Characterization of the complete plastome of Ophrys aveyronensis , a Euro-Mediterranean orchid with an intriguing disjunct geographic distribution

International audienc

Fumarate and nitrate reduction (FNR) dependent activation of the Escherichia coli anaerobic ribonucleotide reductase nrdDG promoter

The nrdDG promoter regulates transcriptional expression of the anaerobic ribonucleotide reductase of Escherichia coli, an essential enzyme required to supply the building blocks for DNA synthesis. In this work, binding of the pleiotropic FNR (fumarate and nitrate reduction) transcriptional regulator to the nrdDG promoter region and the effects of binding on transcription were investigated. Gel retardation analysis with purified FNR* demonstrated FNR interaction at two FNR sites, termed FNR-2 and FNR-1, while studies with altered FNR boxes indicated that the upstream FNR-2 site was essential for anaerobic activation of the nrdDG promoter. Although the FNR-1 site was not absolutely required, it allowed maximal expression of this promoter. These results suggest that the two sites have an additive effect in coordinating nrdDG expression in response to shifting oxygen concentrations. [Int Microbiol 2008; 11(1):49-56

Fumarate and nitrate reduction (FNR) dependent activation of the Escherichia coli anaerobic ribonucleotide reductase nrdDG promoter

The nrdDG promoter regulates transcriptional expression of the anaerobic ribonucleotide reductase of Escherichia coli, an essential enzyme required to supply the building blocks for DNA synthesis. In this work, binding of the pleiotropic FNR (fumarate and nitrate reduction) transcriptional regulator to the nrdDG promoter region and the effects of binding on transcription were investigated. Gel retardation analysis with purified FNR* demonstrated FNR interaction at two FNR sites, termed FNR-2 and FNR-1, while studies with altered FNR boxes indicated that the upstream FNR-2 site was essential for anaerobic activation of the nrdDG promoter. Although the FNR-1 site was not absolutely required, it allowed maximal expression of this promoter. These results suggest that the two sites have an additive effect in coordinating nrdDG expression in response to shifting oxygen concentrations

Ribonucleotide reductases of Salmonella Typhimurium : transcriptional regulation and differential role in pathogenesis

Ribonucleotide reductases (RNRs) are essential enzymes that carry out the de novo synthesis of deoxyribonucleotides by reducing ribonucleotides. There are three different classes of RNRs (I, II and III), all having different oxygen dependency and biochemical characteristics. Salmonella enterica serovar Typhimurium (S. Typhimurium) harbors class Ia, class Ib and class III RNRs in its genome. We have studied the transcriptional regulation of these three RNR classes in S. Typhimurium as well as their differential function during infection of macrophage and epithelial cells. Deletion of both NrdR and Fur, two main transcriptional regulators, indicates that Fur specifically represses the class Ib enzyme and that NrdR acts as a global repressor of all three classes. A Fur recognition sequence within the nrdHIEF promoter has also been described and confirmed by electrophoretic mobility shift assays (EMSA). In order to elucidate the role of each RNR class during infection, S. Typhimurium single and double RNR mutants (as well as Fur and NrdR mutants) were used in infection assays with macrophage and epithelial cell lines. Our results indicate class Ia to be mainly responsible for deoxyribonucleotide production during invasion and proliferation inside macrophages and epithelial cells. Neither class Ib nor class III seem to be essential for growth under these conditions. However, class Ib is able to maintain certain growth in an nrdAB mutant during the first hours of macrophage infection. Our results suggest that, during the early stages of macrophage infection, class Ib may contribute to deoxyribonucleotide synthesis by means of both an NrdR and a Fur-dependent derepression of nrdHIEF due to hydrogen peroxide production and DNA damage associated with the oxidative burst, thus helping to overcome the host defenses

Endophyte infection of Festuca eskia enhances seedling survival to drought and cutting at the expense of clonal expansion

