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Although most land plants are hosts for arbuscular mycorrhizal fungi (AMF), a small number of plant families are arbuscular mycorrhizal (AM) nonhosts. There are indications that strigolactone levels in root exudates of AM nonhost plants are lower than in AM host plants, and it has been shown that in the strigolactone-deficient rmsl mutant (ccd8) of the AM host plant pea, the AMF colonization of roots is highly reduced. Application of the synthetic strigolactone analogue GR24 to this strigolactones-deficient mutant restored AMF colonization of roots. Our objective was to determine whether the application of GR24 to AM nonhost plants can affect their susceptibility to AMF. To test whether GR24 affects AMF colonization in our experimental system, we added GR24 to the strigolactone-deficient pea ccd8 mutant. Application of GR24 increased AMF colonization in the pea mutant to a similar level as in the pea wild type with normal strigolactone levels, showing clearly that in our experimental setup, application of the GR24 positively affects AMF colonization in strigolactone-deficient plants. Observation of cleared roots after application of GR24 to four AM nonhost plant species inoculated with the AMF Glomus intraradices showed that colonization did not occur. Key words: arbuscular mycorrhiza, AM nonhost plants, GR24, strigolactones. Bien que la plupart des plantes terrestres constituent des hotes pour les champignons mycorhiziens arbusculaires (CMA), un petit nombre de familles de plantes ne sont pas des hotes pour les mycorhizes arbusculaire (non hotes MA). Il y a des indications que les teneurs en strigolactones dans les exsudats des plantes non hotes MA seraient moins grandes que chez les plantes hotes MA, et on a egalement demontre que le mutant strigolactones-deficient-rmsl (ccd8) du pois plante hote MA, qu'il y a une forte reduction de la colonisation racinaire MA. L'application d'un analogue de la strigolactone GR24 sur ce mutant strigolactones-deficient restaure la colonisation MA des racines. Les auteurs ont cherche a determiner si l'application de GR24 aux plantes non hotes MA peut affecter leur susceptibilite aux CMA. A cette fin, ils ont verifie si le GR24 affecte la colonisation MA en utilisant leur systeme experimental et en mettant le mutant strigolactone- deficient ccd8 du pois en presence de GR24. L'application de GR24 a augmente la colonisation MA chez le pois mutant, au meme degre que chez le type sauvage du pois, possedant des teneurs normales en strigolactones, demontrant ainsi clairement que dans ce dispositif experimental le GR24 affecte positivement la colonisation MA chez les plants strigolactones- deficients. L'observation de racines eclaircies, apres l'application de GR24, sur quatre especes de plantes non hotes MA inoculees avec le CMA Glomus intraradices, montre que la colonisation n'a pas eu lieux. Mots-cles : mycorhizes arbusculaires, plante AM non hotes, GR24, strigolactones. [Traduit par la Redaction], Introduction Most land plants are arbuscular mycorrhizal fungi (AMF) host plants; however, some plant families (e.g., the Brassicaceae and the Chenopodiaceae) are nonhosts for AMF. Lupinus spp. are exceptions in [...]

Leguminous plants can enter into root nodule symbioses with nitrogen-fixing soil bacteria known as rhizobia. An intriguing but still poorly understood property of the symbiosis is its host specificity, which is controlled at multiple levels involving both rhizobial and host genes. It is widely believed that the host specificity is determined by specific recognition of bacterially derived Nod factors by the cognate host receptor(s), Here we describe the positional cloning of two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively. We show that Rj2 and Rfg1 are allelic genes encoding a member of the Toll-interleukin receptor/nucleotide-binding site/leucine-rich repeat (TIR-NBS-LRR) class of plant resistance (R) proteins. The involvement of host R genes in the control of genotype-specific infection and nodulation reveals a common recognition mechanism underlying symbiotic and pathogenic host-bacteria interactions and suggests the existence of their cognate avirulence genes derived from rhizobia. This study suggests that establishment of a root nodule symbiosis requires the evasion of plant immune responses triggered by rhizobial effectors. soybean | nodulation | nitrogen fixation | defense doi/ 10.1073/pnas.1011957107

