Micro-food web interactions involving bacteria, nematodes, and mycorrhiza enhance tree P nutrition in a high P-sorbing soil amended with phytate

International audiencePhytate is considered a poorly available plant P source but proved to be useful for particular soil bacteria strains. In soil-free conditions, it has been shown that bacteria locked up the mineralized phosphorus from phytate whereas bacterial grazers like nematodes were able to deliver P to plants. Here, we aimed to determine if the interactions between phytate-mineralizing bacteria, bacterial grazer nematodes, and mycorrhizal fungi could increase plant P acquisition from phytate in high P-adsorbing soils. Pinus pinaster was grown in a Cambisol supplemented with phytate. Plants, whether associated or not associated with the ectomycorrhizal fungus Hebeloma cylindrosporum, were either inoculated or not inoculated with the phytase-releasing bacteria Bacillus subtilis and the bacterial-feeding nematode Rhabditis sp. After 100 days, the dual inoculation of bacteria and nematodes significantly increased net plant P accumulation. We observed that, on average, mycorrhizal plants accumulated more P in their shoots than non-mycorrhizal plants. However, the highest plant P acquisition efficiency was found when the three soil organisms were present in the P. pinaster rhizosphere. We conclude that, in a highly inorganic P-fixing soil, plant P acquisition from phytate strongly depends on the grazing of phytate-mineralizing bacteria. Our results confirm the importance of the soil microbial loop to improve plant P nutrition from phytate, which should be considered a route to improve the utilization of this source of poorly available P by plants

Can oak powdery mildew severity be explained by indirect effects of climate on the composition of the Erysiphe pathogenic complex?

Coinfection by several pathogens is increasingly recognized as an important feature in the epidemiology and evolution of plant fungal pathogens. Oak mildew is induced by 2 closely related Erysiphe invasive species E. alphitoides and E. quercicola, which differ in their mode of overwintering. We investigated how climate influences the co-occurrence of the 2 species in oak young stands and whether this is important for the disease epidemiology We studied the frequency of flag-shoots (i.e. shoots developing from infected buds, usually associated with E. quercicola) in 95 oak regenerations over a 6 year period.. Additionally, in 2012 and 2013, the oak mildew severity and the 2 Erysiphe species relative frequencies were determined both in spring and in autumn in 51 regenerations and 43 one-year-old plantations of oaks. Both the frequency of flag-shoots and the proportion of Erysiphe lesions with E. quercicola presence were related to climate. We showed that survival of E. quercicola was improved after mild winters, with increase of both the flag-shoot frequency and the proportion of Erysiphe lesions with E. quercicola presence in spring. However, disease severity was not related to any complementarity effect between the two Erysiphe species causing oak powdery mildew. By contrast, increased E. alphitoides prevalence in spring was associated with higher oak mildew severity in autumn. Our results point out the critical role of between season transmission and primary inoculum to explain disease dynamics which could be significant in a climate warming context

Field reality: short-scale soil heterogeneity impacts mycorrhizal contribution to maize P nutrition under contrasted fertilization

International audienceThe relationship between root colonization by arbuscular mycorrhiza (AM) fungi and phosphorus (P) fertilization remains controversial and soil characteristics are still rarely considered. To assess the contribution of AM to maize P nutrition in contrasted field con-ditions, a range of hybrids were grown at two contrasting P levels of a long-term P-ferti-lizer trial in two adjacent soil types: alkaline and neutral. Root colonization was assessed by microscopic observation and qPCR targeting two common AM species, Funneliformis mosseae and Rhizophagus irregularis. Functionality of AM colonization was assessed by measuring the expression of two P-transporters by qPCR. Regardless of the soil type, AM root colonization was highest in the non-fertilized treat-ment compared to high-P fertilization. Looking closer, this drop was driven by the neutral soil and no variation was observed in the alkaline soil with P fertilization. The non-ferti-lized treatment was particularly harsh for maize development in the neutral soil, trigger-ing the highest expression of the plant P starvation inducible gene (ZmPT1:3), coupled to the lowest plant growth and P content. Under these conditions, the mycorrhizal P uptake pathway was particularly activated with the highest expression of the AM inducible P transporter (ZmPT1;6). On contrary, its expression remained constant among fertilization levels in the alkaline soil. The observed resilience to P fertilization in the alkaline soil could partially be due to high abundance of F. mosseae, which increased under P fertilization. Our results emphasize the importance of considering soil characteristics which differentially impacted AM sym-bioses and their implication in plant nutrition

Habitat partitioning of soil microbial communities along an elevation gradient: from plant root to landscape scale

