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Three common symbiotic ABC-B transporters in Medicago truncatula are regulated by a NIN-independent branch of the symbiosis signalling pathway.

Several ATP-Binding Cassette (ABC) transporters involved in the arbuscular mycorrhizal symbiosis and nodulation have been identified. We describe three previously-unreported ABC subfamily-B transporters, named ABCB for Mycorrhization and Nodulation (AMN1, AMN2, and AMN3), that are expressed early during infection by rhizobia and arbuscular mycorrhizal fungi. These ABCB transporters are strongly expressed in symbiotically infected tissues, including in root hair cells with rhizobial infection threads and arbusculated cells. During nodulation, the expression of these genes is highly induced by rhizobia and purified Nod factors, and was dependent on DMI3, but is not dependent on other known major regulators of infection such as NIN, NSP1, or NSP2. During mycorrhization their expression is dependent on DMI3 and RAM1, but not on NSP1 and NSP2. Therefore, they may be commonly regulated through a distinct branch of the common symbiotic pathway. Mutants with exonic Tnt1-transposon insertions were isolated for all three genes. None of the single or double mutants showed any differences in colonization by either rhizobia or mycorrhizal fungi, but the triple amn1 amn2 amn3 mutant showed an increase in nodule number. Further studies are needed to identify potential substrates of these transporters and understand their roles in these beneficial symbioses.