The interactions between marine prokaryotic and eukaryotic microorganisms are crucial to many biological and biogeochemical processes in the oceans. Often the interactions are mutualistic, as in the symbiosis between phytoplankton, e.g., the dinoflagellate Pfiesteria piscicida and Silicibacter sp. TM1040, a member of the Roseobacter taxonomic lineage. It is hypothesized that an important component of this symbiosis is bacterial production of tropodithietic acid (TDA), a biologically active tropolone compound whose synthesis requires the expression of tdaABCDEF (tdaA-F), as well as six additional genes (cysI, malY, paaIJK, and tdaH). The factors controlling tda gene expression are not known, although growth in laboratory standing liquid cultures drastically increases TDA levels. In this report, we measured the transcription of tda genes to gain a greater understanding of the factors controlling their expression. While the expression of tdaAB was constitutive, tdaCDE and tdaF mRNA increased significantly (3.7- and 17.4-fold, respectively) when cells were grown in standing liquid broth compared to their levels with shaking liquid culturing. No transcription of tdaC was detected when a tdaCp::lacZ transcriptional fusion was placed in 11 of the 12 [Tda.sup.-] mutant backgrounds, with cysI being the sole exception. The expression of tdaC could be restored to 9 of the remaining 11 [Tda.sup.-] mutants--tdaA and tdaH failed to respond--by placing wild-type ([Tda.sup.+]) strains in close proximity or by supplying exogenous TDA to the mutant, suggesting that TDA induces tda gene expression. These results indicate that TDA acts as an autoinducer of its own synthesis and suggest that roseobacters may use TDA as a quorum signal. doi: 10.1128/JB.00410-10

The successful nitrogen-fixing symbiosis between the Gram-negative soil bacterium Sinorhizobium meliloti and its leguminous plant host alfalfa (Medicago sativa) requires the bacterial exopolysaccharide succinoglycan. Succinoglycan and flagellum production, along with the ability to metabolize more than 20 different carbon sources and control the expression of a large number of S. meliloti genes, is regulated by the ExoR--ExoS/Chvl signalling pathway. The ExoR protein interacts with and suppresses the sensing activities of ExoS, the membrane-bound sensor of the ExoS/Chvl two-component regulatory system. Here we show that exoR expression is clearly upregulated in the absence of any functional ExoR protein. This upregulation was suppressed by the presence of the wild-type ExoR protein but not by a mutated ExoR protein lacking signal peptide. The levels of exoR expression could be directly modified in real time by changing the levels of total ExoR protein. The expression of exoR was also upregulated by the constitutively active sensor mutation exoS96, and blocked by two single mutations, exoS* and [exoS.sub.supA], in the ExoS sensing domain. Presence of the wild-type ExoS protein further elevated the levels of exoR expression in the absence of functional ExoR protein, and reversed the effects of exoS96, exoS* and [exoS.sub.supA] mutations. Altogether, these data suggest that ExoR protein autoregulates exoR expression through the ExoS/Chvl system, allowing S. meliloti cells to maintain the levels of exoR expression based on the amount of total ExoR protein. DOI: 10.1099/mic.0.038547-0

Some strains of Phytophthora infestans, the potato late blight pathogen, harbour a small extrachromosomal RNA called PiERE1. A previous study reported that this RNA symbiont does not noticeably affect its host. Here it is revealed that PiERE1 exerts subtle effects on P. infestans, which result in greater thermotolerance during growth and an increase in secondary homothallism, i.e. oospore formation in the absence of the opposite mating type. The interaction can be considered mutualistic since these traits may increase the fitness of P. infestans in nature. Assays of biomarkers for cellular stress revealed that an Hsp70 chaperone was upregulated by PiERE1. A genome-wide search for more members of the Hsp70 family identified ten belonging to the DnaK subfamily, one in the Hsp110/SSE subfamily, and pseudogenes. Four DnaK subfamily genes encoding predicted cytoplasmic or endoplasmic reticulum proteins were upregulated in strains harbouring PiERE1. This may explain the greater thermotolerance conferred by the RNA element, and suggests that Hsp70 may be a useful biomarker for testing organisms for the cellular effects of symbiotic elements. DOI: 10.1099/mic.0.039305-0