International audienceWithin a landscape, multiple habitats exist for soil microbial communities. But how these habitats shape community composition requires an understanding of the way in which microbial diversity is impacted across a broad range of spatial scales. Mountain ecosystems are excellent systems to study microbial communities, because a multitude of climate and soil variables change within a relatively small distance. We investigated microbial community structure in bulk and rhizosphere soils beneath three plant species, Vaccinium myrtillus, Juniperus communis and Picea abies, that structure local plant communities along an elevation gradient in the French Alps. We examined the impact that climate, soil properties, plant diversity and plant root chemical and morphological traits had on microbial αand β-diversities. The most abundant bacterial phyla detected in both bulk and rhizosphere soils were Proteobacteria, Actinobacteria, Acidobacteria and Verrucomicrobia. Along the elevation gradient, bacterial phyla did not display a clear distribution pattern between bulk and rhizosphere soils. For fungi, dominant phyla were Ascomycota and Basidiomycota, and contrasting distribution patterns were found between bulk and rhizosphere soils. Overall, bacterial and fungal α-diversity responded differently to elevation as well to soil compartments (bulk versus rhizosphere soil), revealing no significant patterns in bulk soil beneath any of the structuring plant species, but increasing in the rhizosphere compartment of P. abies just below the treeline. Changes in bacterial β-diversity with elevation were related mostly to soil physical and chemical properties. Bacterial and fungal α-diversity in rhizosphere communities were more related to plant species identity, vegetation diversity and belowground plant traits compared to soil properties, whilst the opposite was found for bulk soil. Our results highlight that environmental changes at the landscape scale (e.g. associated to elevation, soil properties or climate), impact significantly soil microbial communities, but vegetation refines communities at a local scale via the rhizosphere niche

Wheat varietal diversity modulates nitrogen-related enzymatic activities but has limited impact on arbuscular mycorrhizal fungi

International audienceBackground and aims : High-input agriculture involves low within-field crop genetic diversity, while plant diversity in natural ecosystems was shown to promote ecosystem functioning. Increasing intra-specific diversity in agroecosystems is a promising strategy to stabilize crop productivity and promote the associated diversity of soil biota. We investigated the effect of within-field diversity of bread wheat varieties on arbuscular mycorrhizal fungi (AMF) and two enzymatic activities involved in organic nitrogen and phosphorus mineralization.Methods : We set up a field experiment to test whether varietal or functional diversity modulate the abundance and diversity of AMF and the activity of leucine aminopeptidases and phosphatases in the root zone, considering the influence of root morphology. We used sixteen wheat varieties clustered into four groups according to previously measured traits. The abundance of AMF in roots was measured by qPCR, community composition was analyzed by Illumina metabarcoding on two AMF markers (SSU, LSU), and enzymatic activities were quantified by biochemical assays.Results : Soil properties were the primary drivers of all response variables. Varietal diversity affected nitrogen-related soil enzymatic activities but not those related to phosphorus, with a significant increase of leucine-aminopeptidase activities with increasing varietal diversity. Wheat varietal and functional diversity marginally impacted the abundance of AMF, and functional diversity negatively affected AMF diversity on the SSU marker. Mean root traits modulated enzymatic activities, but not AMF communities.Conclusions : Increasing intra-specific crop diversity affects essential soil microbial processes, providing valuable insights for studying the relationship between plant diversity and soil microbiota in agroecosystems

Do perennial alleys help to maintain arbuscular mycorrhizal communities in temperate agroforestry systems ?

National audienceArbuscular mycorrhizal (AM) fungi are crucial for plant nutrition and the sustainability of agroforestery systems. However, the contribution of each agroforestry component (i.e. trees, under-tree herbaceous vegetation -UtHV- and crops) in the establishment and maintenance of AM communities is poorly documented particularly in temperate areas. This study investigates the spatio-temporal dynamics of AM fungi colonizing roots of the three agroforestry components in southwest France. Standing fine root length density and production, AM activity (colonization and extraradical hyphal growth) and diversity of walnut trees, UtHV and soft wheat were assessed over one year in two agroforestry systems at different distances from the perennial tree-UtHV alley. Compared to UtHV, trees showed a higher ability to colonize superficial layers far into the cultivated alleys due to their wider root system in summer. However, due to higher root densities and well established AM fungi observed throughout all the year, UtHV appeared to be more ecologically relevant to maintain an active source of AM inoculum for newly developing crop roots in winter. High degree of root proximity and similarity of AM fungal communities among the three agroforestry components plant to plant interactions

Pearl Millet Genetic Traits Shape Rhizobacterial Diversity and Modulate Rhizosphere Aggregation

