Etude des mécanismes de résistance de la plante modèle Medicago truncatula vis-à-vis de deux agents pathogènes majeurs des légumineuses cultivées (Phoma medicaginis et Aphanomyces euteiches)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Effects of hydroponic culture system and NaCl on interactions between common bean lines and native rhizobia from Tunisian soils

Common bean (Phaseolus vulgaris L.) lines, namely BAT477, CocoT, Flamingo, DOR364 and NAG310, were inoculated with reference strain Rhizobium tropici CIAT899 or native rhizobia, namely Sinorhizobium fredii 1a6 and R. etli 12a3 and then grown in two hydroponic semi-sterile systems: (i) gravel in pots, (ii) aerated nutrient solution in bottles. In the aerated solution system, shoot plant growth was 5 to 9 times higher, depending on the symbiosis, than on gravel. A strain $\times$ line interaction was observed in the gravel system. However, such an interaction was not significant in the aerated system because of large intra-treatment variability. In the aerated system and under NaCl treatment, BAT477 inoculated with local strains (1a6 and 12a3) appeared to be a highly tolerant line. In contrast, NAG310 was the least tolerant line when inoculated by CIAT899. However, the tolerance of NAG310 to NaCl increased when it was inoculated with 12a3, although this strain conferred low nodule growth to Flamingo under NaCl. It is concluded that strain $\times$ line interactions are important and should be considered for selecting the legume most adapted to such an environmental constraint as salinity, and that the aerated solution system is efficient for selecting highly efficient rhizobial symbioses.Effets du système de culture hydroponique et de NaCl sur les interactions entre lignées de haricot et des rhizobia natifs des sols de Tunisie. Les lignées de haricot (Phaseolus vulgaris L.) BAT477, CocoT, Flamingo, DOR364 et NAG310 ont été inoculées avec la souche de référence CIAT899 (Rhizobium tropici) et celles locales 1a6 (R. fredii) et 12a3 (R. etli), et cultivées sur deux systèmes hydroponiques semi-stériles : (i) en pots sur gravier et (ii) en flacons sur solution nutritive aérée. La croissance des parties aériennes est 5 à 9 fois supérieure dans le milieu hydroponique aéré par rapport à celui utilisant le gravier comme support. Une interaction souche $\times$ lignée a été observée sur gravier mais non sur milieu aéré où elle semble être masquée par une importante variabilité. Sous traitement salin en milieu aéré, la lignée BAT477 inoculée avec les souches locales (1a6 et 12a3) se montre particulièrement tolérante. La lignée NAG310 est la moins tolérante quand elle est inoculée avec la souche CIAT899. Par contre, la souche 12a3 lui confère une tolérance au sel pour la production de la biomasse nodulaire contrairement à sa symbiose avec la lignée Flamingo. Il est conclu qu'il est important de prendre en considération les interactions souche $\times$ lignées pour la sélection d'une légumineuse adaptée à une contrainte comme la salinité, et que la culture sur solution aérée est une bonne méthode pour la sélection de symbioses rhizobiennes à fort potentiel de fixation d'azote

Analyse de la diversité génétique et symbiotique des populations naturelles tunisiennes de Medicago truncatula et recherche de QTL liés au stress salin

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Characterization of rhizobia nodulating chickpea in Tunisia

One hundred and fifty rhizobia nodulating chickpea (Cicer arietinum L.) were isolated from soils sampled in different regions of Tunisia. Variability in the time at which nodules appeared after plant inoculation was observed among these isolates. Five isolates induced nodules two weeks after inoculation, whereas, with the remaining 145 isolates, nodules were observed after more than four weeks. On the basis of PCR/RFLP analysis of 16S rDNA, isolates from the first group were classified as Mesorhizobium mediterraneum, and isolates from the second group as Sinorhizobium medicae. The respective taxonomic position of both types of isolates was confirmed by their symbiotic properties. M. mediterraneum isolates did not nodulate Medicago sativa and formed effective nodules on C. arietinum while S. medicae isolates were able to form effective nodules on M. sativa but formed ineffective nodules on C. arietinum, as did reference strains of the species.Caractérisation des rhizobia nodulant le pois chiche en Tunisie. Cent cinquante rhizobia nodulant le pois chiche (Cicer arietinum L.) ont été isolés à partir de sols échantillonnés dans différentes régions de Tunisie. L'inoculation de la plante hôte avec ces isolats montre une variabilité dans le temps d'apparition des premières nodosités. Cinq isolats induisent des nodosités deux semaines après inoculation alors que pour les 145 isolats restants les nodosités ne sont observées qu'après au moins quatre semaines. L'étude par PCR/RFLP de l'ADNr 16S a permis de rattacher les isolats du premier groupe à l'espèce Mesorhizobium mediterraneum et ceux du second groupe à l'espèce Sinorhizobium medicae. La position taxonomique des isolats a été confirmée par leurs propriétés symbiotiques. Les isolats de M. mediterraneum ne nodulent pas Medicago sativa mais forment une symbiose fixatrice avec C. arietinum, ceux de S. medicae, comme les souches références de l'espèce, forment des nodosités efficientes avec M. sativa et inefficientes avec C. arietinum