Mechanisms for controlling symbiont populations are critical for maintaining the associations that exist between a host and its microbial partners. We describe here the transcriptional, metabolic, and ultrastructural characteristics of a diel rhythm that occurs in the symbiosis between the squid Euprymna scolopes and the luminous bacterium Vibrio fischeri. The rhythm is driven by the host's expulsion from its light-emitting organ of most of the symbiont population each day at dawn, The transcriptomes of both the host epithelium that supports the symbionts and the symbiont population itself were characterized and compared at four times over this daily cycle. The greatest fluctuation in gene expression of both partners occurred as the day began. Most notable was an up-regulation in the host of >50 cytoskeleton-related genes just before dawn and their subsequent down-regulation within 6 h. Examination of the epithelium by TEM revealed a corresponding restructuring, characterized by effacement and blebbing of its apical surface. After the dawn expulsion, the epithelium reestablished its polarity, and the residual symbionts began growing, repopulating the light organ. Analysis of the symbiont transcriptome suggested that the bacteria respond to the effacement by up-regulating genes associated with anaerobic respiration of glycerol; supporting this finding, lipid analysis of the symbionts' membranes indicated a direct incorporation of host-derived fatty acids. After 12 h, the metabolic signature of the symbiont population shifted to one characteristic of chitin fermentation, which continued until the following dawn. Thus, the persistent maintenance of the squid-vibrio symbiosis is tied to a dynamic diel rhythm that involves both partners. Euprymna scolopes | microarray | mutualism | Vibrio fischeri | cytoskeleton doi/10.1073/pnas.0909712107

The Sinorhizobium meliloti ExoS/ChvI two-component signaling pathway is required for the development of a nitrogen-fixing symbiosis between S. meliloti and its plant hosts. ExoS/ChvI also has important roles in regulating succinoglycan production, biofilm formation, motility, nutrient utilization, and the viability of free-living bacteria. Previous microarray experiments with an exoS96::Tn5 mutant indicated that ExoS/ChvI influences the expression of a few hundred genes, complicating the investigation of which downstream genes respond directly or indirectly to ExoS/ChvI regulation. To focus our study of ExoS/ChvI transcriptional target genes, we performed transcriptional profiling with chvI gain-of-function and reduced-function strains. The chvI gain-of-function strain that we used contains a dominant gain-of-function chvI allele in addition to wild-type chvI. We identified genes that, relative to their expression level in the wild type, are both upregulated in the chvI gain-of-function strain and downregulated in the reduced-function strain or vice versa. Guided by this focused set of genes, we performed gel mobility shift assays and demonstrated that ChvI directly binds the intergenic regions upstream of ropB1, SMb21440, and SMc01580. Furthermore, DNase I footprint analysis of the region upstream of SMc01580 identified a specific DNA sequence bound by ChvI and allowed the discovery of a possible motif for ChvI binding. Our results provide insight into the mechanism of how ExoS/ChvI regulates its downstream targets and lay a foundation for studying this conserved pathway with critical roles in free-living and symbiotic bacteria. doi: 10.1128/JB.00734-09

The ExpR/Sin quorum-sensing system of the gram-negative soil bacterium Sinorhizobium meliloti plays an important role in the establishment of symbiosis with its host plant Medicago sativa. A mutant unable to produce autoinducer signal molecules (sinI) is deficient in its ability to invade the host, but paradoxically, a strain lacking the quorum-sensing transcriptional regulator ExpR is as efficient as the wild type. We compared the whole-genome expression profile of the wild-type strain with strains missing one of the quorum-sensing regulatory components to identify genes controlled by the ExpR/Sin system throughout the different phases of the bacterial growth cycle, as well as in planta. Our analyses revealed that ExpR is a highly versatile regulator with a unique ability to show different regulatory capabilities in the presence or absence of an autoinducer. In addition, this study provided us with insight into the plant invasion defect displayed by the autoinducer mutant. We also discovered that the ExpR/Sin quorum-sensing system is repressed after plant invasion. Therefore, quorum sensing plays a crucial role in the regulation of many cell functions that ensures the successful invasion of the host and is inactivated once symbiosis is established.