Aims Symbiotic relationships between fungal endophytes and grass species are known to increase stress resistance in the grass host; however, there is little evidence to suggest that the positive effects occur early in the grass life cycle. In this study, we explored the effects of the endophyte Epichloë festucae on the growth and survival of Festuca eskia seedlings under drought and frequent cutting stress. Methods Festuca eskia seedlings were collected from the western part of the plant repartition area in a non-symbiotic population located in a mesic and heavily grazed site (W-NS) and from the eastern part in a symbiotic population from a xeric and lightly grazed site (E-S). The E-S population was experimentally freed from its endophyte (E-F). Two greenhouse experiments were conducted to compare growth and survival between the three seedling types under drought stress and frequent cutting. In the first experiment, 126 seedlings per seedling type (n = 378) were grown for 6 weeks under non-limiting conditions before the cessation of watering. After 3 weeks without irrigation, full irrigation was restored for 10 days to measure the survival rate. Leaf length, leaf elongation rate and survival rate were assessed per population. In the second experiment, 156 seedlings per seedling type (n = 468) were grown under non-limiting conditions. All seedlings were cut to 3 cm high, twice a week, during the first month of growth. Leaf elongation and tillering were monitored on 52 seedlings per seedling type. For each type, seedling survival rate was determined by the number of plants alive after 10 days of regrowth, without cutting.Important FindingsThe drought experiment revealed a phenotypic differentiation to drought in 30 F. eskia populations, suggesting adaptive differentiation: the eastern seedlings showed the highest survival rate. A trade-off between growth and survival was highlighted: the highest drought survival rate was associated with the lowest leaf elongation rate under non-limiting water conditions. Endophyte presence in the eastern population increased seedling drought survival. In contrast, cutting survival rate was similar between W-NS and E-S because the presence of the endophyte increased seedling survival to frequent cutting. However, this positive effect came with a cost: the endophyte reduced seedling tillering rate.8 page(s

Genetically based vertical transmission drives the frequency of the symbiosis between grasses and systemic fungal endophytes

Understanding the variation in hereditary symbiont frequency among host populations is a prerequisite to predict symbiont fixation processes. However, the mechanisms driving this variation remain elusive. Exploring the mechanisms responsible for the observed variability on an ecological time scale requires simultaneous study of fitness differentials between symbiotic (S) and non-symbiotic (NS) hosts, and the symbiont transmission rates to host offspring. We studied these two key mechanisms using a grass-endophyte symbiosis in the alpine grass Festuca eskia. Plants from four native populations varying in endophyte frequencies (ranging from 13% to 100%), and environmental conditions (water availability, and grazing pressure) were transplanted in a common garden. Soil nutrient levels were manipulated to assess genetic and environmental differences within and among populations in fitness-related traits (i.e. traits linked to clonal growth, sexual reproduction and resource acquisition). A fitness differential favouring S over NS plants was detected in all studied populations: in heavily grazed populations, sexual reproduction was higher in S compared with NS plants, whereas in minimally grazed populations, clonal growth increased. Results showed a positive correlation between endophyte transmission rates and population endophyte symbiotic frequencies. The population endophyte transmission rates were not affected by soil resource level. According to selection pressures acting in each population, symbiotic plants appear to perform better in all F. eskia populations. The correlation between endophyte frequencies and transmission, and the positive effect of S on NS plants under our experimental conditions, indicated a predominant role of endophyte transmission in endophyte frequency variation in F. eskia. The endophyte transmission rate variation is genetically based at the population level and can be explained by a trade-off with a specific host trait subjected to strong selection; here, we suspected traits linked to plant resource acquisition. Synthesis: Our study provides evidence for the (i) dominant role of endophyte transmission and its responsibility for endophyte frequency variation in a native grass when compared with fitness differential process between S and NS plants at an ecological time scale, and (ii) genetically based variation in endophyte transmission rates. We also confirm the population specificity of positive endophyte effects in a native grass. Theoretically, two mechanisms might contribute to variation in hereditary symbiont frequency among host populations: the impact of symbioses on host fitness, and symbiont transmission to offspring. We studied both mechanisms using a grass-endophyte symbiosis in a native grass.10 page(s