International audienceRoot exudation contributes to soil carbon allocation and also to microbial C and energy supply, which subsequently impacts soil aggregation around roots. Biologically-driven soil structural formation is an important driver of soil fertility. Plant genetic determinants of exudation and more generally of factors promoting rhizosphere soil aggregation are largely unknown. Here, we characterized rhizosphere aggregation in a panel of 86 pearl millet inbred lines using a ratio of root-adhering soil dry mass per root tissue dry mass (RAS/RT). This ratio showed significant variations between lines, with a roughly 2-fold amplitude between lowest and highest average values. For 9 lines with contrasting aggregation properties, we then compared the bacterial diversity and composition in root-adhering soil. Bacterial α-diversity metrics increased with the " RAS/RT ratio. " Regarding taxonomic composition, the Rhizobiales were stimulated in lines showing high aggregation level whereas Bacillales were more abundant in lines with low ratio. 184 strains of cultivable exopolysaccharides-producing bacteria have been isolated from the rhizosphere of some lines, including members from Rhizobiales and Bacillales. However, at this stage, we could not find a correlation between abundance of EPS-producing species in bacterial communities and the ratio RAS/RT. These results illustrated the impact of cereals genetic trait variation on soil physical properties and microbial diversity. This opens the possibility of considering plant breeding to help management of soil carbon content and physical characteristics through carbon rhizodeposition in soil

An interdisciplinary approach to increase wheat within-field diversity and promote agro-ecosystem services

International audienceOne major challenge for increasing agriculture sustainability is to better mobilize crop genetic diversity, as prone by agroecology. A simple way to increase within-field diversity is to use cultivar mixtures, and this has been successfully applied to a few crops in the past. Despite numerous scientific papers documeting the value of cultivar mixtures in wheat and other cereals, especially to control diseases, their cultivation has remained marginal throughout the world. To understand the origin of this gap between scientific knowledge and agricultural practices, the French project Wheatamix explored the synergies mobilized by cultivar mixtures, their impact on various ecosystem services, and their potential to reinforce the sustainability, resilience, and multi-functionality of agriculture. It focused on the agro-ecological and socio-economic impacts of variety associations at different scales, from the plant level up to the wheat supply chain. The project aims at developing new blending and breeding methods to design performing mixtures.To understand how plant-to-plant interactions shape wheat mixtures performances, Wheatamix has set five objectives: 1) describe the variability of morphological and ecological traits in a panel of 57 varieties; 2) explore variability by blending 16 contrasted varieties from the panel into 72 mixtures, composed of 2, 4, and 8 components; 3) study the ecosystem services provided; 4) assess the technical and economic performances in farmer conditions; 5) evaluate the impact of cultivar mixtures on the wheat supply chain. To achieve these goals, this project has developed an interdisciplinary approach, mobilizing agronomy, ecology, economics, ecophysiology, epidemiology, genetics, and management sciences. The project brought together scientists from 10 labs, as well as agricultural advisers and farmers from 6 French counties. The project first described the functional diversity of 57 varieties, highlighting the effects of modern breeding on trait variability, that lowered variability of traits subject to direct selection, and impacted both plant architecture, physiological traits as nutrient absorption, but also trade-off between traits. Wheatamix then surveyed how variation in mixture diversity impacted wild communities. A first result highlighted the low abundance of macro-organisms in this experiment: no relationship was found between the number of varieties in a mixture and the diversity/abundance of earthworms, weeds, mycorrhizae, springtails, beetles, nematodes. However, a significant effect of mixture diversity on the abundance of some spiders, and on nitrifying bacteria, was observed. Coming to ecosystem services, disease regulation (rust and septoria) has been confirmed as the most strongly and positively affected by varietal associations, raising also the strong effects of architectural variability of the canopy (septoria). Diversity also contributed to higher predation rates on aphids. Lastly, soil nitrification and denitrification activities were significantly affected by mixture diversity on 4 surveyed sites, contributing to a shift in plant nutrition and positive effect of greenhouse gas emission.Co-design of variety mixtures was carried out with farmers, technical advisers, and scientists. For three years, 30 farmers in the Paris basin proposed varietal blends and measured their performance on their farms. This exchange first highlighted that the first goals for farmers was to i) secure their production ii) simplify plot management. Then co-design workshops allowed to propose assembly rules and design mixtures, resulting in a wide diversity of sown mixtures. Field trials revealed that in more than 70% of the cases, the mixture had a higher yield than the mean of its components. This work highlighted farmers needs and resulted in a Multicriteria Evaluation Tool, helping farmers and advisers to design mixtures. The survey of the wheat supply chain finally highlighted the need for a concerted innovation among the various actors. Finally, Wheatamix also developed new statistical method to infer mixing ability, allowing both to blend the best mixers, and also to propose new breeding methods.Coupling various disciplines and approaches, such as ecophysiological modeling of plant competition (FSPM WALTer), field and controlled experiments, theoretical framework in ecology (sampling vs complementarity effects, functional traits and tradeoff), and mixture co-design and surveys with stakeholders, Wheatamix has allowed to understand the interest of cultivar mixtures for farmers. Wheat cultivar mixtures are experiencing an exponential growth: they only represented 2% of bread wheat sown in 2010, and are presently at 8%, raking at the first position on the cultivar list. Wheatamix emphasizes the need for an interdisciplinary approach when addressing agroecological subjects, and illustrates the strong mutual benefices between agronomic and ecological sciences