Medicago Plants Control Nodulation by Regulating Proteolysis of the Receptor-Like Kinase DMI2.

Abstract

Plants use receptor-like kinases to monitor environmental changes and transduce signals into plant cells. The Medicago truncatula (hereafter Mtruncatula) DOES NOT MAKE INFECTIONS2 (DMI2) protein functions as a coreceptor of rhizobial signals to initiate nodule development and rhizobial infection during nitrogen-fixing symbiosis, but the mechanisms regulating DMI2 protein level and folding are still unknown. Here, we report that DMI2 protein abundance changes during nitrogen-fixing symbiosis. DMI2 accumulates in the nodules and is induced by rhizobia treatment through a posttranscriptional process. However, DMI2 induction is independent of the perception of Nod factor, a group of lipochitooligosaccharides secreted by rhizobia. The stability of the DMI2 protein is controlled by the proteasome pathway: in rhizobia-free environments, DMI2 is degraded by the proteasome, but during rhizobial infection, DMI2 is protected from the proteasome, resulting in protein accumulation. Furthermore, proteasome inhibitor-promoted accumulation of DMI2 protein in Medicago roots induces the expression of two early nodulation marker genes, supporting the hypothesis that DMI2 accumulation activates downstream symbiosis signaling. The extracellular region of DMI2 contains two malectin-like domains (MLDs) and a leucine-rich repeat. When conserved amino acids in the MLDs are mutated, DMI2 fails to restore nodule development in dmi2 mutants, and point-mutated MLD proteins are degraded constitutively, suggesting that the MLD may be vital for the accumulation of DMI2. Our findings suggest that legumes control nodule development through modulating the protein level of DMI2, revealing a layer of regulation in the interaction between plants and rhizobia in nitrogen-fixing symbiosis.