Interplay between endophyte prevalence, effects and transmission : insights from a natural grass population

Two main mechanisms are thought to affect the prevalence of endophyte-grass symbiosis in host populations: the mode of endophyte transmission, and the fitness differential between symbiotic and non-symbiotic plants. These mechanisms have mostly been studied in synthetic grass populations. If we are to improve our understanding of the ecological and evolutionary dynamics of such symbioses, we now need to determine the combinations of mechanisms actually operating in the wild, in populations shaped by evolutionary history. We used a demographic population modeling approach to identify the mechanisms operating in a natural stand of an intermediate population (i.e. 50% of plants symbiotic) of the native grass Festuca eskia. We recorded demographic data in the wild over a period of three years, with manipulation of the soil resources for half the population. We developed two stage-structured matrix population models. The first model concerned either symbiotic or non-symbiotic plants. The second model included both symbiotic and non-symbiotic plants and took endophyte transmission rates into account. According to our models, symbiotic had a significantly higher population growth rate than non-symbiotic plants, and endophyte prevalence was about 58%. Endophyte transmission rates were about 0.67 or 0.87, depending on the growth stage considered. In the presence of nutrient supplementation, population growth rates were still significantly higher for symbiotic than for non-symbiotic plants, but endophyte prevalence fell to 0%. At vertical transmission rates below 0.10–0.20, no symbiosis was observed. Our models showed that a positive benefit of the endophyte and vertical transmission rates of about 0.6 could lead to the coexistence of symbiotic and non-symbiotic F. eskia plants. The positive effect of the symbiont on host is not systematically associated with high transmission rates of the symbiont over short time scales, in particular following an environmental change.17 page(s

Endophytic fungus fine-tunes the persistence strategy of its alpine host grass in response to soil resource levels

An understanding of hereditary endophytic fungi, and the effects on grass persistence strategies (i.e. relative investment in sexual reproduction and vegetative growth) under natural conditions may help to predict how some alpine ecosystems will respond to environmental change. Grass persistence and endophyte maintenance in host populations are closely related, but could become independent due to endophyte loss mechanisms. We used native grass and endophyte populations to test the hypothesis that fungal endophytes manipulate grass persistence strategies to secure endophyte maintenance in plant populations. Two conditions were required to verify this hypothesis: 1) the fungus caused alterations in host plant strategies; and 2) plant phenotypic changes induced by the fungal endophyte increased endophyte transmission. We compared symbiotic (S) and non-symbiotic (NS) persistence strategies of Festuca eskia (Poaceae), an alpine grass infected by the asexual form of the fungal endophyte Epichloë festucae. We characterised endophyte transmission efficiency, and described vegetative growth and sexual reproduction in a field population that naturally supports approximately 50% S plants. We built a demographic model to estimate plant vegetative growth rates. A correlation between plant persistence strategy, and fungal maintenance was evaluated by increasing soil resource levels. Under natural conditions, S and NS plants exploited different persistence strategies in the same population; S plants exhibited greater vegetative growth than their NS counterparts, while maintaining the same reproductive output. In response to higher soil resource levels, S plants shifted in persistence strategies and phenology, whereas NS plants maintained the same strategies. Therefore, results suggested the fungal endophyte fine-tuned host persistence strategies according to soil resource level. Finally, we found no direct relationship between the changes induced by fungal endophyte and endophyte transmission. Consequently, fungal endophytes affected host persistence strategies, but did not directly increase endophyte transmission.10 page(s

Endophytic symbiosis fine-tunes the persistence strategy of its alpine host grass in response to edaphic resource levels

National audienc

Teamwork, soft skills, and research training

We provide a list of soft skills that are important for collaboration and teamwork, based on our own experience and from an opinion survey of team leaders. Each skill can be learned to some extent. We also outline workable short courses for graduate schools to strengthen teamwork and collaboration skills among research students.4 page(s