We report here the first characterization of the Sinorhizobium meliloti open reading frame SMc01113. The SMc01113 protein is a member of a highly conserved protein family, universal among bacteria. We demonstrate that the SMc01113 gene is absolutely required for S. meliloti symbiosis with alfalfa and also for the protection of the bacterium from a wide range of environmental stresses.

Obligate symbioses with nutrient-provisioning bacteria have originated often during animal evolution and have been key to the ecological diversification of many invertebrate groups. To date, genome sequences of insect nutritional symbionts have been restricted to a related cluster within Gammaproteobacteria and have revealed distinctive features, including extreme reduction, rapid evolution, and biased nucleotide composition. Using recently developed sequencing technologies, we show that Sulcia muelleri, a member of the Bacteroidetes, underwent similar genomic changes during coevolution with its sap-feeding insect host (sharpshooters) and the coresident symbiont Baumannia cicadellinicola (Gammaproteobacteria). At 245 kilobases, Sulcia's genome is approximately one tenth of the smallest known Bacteroidetes genome and among the smallest for any cellular organism. Analysis of the coding capacities of Sulcia and Baumannia reveals striking complementarity in metabolic capabilities. Bacteroidetes| insects | pyrosequencing | Sharpshooters | genome reduction

Legumes form symbiotic associations with both mycorrhizal fungi and nitrogen-fixing soil bacteria called rhizobia. Several of the plant genes required for transduction of rhizobial signals, the Nod factors, are also necessary for mycorrhizal symbiosis. Here, we describe the cloning and characterization of one such gene from the legume Medicago truncatula. The DMI1 (does not make infections) gene encodes a novel protein with low global similarity to a ligand-gated cation channel domain of archaea. The protein is highly conserved in angiosperms and ancestral to land plants. We suggest that DMI1 represents an ancient plant-specific innovation, potentially enabling mycorrhizal associations., Mycorrhizal and rhizobial associations represent the two most important symbiotic relations between higher plants and microorganisms, providing access to otherwise limiting supplies of phosphate and nitrogen, respectively. Although better characterized [...]

Results reveal that BacA protein function affects the bacterial cell envelope structural components including lipopolysaccharide in Sinorhizobium meliloti. Data suggest that such cell envelope changes appear to be essential for the bacteium to establish infection and symbiosis.

The authors review the publications on the rhizobium-plant interactions. The topics of interest include NH (sub)4 (super)+ as a regulatory signal in symbiosis and NH (sub)4 (super)+ metabolism.

Results reveal that the endogenous ner gene is not involved in the secondary phenotype switching but inactivation of the primary form ner affects outer membrane protein profile in Photorhabdus. Data show accumulation of outer membrane proteins in the stationary phase.

A comparative analysis of the sequence of R7A symbiosis island with the putative strain MAFF303099 symbiosis island uncovers novel symbiotic genes, which have bearing on the evolution of the two mesorhizobium loti symbiosis islands. Data indicate that most of the 414 genes of the R7A island are strain-specific and clustered.

Buchnera is a mutualistic intracellular symbiont of aphids. Their association began about 200 million years ago, with host and symbiont lineages evolving in parallel since that time. During this coevolutionary process, Buchnera has experienced a dramatic decrease of genome size, retaining only essential genes for its specialized lifestyle. Previous studies reported that genome size in Buchnera spp. is very uniform, suggesting that genome shrinkage occurred early in evolution, and that modern lineages retain the genome size of a common ancestor. Our physical mapping of Buchnera genomes obtained from five aphid lineages shows that the genome size is not conserved among them, but has been reduced down to 450 kb in some species. Here we show evidence of six species with a genome size smaller than Mycoplasma genitalium, the smallest bacterial genome reported thus far (580 kb). Our findings strongly suggest that the Buchnera genome is still experiencing a reductive process toward a minimum set of genes necessary for its symbiotic lifestyle.

