Medicago truncatula-Sinorhizobium meliloti-Fusarium oxysporum Tripartite Interaction Alters Nodulation and Nitrogen Fixation

Plants encounter a myriad of microorganisms at the root-soil interface that can invade with detrimental or beneficial outcomes. Promoting a symbiotic relationship may interfere with the restriction responses to pathogens. In this study, we established a tripartite Medicago truncatula (Mt)-Sinorhizobium meliloti (Sm)-Fusarium oxysporum (Fo) interaction to study the effect of the interplay between symbiosis and defense on the symbiosis establishment and efficiency in a resistant (A17) and susceptible (TN1.11) genotype to Fo infection. Our results showed that Sm induced the expression of allene oxide cyclase 1 and 2 (MtAoc1) and (MtAoc2) at 6 and 24 h post infection (hpi) in A17 and at 24 hpi in TN1.11. This up-regulation of MtAoc1 and MtAoc2 was also observed when co-inoculating TN1.11 genotype with Fo and Sm. Moreover, in this later treatment the expression level of the gene of the common signaling pathway doesn't make infection 3 (DIM3) was reduced compared to Sm-inoculated plants. This reduction of DIM3 transcripts was concomitant to a decrease in nodule number (NN), nodule fresh weight, and nitrogen-fixing rate (NFR), and to an accumulation of spermine in nodules. This defectiveness in symbiosis parameters, in the co-inoculation treatment, was also observed in A17 genotype, along with an accumulation of phytohormones in leaves and polyamines in nodules. Our results suggest that in the tripartite interaction, the high expression of MtAoc and the decrease of DMI3 transcripts along with the accumulation of phytohormones and polyamines in leaves overlapped with symbiosis establishment and functioning and thus, reduced NN and NFR.

Automated summary

Plants encounter various microorganisms at the root-soil interface, which can have harmful or beneficial effects. By studying a tripartite interaction between Medicago truncatula, Sinorhizobium meliloti, and Fusarium oxysporum, researchers found that the high expression of certain genes induced by symbiotic bacteria interfered with the establishment and functioning of symbiosis. Additionally, the down-regulation of a common signaling pathway gene was observed, along with a decrease in symbiosis parameters and an accumulation of certain compounds.