Abstract
LYK3 is a lysin motif receptor-like kinase of Medicago truncatula, which is essential for the establishment of the nitrogen-fixing, root nodule symbiosis with Sinorhizobium meliloti. LYK3 is a putative receptor of S. meliloti Nod factor signals, but little is known of how it is regulated and how it transduces these symbiotic signals. In a screen for LYK3-interacting proteins, we identified M. truncatula Plant U-box protein 1 (PUB1) as an interactor of the kinase domain. In planta, both proteins are localized and interact in the plasma membrane. In M. truncatula, PUB1 is expressed specifically in symbiotic conditions, is induced by Nod factors, and shows an overlapping expression pattern with LYK3 during nodulation. Biochemical studies show that PUB1 has a U-box-dependent E3 ubiquitin ligase activity and is phosphorylated by the LYK3 kinase domain. Overexpression and RNA interference studies in M. truncatula show that PUB1 is a negative regulator of the LYK3 signaling pathway leading to infection and nodulation and is important for the discrimination of rhizobia strains producing variant Nod factors. The potential role of PUB E3 ubiquitin ligases in controlling plant-microbe interactions and development through interacting with receptor-like kinases is discussed.
Highlights
The ability of legumes to form a symbiosis with nitrogen-fixing bacteria termed rhizobia is of immense agronomic and ecological importance (Graham and Vance, 2003)
From the cDNA sequences of our clones and those in the M. truncatula Expressed Sequence Tag (MtEST) database, it is clear that the C terminus of the protein has not been correctly deduced from the automatic gene discovery/annotation programs in the M. truncatula genome sequencing project; our sequence has been submitted to the GenBank databases and can be found under accession number BK007068
LYK3 is essential for nodulation of M. truncatula but occupies an enigmatic position in the Nod factors (NFs) signaling pathways as little is known of how it links to downstream responses
Summary
The ability of legumes to form a symbiosis with nitrogen-fixing bacteria termed rhizobia is of immense agronomic and ecological importance (Graham and Vance, 2003). The symbiosis is manifested by the production of nodules on the roots of the plant in which the bacteria fix nitrogen in exchange for plant carbohydrate and a safe niche. Genetic studies on both the bacterium and the plant have identified Nod factor signaling as an essential step in the establishment of the symbiosis in most, but not all studied legume–rhizobia interactions (Denarieet al., 1996; Oldroyd and Downie, 2008; Masson-Boivin et al, 2009). The structure of the fatty acid and the type of chemical substitutions on the chitin fragment backbone are important elements for host–microbe recognition, infection, and nodulation. The NFs produced by Sinorhizobium meliloti, which nodulates Medicago truncatula and related genera, are mainly
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