LysM domains mediate lipochitin-oligosaccharide recognition and Nfr genes extend the symbiotic host range.

Legume-Rhizobium symbiosis is an example of selective cell recognition controlled by host/non-host determinants. Individual bacterial strains have a distinct host range enabling nodulation of a limited set of legume species and vice versa. We show here that expression of Lotus japonicus Nfr1 and Nfr5 Nod-factor receptor genes in Medicago truncatula and L. filicaulis, extends their host range to include bacterial strains, Mesorhizobium loti or DZL, normally infecting L. japonicus. As a result, the symbiotic program is induced, nodules develop and infection threads are formed. Using L. japonicus mutants and domain swaps between L. japonicus and L. filicaulis NFR1 and NFR5, we further demonstrate that LysM domains of the NFR1 and NFR5 receptors mediate perception of the bacterial Nod-factor signal and that recognition depends on the structure of the lipochitin-oligosaccharide Nod-factor. We show that a single amino-acid variation in the LysM2 domain of NFR5 changes recognition of the Nod-factor synthesized by the DZL strain and suggests a possible binding site for bacterial lipochitin-oligosaccharide signal molecules.

A nucleoporin is required for induction of Ca2+ spiking in legume nodule development and essential for rhizobial and fungal symbiosis.

Nuclear-cytoplasmic partitioning and traffic between cytoplasmic and nuclear compartments are fundamental processes in eukaryotic cells. Nuclear pore complexes mediate transport of proteins, RNAs and ribonucleoprotein particles in and out of the nucleus. Here we present positional cloning of a plant nucleoporin gene, Nup133, essential for a symbiotic signal transduction pathway shared by Rhizobium bacteria and mycorrhizal fungi. Mutation of Nup133 results in a temperature sensitive nodulation deficient phenotype and absence of mycorrhizal colonization. Root nodules developing with reduced frequency at permissive temperatures are ineffective and electron microscopy show that Rhizobium bacteria are not released from infection threads. Measurement of ion fluxes using a calcium-sensitive dye show that Nup133 is required for the Ca2+ spiking normally detectable within minutes after application of purified rhizobial Nod-factor signal molecules to root hairs. Localization of NUP133 in the nuclear envelope of root cells and root hair cells shown with enhanced yellow fluorescent protein fusion proteins suggests a novel role for NUP133 nucleoporins in a rapid nuclear-cytoplasmic communication after host-plant recognition of symbiotic microbes. Our results identify a component of an intriguing signal process requiring interaction at the cell plasma membrane and at intracellular nuclear and plastid organelle-membranes to induce a second messenger.