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Legume plants host nitrogen-fixing endosymbiotic Rhizobium bacteria in root nodules. In Medicago truncatula, the bacteria undergo an irreversible (terminal) differentiation mediated by hitherto unidentified plant factors. We demonstrated that these factors are nodule-specific cysteine-rich (NCR) peptides that are targeted to the bacteria and enter the bacterial membrane and cytosol Obstruction of NCR transport in the dnf1-1 signal peptidase mutant correlated with the absence of terminal bacterial differentiation. On the contrary, ectopic expression of NCRs in legumes devoid of NCRs or challenge of cultured rhizobia with peptides provoked symptoms of terminal differentiation. Because NCRs resemble antimicrobial peptides, our findings reveal a previously unknown innovation of the host plant, which adopts effectors of the innate immune system for symbiosis to manipulate the cell fate of endosymbiotic bacteria. 10.1126/science.1184057

Root endosymbioses vitally contribute to plant nutrition and fitness worldwide. Nitrogen-fixing root nodulation, confined to four plant orders, encompasses two distinct types of associations, the interaction of legumes (Fabales) with rhizobia bacteria and actinorhizal symbioses, where the bacterial symbionts are actinomycetes of the genus Frankia. Although several genetic components of the host-symbiont interaction have been identified in legumes, the genetic basis of actinorhiza formation is unknown. Here, we show that the receptor-like kinase gene SymRK, which is required for nodulation in legumes, is also necessary for actinorhiza formation in the tree Casuarina glauca. This indicates that both types of nodulation symbiosis share genetic components. Like several other legume genes involved in the interaction with rhizobia, SymRK is also required for the interaction with arbuscular mycorrhiza (AM) fungi. We show that SymRK is involved in AM formation in C. glauca as well and can restore both nodulation and AM symbioses in a Lotus japonicus symrk mutant. Taken together, our results demonstrate that SymRK functions as a vital component of the genetic basis for both plant-fungal and plant-bacterial endosymbioses and is conserved between legumes and actinorhiza-forming Fagales. actinorhizal symbioses | Casuarina glauca | mycorrhizae | signaling

Research has been conducted on the hematophagous glossiphoniid leeches. The symbiotic bacteria associated with these leeches have been characterized and the results of this molecular characterization are reported.

Photorhabdus luminescens, a bacterium with alternate pathogenic and symbiotic phases of its lifestyle, represents a source of novel genes associated with both virulence and symbiosis. This entomopathogen lives in a 'symbiosis of pathogens' with nematodes that invade insects. Thus the bacteria are symbiotic with entomopathogenic nematodes but become pathogenic on release from the nematode into the insect blood system. Within the insect, the bacteria need to both avoid the peptide- and cellular- (hemocyte) mediated immune response and also to kill the host, which then acts as a reservoir for bacterial and nematode reproduction. However, the mechanisms whereby Photorhabdus evades the insect immune system and kills the host are unclear. Here we show that a single large Photorhabdus gene, makes caterpillars floppy (mcf), is sufficient to allow Esherichia coli both to persist within and kill an insect. The predicted high molecular weight Mcf toxin has little similarity to other known protein sequences but carries a BH3 domain and triggers apoptosis in both insect hemocytes and the midgut epithelium. insecticide | Photorhabdus | pathogenicity | apoptosis | toxin

Phylogenetic and chromosomal gene analyses reveal a strictly vertical transmission of plasmids among Buchnera aphidicola strains associated with aphid hosts.

Results demonstrate three distinct intracellular symbiotic bacteria in the bamboo pseudococcid by a molecular phylogenetic analysis and in situ hybridization.

The squid symbiont Vibrio fischeri has only one gene for catalase, katA, the protein product of which is homologous to a defined group III catalases. This enzyme is located in the periplasm of Vibrio indicating that it may be responsible for detoxifying oxygen radicals from the environment. Mutants deficient in katA are unable to colonize the light organs of juvenile squids suggesting that the enzyme has an important function in the symbiotic relationship between Vibrio and its host.

The uptake of ammonium, free amino acids (FAA) and nitrate by mussels containing endosymbiotic methanotrophs were characterized. In the presence of methane, themussels utilized ammonium and FAA, but not nitrate. Ammonium may repress the ability to utilize nitrate. However, the results show that ammonium uptake can support the nitrogen needs of the mussel-bacteria system. Furthermore, FAA could also provide as much nitrogen as ammonium. Therefore, the nitrogen needs of the methane mussel symbiosis are met by ammonium, FAA as well as particulates.

The effects of flooding and nitrogen fertilization on ergot alkaloid production within the tall fescue-Acremonium coenophialum symbiotic interactionwere determined. The results suggest that flooding does not appreciably affect ergot alkaloid biosynthesis by the fungus. However, under moderate water deficit ergot synthesis is significantly affected by the type and rate of nitrogen available. Under drought conditions, nitrogen availability does not affect ergot biosynthesis because of decreased permeability of roots to nitrogen.

Research has been conducted on protist endosymbionts from the acid mine drainage. The authors have investigated these endosymbionts via the use of culture-independent approaches.

Results indicate that in the mealybugs population the primary endosymbionts are infected several times with different precursors of the secondary symbionts and that such an association is stable and leads to cospeciation. Evolutionary relationships of the primary and secondary symbionts of mealybug species are discussed.

Endosymbionts and their surrounding membranes have been dubbed `symbiosomes'. The boundary layer between the host cell and the symbiont is the symbiosome membrane. All cell to cell communication between the host and the symbiont must occur through this layer. Thus information on the origin and molecular structure of this layer is critical. In many dinoflagellate/cnidarian symbioses the symbiosome membrane consists of many layers of apparent membranes. This condition is evident in the association between Aiptasia pallida and its symbiont Symbiodinium bermudense. The origin of these membranes has been attributed to the host symbiont and to both by various authors. As to date no evidence has been presented to confirm any of these proposed origins. Using extracted symbiosomes as a source of antigen two monoclonal antibodies were produced an IgG and an IgM both of which show positive binding to the symbiosome membrane in immunofluorescent studies. Utilizing nanogold conjugated antibodies and the TEM we have demonstrated that the multiple layers of membranes closest to the symbiont are of symbiont origin while only the outer most membrane is supplied by the host.

Pogonophorans are gutless tube-dwelling worms that derive their nutritional needs from endosymbiotic bacteria. Although some recent studies have focused on the genetics of hydrothermal vent dwelling vestimentiferans, little is known about the genetic diversity or phylogenetic relationships of the perviate pogonophorans. Using a multi-gene approach (COI, 18S rDNA, 28S rDNA), the evolutionary history of pogonophorans was reconstructed using standard phylogenetic techniques. Interestingly, much more genetic diversity was found in the perviates than in the vestimentiferans. These reconstructions suggest that the vestimentiferans are nested within a paraphyletic clade of perviates. This finding adds further support to idea that Siboglinid evolution was driven by an increasing ability to adapt to reducing environments.