Research describes the population dynamics associated with double and single Wolbachia infections in Drosophila simulans. Data reveal that the population predominantly consists of doubly infected flies and both male and female components are involved in the phenotypic expression of incompatibility. Furhter, infection status does not correlate with mitochondrial sequence polymorphism.

Results demonstrate that the induction of fixNOQPd operon in Rhizobium etli involves a regulatory circuit consisting of at least three transcriptional regulators of the cyclic AMP receptor protein-Fnr family. Data indicate that the regulatory pathway plays an important role in the optimal production of terminal oxidase during symbiosis.

Research describes the Sodalis glossinidius genomic size, its overall coding capacity, and the plasmid it harbors. Inflence of symbiotic association with the tsetse fly on the genomic size and content of the bacterium is also discussed.

The Mesorhizobium sp. strain R7A bio operon on its acquired symbiosis island of has been found to include a novel gene that is part of the pimeloyl-coA synthesis. The reason the vitamin biosynthetic loci are on an acquired genetic element not in nonsymbiotic mesorhizobia is not yet known. The symbiosis island is a 500-kb chromosomal genetic elements that when transferred converts nonsymbiotic mesorhizobia to symbionts that can nodulate and fix nitrogen with Lotus corniculatus. A 13.2-kb region of the island that changed the nonsymbionts to nicotinate and biotin prototrophy has been characterized.

A genetic analysis of a 410-kilobase region of Bradyrhizobium japonicum chromosome carrying nitrogen fixation and nodulation genes is presented. Data reveal that the fragment contains all nitrogen fixation genes and 35% of proteins encoded by the fragment resemble rhizobial proteins.

Research demonstrates hemN-like gene (hemN) 1 and 2 in Bradyrhizobium japonicum involved in the heme biosynthesis in bacteria. Mutants of only hemN1 gene are able to grow anaerobically under nitrate-respiring conditions and fix nitrogen in symbiotic association with soybean.

Research shows that a transposon-induced mutant of rhizobium tropici encode a product very similar to glycogen synthase and lacks glycogen. The mutant exhibits a decrease in high-molecular weight exopolysaccharide and increased symbiotic nitrogen fixation and respiration.

Analysis of Euprymna scolopes mutant, which is defective in symbiotic association traits, exhibits poor colonization by the marine bacterium Vibrio fischeri due to a genetic barrier. Sequence analysis reveals that the inactivated locus, rscS, in the mutant encodes a protein resembling a hybrid two-component regulatory sensor system.

Causes of male-killing have been studied in a sample of the two-spot ladybird, Adalia bipunctata (Coleoptera: Coccinellidae) from Russia. Thirty-six matrilines were tested looking for the four male-killing symbionts that are known to infect the species. All four were found. They are Spiroplasma, Rickettsia, and two Wolbachia strains. Possible explanations are offered.

Results reveal that the nitrogen fixation regulatory protein NifA is an activator of three other genes, termed nrg for NifA-regulated gene, besides being an activator of nitrogen fixation genes in Bradyrhizobium japonicum as shown by competitive DNA-RNA hybridization.

An investigation of mitochondrial DNA variability and of Wolbachia infection carried out in two sibling woodlice species in seven populations is discussed. Host reproductive changes caused by Wolbachia enhance spread of a symbiont in infected populations and the infected cytoplasm goes up in frequency, hitchiking, as it were, at the expense of uninfected cytoplasm and usually reducing genetic polymorphism.

Bacterial genetic expression is influenced by changing environmental conditions as shown by a study which characterized the regulation of the genetic expression of Rhizobium etli rpoN2 gene. The rpoN2 gene clustered with nitrogen fixation genes was localized approximately 3.5 kb upstream from the R. etli fixA gene. The genetic transcription was highly active in free-living microaerobic conditions which imply that a symbiosis-specific signal regulator is behind the regulation.

Two isolates of the root-nodule bacteria Bradyrhizobium sp. were inoculated on three lineages and the lineage crosses of the legume Amphicarpea bracteata to determine the symbiotic variability among the three strains of A. bracteata. Results revealed that a single genetic locus control the formation of nodule between the narrow host-range Bradyrhizobium and the A. bracteata lineage able to sustain this bacteria.

