Sinorhizobium meliloti infection in the wild type (WT). However, the expression of these genes was down-regulated in both the roots and shoots of an infection-resistant mutant, dmi3. The different expression patterns between the WT and the mutant roots indicated that auxin relocation may be involved in rhizobial infection responses. Furthermore, IAA contents were significantly up-regulated in the shoots and down-regulated in the roots after Sinorhizobium meliloti infection in the WT. Inoculation of roots with rhizobia may reduce the auxin loading from shoots to roots by inhibiting the expression of most auxin transporter genes. However, the rate of change of gene expression and IAA contents in the dmi3 mutant were obviously lower than in the WT. The identification and expression analysis of auxin transporter genes helps us to understand the roles of auxin in the regulation of nodule formation in M. truncatula.