Protective Role of Native Rhizospheric Soil Microbiota Against the Exposure to Microcystins Introduced into Soil-Plant System via Contaminated Irrigation Water and Health Risk Assessment.

Microcystins (MCs) produced in eutrophic waters may decrease crop yield, enter food chains and threaten human and animal health. The main objective of this research was to highlight the role of rhizospheric soil microbiota to protect faba bean plants from MCs toxicity after chronic exposure. Faba bean seedlings were grown in pots containing agricultural soil, during 1 month under natural environmental conditions of Marrakech city in Morocco (March-April 2018) and exposed to cyanobacterial extracts containing up to 2.5 mg·L-1 of total MCs. Three independent exposure experiments were performed (a) agricultural soil was maintained intact "exposure experiment 1"; (b) agricultural soil was sterilized "exposure experiment 2"; (c) agricultural soil was sterilized and inoculated with the rhizobia strain Rhizobium leguminosarum RhOF34 "exposure experiment 3". Overall, data showed evidence of an increased sensitivity of faba bean plants, grown in sterilized soil, to MCs in comparison to those grown in intact and inoculated soils. The study revealed the growth inhibition of plant shoots in both exposure experiments 2 and 3 when treated with 2.5 mg·L-1 of MCs. The results also showed that the estimated daily intake (EDI) of MCs, in sterilized soil, exceeded 2.18 and 1.16 times the reference concentrations (0.04 and 0.45 µg of microcysin-leucine arginine (MC-LR). Kg-1 DW) established for humans and cattle respectively, which raises concerns about human food chain contamination.

Phytobeneficial bacteria improve saline stress tolerance in Vicia faba and modulate microbial interaction network.

Increased global warming, caused by climate change and human activities, will seriously hinder plant development, such as increasing salt concentrations in soils, which will limit water availability for plants. To ensure optimal plant growth under such changing conditions, microorganisms that improve plant growth and health must be integrated into agricultural practices. In the present work, we examined the fate of Vicia faba microbiota structure and interaction network upon inoculation with plant-nodulating rhizobia (Rhizobium leguminosarum RhOF125) and non-nodulating strains (Paenibacillus mucilaginosus BLA7 and Ensifer meliloti RhOL1) in the presence (or absence) of saline stress. Inoculated strains significantly improved plant tolerance to saline stress, suggesting either a direct or indirect effect on the plant response to such stress. To determine the structure of microbiota associated with V. faba, samples of the root-adhering soil (RAS), and the root tissues (RT) of seedlings inoculated (or not) with equal population size of RhOF125, BLA7 and RhOL1 strains and grown in the presence (or absence) of salt, were used to profile the microbial composition by 16S rRNA gene sequencing. The inoculation did not show a significant impact on the composition of the RT microbiota or RAS microbiota. The saline stress shifted the RAS microbiota composition, which correlated with a decrease in Enterobacteriaceae and an increase in Sphingobacterium, Chryseobacterium, Stenotrophomonas, Agrobacterium and Sinorhizobium. When the microbiota of roots and RAS are considered together, the interaction networks for each treatment are quite different and display different key populations involved in community assembly. These findings indicate that upon seed inoculation, community interaction networks rather than their composition may contribute to helping plants to better tolerate environmental stresses. The way microbial populations interfere with each other can have an impact on their functions and thus on their ability to express the genes required to help plants tolerate stresses.

Characterization of plant growth promoting rhizobacteria and their benefits on growth and phosphate nutrition of faba bean and wheat.

In recent years, more attention has been paid to plant growth promoting (PGP) rhizobacteria use as a biofertilizer alternative to chemical fertilizers, which might cause damage to the environment. The main objective of this work was to evaluate the field application of PGP bacteria and rhizobial strains on the productivity of two food crops extensively used in Morocco; Vicia faba L. and Triticum durum L. A field experiment with four treatments was designed: (1) control without inoculation, (2) PGP bacteria alone (P), (3) rhizobia alone (R) and (4) a mixture of PGP-rhizobia (PR). Furthermore, the PGP strains were tested for their ability to solubilize complex mineral phosphorus and potassium and for their production of indole acetic acid and exopolysaccharides. The strains showed several plant growth promoting traits. Field inoculation by these rhizobacteria improved phosphorus uptake and the agronomic parameters of faba bean and wheat plants, such as biomass of shoots and roots, as well as the weight of bean pods and wheat spikes. The most pronounced effect was displayed by rhizobial strains or the combination of PGP-rhizobia. The rhizobacterial inoculation significantly stimulated the growth of both crops and could be used as potential biofertilizers to optimize growth and phosphorus retention capacity.

Phenotypic and genetic diversity of Moroccan rhizobia isolated from Vicia faba and study of genes that are likely to be involved in their osmotolerance.

Rhizobia are symbiotic nitrogen-fixing bacteria in root nodules of legumes. In Morocco, faba bean (Vicia faba L.), which is the main legume crop cultivated in the country, is often grown in marginal soils of arid and semi-arid regions. This study examines the phenotypic diversity of rhizobia nodulating V. faba isolated from different regions in Morocco for tolerance to some abiotic stresses. A total of 106 rhizobia strains isolated from nodules were identified at the species level by analysing 16S rDNA. Additionally, for selected strains recA, otsA, kup and nodA fragments were sequenced. 102 isolates are likely to belong to Rhizobium leguminosarum or R. laguerreae and 4 isolates to Ensifer meliloti. All strains tolerating salt concentrations of 428 or 342mM NaCl as well as 127 or 99mM Na2SO4 were highly resistant to alkaline conditions (pH 10) and high temperature (44°C). Three strains: RhOF4 and RhOF53 (both are salt-tolerant) and RhOF6 (salt-sensitive) were selected to compare the influence of different levels of salt stress induced by NaCl on growth and on trehalose and potassium accumulation. We find a direct correlation between the trehalose contents of the rhizobial strains and their osmotolerance.

Microcystin-tolerant Rhizobium protects plants and improves nitrogen assimilation in Vicia faba irrigated with microcystin-containing waters.

Irrigation of crops with microcystins (MCs)-containing waters-due to cyanobacterial blooms-affects plant productivity and could be a way for these potent toxins entering the food chain. This study was performed to establish whether MC-tolerant rhizobia could benefit growth, nodulation, and nitrogen metabolism of faba bean plants irrigated with MC-containing waters. For that, three different rhizobial strains-with different sensitivity toward MCs-were used: RhOF96 (most MC-sensitive strain), RhOF125 (most MC-tolerant strain), or Vicz1.1 (reference strain). As a control, plants grown without rhizobia and fertilized by NH4NO3 were included in the study. MC exposure decreased roots (30-37 %) and shoots (up to 15 %) dry weights in un-inoculated plants, whereas inoculation with rhizobia protects plants toward the toxic effects of MCs. Nodulation and nitrogen content were significantly impaired by MCs, with the exception of plants inoculated with the most tolerant strain RhOF125. In order to deep into the effect of inoculation on nitrogen metabolism, the nitrogen assimilatory enzymes (glutamine synthetase (GS) and glutamate synthase (GOGAT)) were investigated: Fertilized plants showed decreased levels (15-30 %) of these enzymes, both in shoots and roots. By contrast, inoculated plants retained the levels of these enzymes in shoots and roots, as well as the levels of NADH-GOGAT activity in nodules. We conclude that the microcystin-tolerant Rhizobium protects faba bean plants and improves nitrogen assimilation when grown in the presence of MCs.