Fluorescent pseudomonads from the micorrhizosphere of Douglas fir with Liccaria bicolor were analyzed to determine the effects of the Douglas fir mycorrhizosphere on the phenotypic traits of the pseudomonads. Genotypic characterization of isolates of Pseudomonas fluorescens from Douglas fir indicated the stimulatory effects of the mycorrhizosphere on the size of the populations of fluorescent pseudomonads in the soil. The micorrhizosphere of Douglas fir also counterselected Pseudomonas fluorescens V and II strains compared to Pseudomonas fluorescens bv. I strains.

The chromosomal katA gene of Rhizobium meliloti was cloned by utilizing a catalase-specific deoxyribonucleic acid probe to determine the role of KatA peroxide-inducible catalase in nodulation and plant-microbe symbiosis. The katA gene is composed of an extended N-terminal arm with C-terminal alpha helices that contains a beta-barrel core. The KatA monofunctional catalase that is encoded by the gene acts as a major component of the adaptive response which facilitates nodulation and Medicago-Rhizobium symbiosis.

Rhizobium fredii USDA257 is a Gram-negative soil bacterium from China that fixes nitrogen in symbiosis with primitive soybean (Glycine max (L.) Merr.) cultivars such as Peking, but not with advanced cultivars such as McCall. Mutation of either of two bacterial loci, nolBTUVWX or nolC, removes this cultivar-specificity constraint, allowing the bacterium to nodulate advanced cultivars. We have discovered that USDA257 forms nodules on Erythrina costaricensis M. Micheli, but not on six other species of this genus, which includes trees and shrubs from the tropics. Inactivation of nolBU or nolC broadens the symbiosis to include other Erythrina species. Whereas nolBU-mutants acquire the capacity to nodulate E. fusca Loureiro, E. variegata L., and E. vespertilio Benth., nolC-mutants could nodulate only E. variegata. Nodules are determinate and often clustered in root axils near the root crown. E. variegata nodules containing a nolBU-mutant consist of well-developed inner and outer cortical layers, as well as a bacteroid zone. The cortical layers are separated from one another by a meristematic region that is associated with large darkly staining cells, and cells of the inner cortex contain numerous starch grains. Histochemical staining of E. variegata roots inoculated with a strain containing a nolB-lacZ gene fusion confirms that nolB and nolC are expressed during the initial stages of bacterial interaction with the host. R. fredii enters elongated root-hairs via infection threads, which ramify within root-hairs and are associated with meristematic activity in the adjacent underlying cortical cells.

The NodD regulatory protein acts in conjunction with plant signals to positively regulate the expression of nodulation genes. The nodD gene was used to probe the DNA of Rhizobium tropici CIAT899 and Leucaena leucocephala BR816. The results showed that presence of four copies of nodD in BR816 and five copies in CIAT899. These results were confirmed by cloning experiments. The implications of multiple copies of nodD genes to the broad host range of these species are discussed.

The contribution of each of the plasmids (pa to pf) of Rhizobium leguminosarum bv. phaseoli to symbiotic and growth functions was determined. Plasmid pd that carried nod and nif genes and plasmid pb that carried genes involved in LPS production were indispensable for nodulation. The plasmid-cured (pb, pc, pe and pf) and deleted (pe) derivatives had reduced growth rates in minimal and rich media than the wild-type strains. They were also less competitive, although complementation with corresponding wild-type plasmids restored their competitiveness.

The Rhizobium leguminosarum biovar vicae rhiA-rhiR region comprises plasmid-encoded genes which are expressed in the rhizosphere during symbiotic interactions. This region was characterized molecularly, resulting in the identification of four genes, rhiABCR. Sequence analysis showed that RhiA is similar to the E. coli lactose carrier encoded by lacY. In addition, RhiR is similar to the bacterial proteins LuxR and UvrC-28K. Their sequences also suggest that RhiA and RhiB are cytoplasmic, while RhiC is periplasmic. These genes are probably involved in the early stages of symbiotic interaction between the rhizobium and its host.