Rhizobium gallicum as an efficient symbiont for bean cultivation

Rhizobia are soil bacteria that fix atmospheric nitrogen in symbiosis with legumes in specialized organs called nodules. The legumes thus acquire the autonomy to grow in nitrogen-deficient soils. When nitrogen fixation by indigenous rhizobia is limited, field inoculation with efficient and competitive strains is an economically feasible way to increase production. When the inoculant is made from native strains of rhizobia the success of inoculation should be increased, since local strains are better adapted than commercial inoculants. Here, a Rhizobium gallicum strain, 8a3, previously selected for its competitiveness and symbiotic effectiveness with common bean under laboratory conditions, was tested in field trials in Tunisia. The experiments were conducted in six fields using three common bean cultivars. The majority of the fields showed a low density of the native rhizobia and inefficient nodulation by Sinorhizobium meliloti, a known symbiont of Medicago. Our results show that inoculation with R. gallicum strain 8a3 induced an increase in nodule numbers accompanied by a more than twofold increase in shoot dry yield. Monitoring of the nodulation occupancy through the fingerprinting of the repetitive extragenic palindromic sequences (REP-PCR) showed that strain 8a3 was competitive even in the soil showing a high population density of indigenous R. gallicum, and occupied more than 40% of the nodules. Moreover, in vitro antibiosis assays indicated that strain 8a3 produces antimicrobial activity on agar medium against indigenous common bean rhizobia, including the inefficient strains of S. meliloti. These results point out the benefits that could be achieved by selecting efficient and competitive strains among natural populations of rhizobia

Agrobacterium strains isolated from root nodules of common bean specifically reduce nodulation by Rhizobium gallicum

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699International audienceIn a previous work, we showed that non-nodulating agrobacteria strains were able to colonize root nodules of common bean. Both rhizobia and agrobacteria co-existed in the infected nodules. No impact on symbiosis was found in laboratory conditions when using sterile gravel as a support for growth. In this study, soil samples originating from different geographic and agronomic regions in Tunisia were inoculated with a mixture of agrobacteria strains isolated previously from root nodules of common bean. A significant effect on nodulation and vegetal growth of common bean was observed. Characterization of nodulating rhizobia and comparison with non-inoculated controls showed a biased genetic structure. It seemed that Rhizobium gallicum was highly inhibited, whereas nodulation by Sinorhizobium medicae was favored. Co-inoculation of non-sterile soils with R. gallicum and agrobacteria confirmed these findings. In vitro antibiosis assays indicated that agrobacteria exercised a significant antagonism against R. gallicum

Antioxidant gene-enzyme responses in Medicago truncatula genotypes with different degree of sensitivity to salinity

Antioxidant responses and nodule function of Medicago truncatula genotypes differing in salt tolerance were studied. Salinity effects on nodules were analysed on key nitrogen fixation proteins such as nitrogenase and leghaemoglobin as well as estimating lipid peroxidation levels, and were found more dramatic in the salt-sensitive genotype. Antioxidant enzyme assays for catalase (CAT, EC 1.11.1.6), superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and guaiacol peroxidase (EC 1.11.1.7) were analysed in nodules, roots and leaves treated with increasing concentrations of NaCl for 24 and 48 h. Symbiosis tolerance level, depending essentially on plant genotype, was closely correlated with differences of enzyme activities, which increased in response to salt stress in nodules (except CAT) and roots, whereas a complex pattern was observed in leaves. Gene expression responses were generally correlated with enzymatic activities in 24-h treated roots in all genotypes. This correlation was lost after 48 h of treatment for the sensitive and the reference genotypes, but it remained positively significant for the tolerant one that manifested a high induction for all tested genes after 48 h of treatment. Indeed, tolerance behaviour could be related to the induction of antioxidant genes in plant roots, leading to more efficient enzyme stimulation and protection. High induction of CAT gene was also distinct in roots of the tolerant genotype and merits further consideration. Thus, part of the salinity tolerance in M. truncatula is related to induction and sustained expression of highly regulated antioxidant mechanism

Alternative oxidase 1 (Aox1) gene expression in roots of Medicago truncatula is a genotype-specific component of salt stress tolerance

Alternative oxidase (AOX) is the central component of the non-phosphorylating alternative respiratory pathway in plants and may be important for mitochondrial function during environmental stresses. Recently it has been proposed that Aox can be used as a functional marker for breeding stress tolerant plant varieties. This requires characterization of Aox alleles in plants with different degree of tolerance in a certain stress, affecting plant phenotype in a recognizable way. In this study we examined Aox1 gene expression levels in Medicago truncatula genotypes differing in salt stress tolerance, in order to uncover any correlation between Aox expression and tolerance to salt stress. Results demonstrated a specific induction of Aox1 gene expression in roots of the tolerant genotype that presented the lowest modulation in phenotypic and biochemical stress indices such as morphologic changes, protein level, lipid peroxidation and ROS generation. Similarly, in a previous study we reported that induction of antioxidant gene expression in the tolerant genotype contributed to the support of the antioxidant cellular machinery and stress tolerance. Correlation between expression patterns of the two groups of genes was revealed mainly in 48 h treated roots. Taken together, results from both experiments suggest that M. truncatula tolerance to salt stress may in part due to an efficient control of oxidative balance thanks to (i) induction of antioxidant systems and (ii) involvement of the AOX pathway. This reinforces the conclusion that differences in antioxidant mechanisms can be essential for salt stress tolerance in M. truncatula and possibly the corresponding genes, especially Aox, could be utilized as functional marker

Perspectives on Males and Singing

Over the centuries, there has been reluctance among boys and men to become involved in some forms of singing. Perspectives on Males and Singing tackles this conundrum head-on as the first academic volume to bring together leading thinkers and practitioners who share their insights on the involvement of males in singing. The authors share research that analyzes the axiomatic male disinclination to sing, and give strategies designed to engage males more successfully in performing vocal music emphasizing the many positive effects it can have on their lives. Inspired by a meeting at the Australian symposium ‘Boys and Voices’, which focused on the engagement of boys in singing, the volume includes contributions from leading authorities in Australia, New Zealand, Canada, the United States and Europe