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NAD1 Controls Defense-Like Responses in Medicago truncatula Symbiotic Nitrogen Fixing Nodules Following Rhizobial Colonization in a BacA-Independent Manner.

ABSTRACT: Legumes form endosymbiotic interaction with host compatible rhizobia, resulting in the development of nitrogen-fixing root nodules. Within symbiotic nodules, rhizobia are intracellularly accommodated in plant-derived membrane compartments, termed symbiosomes. In mature nodule, the massively colonized cells tolerate the existence of rhizobia without manifestation of visible defense responses, indicating the suppression of plant immunity in the nodule in the favur of the symbiotic partner. Medicago truncatulaDNF2 (defective in nitrogen fixation 2) and NAD1 (nodules with activated defense 1) genes are essential for the control of plant defense during the colonization of the nitrogen-fixing nodule and are required for bacteroid persistence. The previously identified nodule-specific NAD1 gene encodes a protein of unknown function. Herein, we present the analysis of novel NAD1 mutant alleles to better understand the function of NAD1 in the repression of immune responses in symbiotic nodules. By exploiting the advantage of plant double and rhizobial mutants defective in establishing nitrogen-fixing symbiotic interaction, we show that NAD1 functions following the release of rhizobia from the infection threads and colonization of nodule cells. The suppression of plant defense is self-dependent of the differentiation status of the rhizobia. The corresponding phenotype of nad1 and dnf2 mutants and the similarity in the induction of defense-associated genes in both mutants suggest that NAD1 and DNF2 operate close together in the same pathway controlling defense responses in symbiotic nodules.

SUBMITTER: Domonkos A

PROVIDER: S-EPMC5748705 | biostudies-literature | 2017 Dec

REPOSITORIES: biostudies-literature

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Project description:Symbiotic bacteria (rhizobia) are maintained and conditioned to fix atmospheric nitrogen in infected cells of legume root nodules. Rhizobia are confined to the asymmetrical protrusions of plasma membrane (PM): infection threads (IT), cell wall-free unwalled droplets and symbiosomes. These compartments rapidly increase in surface and volume due to the microsymbiont expansion, and remarkably, the membrane resources of the host cells are targeted to interface membrane quite precisely. We hypothesized that the change in the membrane tension around the expanding microsymbionts creates a vector for membrane traffic toward the symbiotic interface. To test this hypothesis, we selected calcium sensors from the group of synaptotagmins: MtSyt1, Medicago truncatula homolog of AtSYT1 from Arabidopsis thaliana known to be involved in membrane repair, and two other homologs expressed in root nodules: MtSyt2 and MtSyt3. Here we show that MtSyt1, MtSyt2, and MtSyt3 are expressed in the expanding cells of the meristem, zone of infection and proximal cell layers of zone of nitrogen fixation (MtSyt1, MtSyt3). All three GFP-tagged proteins delineate the interface membrane of IT and unwalled droplets and create a subcompartments of PM surrounding these structures. The localization of MtSyt1 by EM immunogold labeling has shown the signal on symbiosome membrane and endoplasmic reticulum (ER). To specify the role of synaptotagmins in interface membrane formation, we compared the localization of MtSyt1, MtSyt3 and exocyst subunit EXO70i, involved in the tethering of post-Golgi secretory vesicles and operational in tip growth. The localization of EXO70i in root nodules and arbusculated roots was strictly associated with the tips of IT and the tips of arbuscular fine branches, but the distribution of synaptotagmins on membrane subcompartments was broader and includes lateral parts of IT, the membrane of unwalled droplets as well as the symbiosomes. The double silencing of synaptotagmins caused a delay in rhizobia release and blocks symbiosome maturation confirming the functional role of synaptotagmins. IN CONCLUSION:synaptotagmin-dependent membrane fusion along with tip-targeted exocytosis is operational in the formation of symbiotic interface.

Project description:BackgroundRhizobia are soil bacteria that establish symbiotic relationships with legumes and fix nitrogen in root nodules. We recently reported that several nitrogen-fixing rhizobial strains, belonging to Rhizobium phaseoli, R. trifolii, R. grahamii and Sinorhizobium americanum, were able to colonize Phaseolus vulgaris (common bean) seeds. To gain further insight into the traits that support this ability, we analyzed the genomic sequences and proteomes of R. phaseoli (CCGM1) and S. americanum (CCGM7) strains from seeds and compared them with those of the closely related strains CIAT652 and CFNEI73, respectively, isolated only from nodules.ResultsIn a fine structural study of the S. americanum genomes, the chromosomes, megaplasmids and symbiotic plasmids were highly conserved and syntenic, with the exception of the smaller plasmid, which appeared unrelated. The symbiotic tract of CCGM7 appeared more disperse, possibly due to the action of transposases. The chromosomes of seed strains had less transposases and strain-specific genes. The seed strains CCGM1 and CCGM7 shared about half of their genomes with their closest strains (3353 and 3472 orthologs respectively), but a large fraction of the rest also had homology with other rhizobia. They contained 315 and 204 strain-specific genes, respectively, particularly abundant in the functions of transcription, motility, energy generation and cofactor biosynthesis. The proteomes of seed and nodule strains were obtained and showed a particular profile for each of the strains. About 82 % of the proteins in the comparisons appeared similar. Forty of the most abundant proteins in each strain were identified; these proteins in seed strains were involved in stress responses and coenzyme and cofactor biosynthesis and in the nodule strains mainly in central processes. Only 3 % of the abundant proteins had hypothetical functions.ConclusionsFunctions that were enriched in the genomes and proteomes of seed strains possibly participate in the successful occupancy of the new niche. The genome of the strains had features possibly related to their presence in the seeds. This study helps to understand traits of rhizobia involved in seed adaptation.

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NAD1 Controls Defense-Like Responses in Medicago truncatula Symbiotic Nitrogen Fixing Nodules Following Rhizobial Colonization in a BacA-Independent Manner.

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