A Mutation in Mediator Subunit MED16A Suppresses Nodulation and Increases Arbuscule Density in Medicago truncatula

Nodulation in legumes is regulated to maintain carbon–nitrogen homeostasis. A systemic negative feedback pathway, Autoregulation of Nodulation (AON) controls nodule number by suppressing further nodulation as early as 48–72 h post inoculation. In order to identify new genes involved in AON, a screen for suppressors of sunn-1 , a Medicago truncatula AON mutant with a hypernodulation phenotype, was performed and identified a gene not previously associated with signaling in the legume-rhizobia symbiosis . We discovered that sunn-1 plants carrying the mutation s uppressor o f s unn-1 ( sos1 ) displayed wild-type nodule numbers. At 3 days post inoculation (dpi), sos1 sunn-1 plants make many nodulation foci and infection threads but lack the emerging nodules observed in wild-type and sunn-1 mutants. At 10 dpi, in contrast to sunn-1 , no significance in nodule number was observed between wildtype and the sos1 sunn-1 plants. Using grafting, we showed that sos1 suppression of the sunn-1 nodule phenotype is dependent on the genotype of the root. The lesion a C to T transition resulting in an R to H a change mapped to Mediator 16A (MED16A) and expression of this gene in composite hairy roots led to recovery of hypernodulation in sos1 sunn-1 plants. Induction of expression of the MtNIN transcription factor involved in regulating nodulation is reduced in sos1 sunn-1 compared to sunn-1. Furthermore, sos1 plants display fewer nodules than wild-type but higher arbuscular density when colonized by an arbuscular mycorrhizal fungus, and shorter roots in the absence of rhizobia. Collectively, our findings show that effects of mutation of Mediator 16A are not specific for symbiosis but involve multiple root response pathways, making the sos1 mutant an important tool for dissection of regulons controlling rhizobial and mycorrhizal responses, root growth and possibly other response pathways.