Abstract
Recognition of Nod factors by LysM receptors is crucial for nitrogen-fixing symbiosis in most legumes. The large families of LysM receptors in legumes suggest concerted functions, yet only NFR1 and NFR5 and their closest homologs are known to be required. Here we show that an epidermal LysM receptor (NFRe), ensures robust signalling in L. japonicus. Mutants of Nfre react to Nod factors with increased calcium spiking interval, reduced transcriptional response and fewer nodules in the presence of rhizobia. NFRe has an active kinase capable of phosphorylating NFR5, which in turn, controls NFRe downstream signalling. Our findings provide evidence for a more complex Nod factor signalling mechanism than previously anticipated. The spatio-temporal interplay between Nfre and Nfr1, and their divergent signalling through distinct kinases suggests the presence of an NFRe-mediated idling state keeping the epidermal cells of the expanding root system attuned to rhizobia.
Highlights
Perception of Nod factors by LysM receptor kinases, NFR1 and NFR5 in Lotus japonicus (Broghammer et al, 2012), triggers tightly coordinated events leading to root nodule symbiosis (Madsen et al, 2003; Radutoiu et al, 2003)
Some or several of these responses are retained in the Ljnfr1, Mtlyk3 and Pssym37 mutants (Radutoiu et al, 2003; Smit et al, 2007; Zhukov et al, 2008) raising the possibility that modular receptor complex formation regulated in a spatio-temporal manner might contribute to Nod factor
Pure protein was obtained after four steps of purification and the homogeneity was confirmed by size exclusion chromatography (Figure 1—figure supplement 2)
Summary
Perception of Nod factors by LysM receptor kinases, NFR1 and NFR5 in Lotus japonicus (Broghammer et al, 2012), triggers tightly coordinated events leading to root nodule symbiosis (Madsen et al, 2003; Radutoiu et al, 2003). Murakami et al found that when NRFe was mutated, the Nod factor signaling inside the root was less activated and fewer nodules formed, suggesting NRFe plays an important role in this symbiosis. We show that LYS1 is an epidermal LysM receptor contributing to the NFR1-NFR5 mediated signalling in a spatio-temporal manner This gene is primarily expressed in epidermal cells of the susceptible zone where roots are competent for initiation of symbiosis, and has a restricted signalling capacity leading to Nin activation in the outer root cell layers. Our findings provide evidence for a complex Nod factor signalling where LYS1 activity in the outer root cell layers aids in maintaining a normal calcium spiking interval in the root hairs, integral transcript responses in the susceptible root zone, and initiation of nodule primordia on the expanding root system. The Lys gene is renamed Nfre, in accordance with the identified role of this gene during Nod factor signalling in the epidermal layer
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