Host-specific competitiveness to form nodules in Rhizobium leguminosarum symbiovar viciae

Fabeae legumes such as pea and faba bean form symbiotic nodules with a large diversity of soil Rhizobium leguminosarum symbiovar viciae (Rlv) bacteria. However, bacteria competitive to form root nodules (CFN) are generally not the most efficient to fix dinitrogen, resulting in a decrease in legume crop yields. Here, we investigate differential selection by host plants on the diversity of Rlv. A large collection of Rlv was collected by nodule trapping with pea and faba bean from soils at five European sites. Representative genomes were sequenced. In parallel, diversity and abundance of Rlv were estimated directly in these soils using metabarcoding. The CFN of isolates was measured with both legume hosts. Pea/faba bean CFN were associated to Rlv genomic regions. Variations of bacterial pea and/or faba bean CFN explained the differential abundance of Rlv genotypes in pea and faba bean nodules. No evidence was found for genetic association between CFN and variations in the core genome, but variations in specific regions of the nod locus, as well as in other plasmid loci, were associated with differences in CFN. These findings shed light on the genetic control of CFN in Rlv and emphasise the importance of host plants in controlling Rhizobium diversity

Inoculation requirement of pea and faba bean and selection of Rlv strains

Soil is an heterogeneous and fluctuating environment. Water limitation in agronomic soils is often partial but lead locally to the suppression of symbiotic activity and therefore to reduction of N acquisition by the plant. A split-root system was developed in pea to evaluate the capacities of Pea-Rhizobium leguminosarum bv viciae (Rlv) symbiotic associations to compensate a local reduction of water availability. Root systems are split in two parts watered separately. Soil water content was monitored. Water limitation was exerted by stopping watering of a half root system. Arrest of nitrogen fixation is very rapidly observed in root directly exposed to water stress. This local and partial water limitation of the plant triggered a systemic signaling on well-watered roots of the same plant. The compensatory response occurs by increasing nodule biomass (and therefore nitrogen fixation capacities) of these roots not directly exposed to the water limitation. In preliminary experiments pea plants were inoculated with a reference Rlv strain. Response of and root and nodule development that compensate the local water limitation was confirmed. Interestingly, the compensatory response varies according to the Rlv strain as another Rlv strain, displaying similar nitrogen fixation efficiency in watered conditions, was not able to promote equivalent compensatory response. This experimental system has been used as a screen to select Rlv strains able to promote rapid compensatory responses to water stress

Is co-inoculation of strawberry plants with plant growth-promoting bacteria and arbuscular mycorrhizal fungi leading to an improvement of growth and fruit quality?

National audienc

Sugar transport and partitioning during arbuscular mycorrhizal symbiosis (from basic to applied science)

SPE IPMThe nutrient exchanges between plant and fungus are the key elements of the arbuscular mycorrhizal (AM) symbiosis. The fungus improves the plant uptake of mineral nutrients, mainly phosphate, while the plant provides the fungus with photosynthetically assimilated carbohydrates. Although knowledge about the mechanisms underlying nutrient exchanges between the symbiotic partners still remains very limited, recent advances in mycorrhiza research have allowed identification and functional characterization of fungal sugar transport systems. The present thesis firstly focused on the identification and characterization of sugar transporters from the model arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis as well as on the understanding of their roles in the tight interaction between the microsymbiont and its plant host. Based on the available preliminary genomic data of R. irregularis, RiMST5 and RiMST6 were identified as new monosaccharide transporters, bringing to six the current total number of glomeromycota MSTs. Functional analyses by yeast complementation and expression profiling by quantitative RT-PCR revealed that RiMST6 is a high-affinity glucose transporter, and that together with RiMST5, have versatile roles and can be recruited during stressful conditions. Besides getting knowledge on the nutrient transporters that operate in mycorrhizal symbiosis, understanding their regulation patterns during the multitrophic interactions that take place in the rhizosphere of host plants is also required to integrate plant growth responses in the context of quality production and yield. To make a link between the fundamental and applied approaches a concatenation of experiments performed over a two-year time period has been carried out. The effects of different combinations of AMF and Plant Growth Promoting Rhizobacteria (PGPR) on Fragaria x ananassa growth and yield have been tested under semi-controlled conditions. We showed that both fruit yield and quality can be improved by one PGPR and one AMF co-inoculated strain and the most satisfactory combination with regard to yield benefits has been selected. Moreover, we highlighted the complexity of such a system in which not only the selected partners are important for fruit yield and quality, but also environmental conditions. To go deeply into the specific effects of each microorganism on plant growth and production, F. x ananassa has been inoculated by the selected combination, namely R. irregularis and Pseudomonas fluorescens Pf4. In this experiment, co-inoculation did not display significant positive effects on plant development and yield. However, we highlighted that in addition to the different partners, all the environmental parameters have to be taken into consideration and that the commercialization of an adapted combination must be tested under field conditions for several years. Additionally, molecular mechanisms and actors, e.g. F. x ananassa sugar transporters (FaSUTs), determining sugar partitioning on strawberry plants have been investigated. Based on the conservation of transport capacity, expression profiles and subcellular localization between SUTs orthologs, we suggest the lack of principal phloem loader in F. x ananassa and a main role of SUT1 and SUT2 proteins in phloem unloading. Finally, the expression analysis of FaSUTs highlighted a tight regulation of sugar transport upon AMF and PGPR inoculation, which is undoubtedly related to the plant physiological status. In summary, this study is a combination between fundamental and applied approaches, which provides (i) new insights into transport processes between plant and fungus in the course of the AM symbiosis and (ii) a dataset relative to AMF/PGPR inoculation effects on plant growth and yield that can allow a future commercialization

Identification de transporteurs de sucres marqueurs de la mycorhize à arbuscules

National audienc

Identification of sugar transporters in arbuscular mycorrhiza, from basic to applied science

National audienceOur study focuses on sugar transporters from both plant and fungal partners at the symbiotic interface to better understand biotrophic exchange systems. Thereby, a collection of putative contigs and ESTs of hexose transporters from Medicago truncatula and Fragaria x ananassa will be processed. The full length sequences will be cloned for functional complementation and uptake experiments in transport deficient yeast mutants. This work also investigates the influence of different mycorrhizal fungi on (1) the expression level of sugar transporters and (2) the economically relevant part of F. x ananassa by analyzing its impact on the plant and the fruit. To this aim, combination of phenological observations, transcriptomics (qRT-PCR and Microarrays) and biochemical analyses will be performed. The need of the complete characterization of hexose transporters is reinforced by the recent demonstration of enhanced sugar content in mycorrhizal strawberries (G. Berta and G. Lingua; personal communication)

SbAMT3;1 et SbAMT4 : deux transporteurs d'ammonium du Sorgho induits localement lors de la mycorhization

National audienc

Identification of sugar transporters in arbuscular mycorrhiza

National audienceOur study focuses on sugar transporters from both plant and fungal partners at the symbiotic interface to better understand biotrophic exchange systems. Thereby, a collection of putative contigs and ESTs of hexose transporters from Medicago truncatula and Fragaria x ananassa will be processed. The full length sequences will be cloned for functional complementation and uptake experiments in transport deficient yeast mutants. This work also investigates the influence of different mycorrhizal fungi on (1) the expression level of sugar transporters and (2) the economically relevant part of F. x ananassa by analyzing its impact on the plant and the fruit. To this aim, combination of phenological observations, transcriptomics (qRT-PCR and Microarrays) and biochemical analyses will be performed. The need of the complete characterization of hexose transporters is reinforced by the recent demonstration of enhanced sugar content in mycorrhizal strawberries (G. Berta and G. Lingua; personal communication)

Biotrophic transportome in the arbuscular mycorrhiza

Understanding mechanisms underlying high nutrients use efficiency and carbon allocation in a context of mycorrhizal interactions is critical for sound management of croplands taking care of ecosystem services rendered by mycorrhizal fungi. Transport processes across the polarised membrane interfaces are of major importance in the functioning of the established mycorrhizal association as the symbiotic relation is based on a 'fair-trade' between fungus and host plant. Uptake and exchanges of nutrient and/or metabolites, at biotrophic interfaces are controlled by membrane transporters and their regulation patterns are essential in determining the outcome of plant fungal interactions and in adapting to changes in soil nutrient quantity and/or quality. The talk will present the current state of art with a special focus on S and C transports

Is co-inoculation with plant growth promoting bacteria and arbuscular mycorrhizal fungi increasing strawberry production yields?

International audienc