The potential of plant growth-promoting bacteria isolated from arid heavy metal contaminated environments in alleviating salt and water stresses in alfalfa.

Co-evolution of plant beneficial microbes in contaminated environments enhances plant growth and mitigates abiotic stress. However, few studies on heavy metal (HM) tolerant plant growth-promoting bacteria (PGPB) promoting crop growth in Morocco's farming areas affected by drought and salinity are available. Plant associated bacteria tolerant to HM and able to produce indole acetic acid and siderophores, display ACC-deaminase activity and solubilize phosphate, were isolated from long-term metal exposed environments. Tolerance to HM and biofilms formation in the absence or presence of HM were assessed. A consortium including two Ensifer meliloti strains (RhOL6 and RhOL8), one Pseudomonas sp. strain (DSP17), and one Proteus sp. strain (DSP1), was used to inoculate alfalfa (Medicago sativa) seedlings under various conditions, namely, salt stress (85 mM) and water stress (30% water holding capacity). Shoot and root dry weights of alfalfa were measured 60 days after sowing. In the presence of HM, DSP17 showed the greatest auxin production, whereas RhOL8 had the highest ACC-deaminase activity and DSP17 formed the densest biofilm. Root dry weight increased 138% and 195% in salt and water stressed plants, respectively, regarding non-inoculated controls. Our results confirm the improvement of alfalfa growth and mitigation of salt and drought stress upon inoculation.

Mycorrhizas and Trichoderma fungi increase the accumulation of secondary metabolites in grain legume leaves and suppress foliar diseases in field-grown conditions of the humid forest of Cameroon.

BACKGROUND: Arbuscular mycorrhizal and Trichoderma fungi alter the synthesis of secondary metabolites of plants and confer tolerance from pathogens attacks. However, there is less supportive evidence from on-field studies confirming the above-mentioned hypothesis, particularly for the humid forest zone of Cameroon where pathogens are important sources of yield losses for legumes such as soybean and common bean. MATERIALS AND METHODS: We evaluated the impacts of mycorrhiza isolates of Rhizophagus intraradices (Ri) and Trichoderma asperellum (Ta) fungi and their co-inoculations (Ta x Ri) in the synthetizing of leaves secondary metabolites, foliar disease symptoms, growth, N and P uptake, and yields of three genotypes of soybean (TGx 1485-1D, TGx 1990-93 F, and TGx 1990-97 F) and common beans (NUA-99, DOR-701, and PNN) under field conditions of Cameroon. RESULTS: We found that common bean plants showed a lower foliar infection rate but a higher increase in root colonization intensity, shoot dry weight, and N and P uptakes than soybeans when inoculated with Ri and Ta treatment. However, the grain yield of soybean soybean was higher (2000 kg ha 1) than the common bean plants for the Ri × Ta treatment. The soybean genotype TGx 1990-93F had increased root colonization intensity and the lowest foliar infection rate, making it stronger and tolerant to pathogen attacks when co-inoculated with Ri × Ta fungi (F). Bean plants inoculated with Ri and the co-inoculated with Ri × Ta demonstrated lower symptoms of foliar attack, and increased root colonization, particularly the PNN variety. The total amino acid and proline accumulations were higher for soybean than common bean plants due to fungi inoculations, and soybean genotypes accumulated more excellent contents of amino acid and proline in the control (10.1 mg g- 1 fwt) that significantly increased under the Ri × Ta inoculation (13.4 mg g- 1 fwt). CONCLUSIONS: Common bean plants inoculated with Ta and Ri fungi accumulated higher phenolic compounds in their leaves that aided them in overcoming the pathogen attacks than soybean plants.

Plants-Microorganisms-Based Bioremediation for Heavy Metal Cleanup: Recent Developments, Phytoremediation Techniques, Regulation Mechanisms, and Molecular Responses.

Rapid industrialization, mine tailings runoff, and agricultural activities are often detrimental to soil health and can distribute hazardous metal(loid)s into the soil environment, with harmful effects on human and ecosystem health. Plants and their associated microbes can be deployed to clean up and prevent environmental pollution. This green technology has emerged as one of the most attractive and acceptable practices for using natural processes to break down organic contaminants or accumulate and stabilize metal pollutants by acting as filters or traps. This review explores the interactions between plants, their associated microbiomes, and the environment, and discusses how they shape the assembly of plant-associated microbial communities and modulate metal(loid)s remediation. Here, we also overview microbe-heavy-metal(loid)s interactions and discuss microbial bioremediation and plants with advanced phytoremediation properties approaches that have been successfully used, as well as their associated biological processes. We conclude by providing insights into the underlying remediation strategies' mechanisms, key challenges, and future directions for the remediation of metal(loid)s-polluted agricultural soils with environmentally friendly techniques.

Compost and mycorrhizae application as a technique to alleviate Cd and Zn stress in Medicago sativa.

Soil pollution by heavy metals, in the last decades, has become a worldwide major concern for which finding a solution is becoming more important to conserve soil for future generations. This study used an ecotoxicology approach to evaluate the effectiveness of compost and arbuscular mycorrhizal fungus (AMF) and their combination on Medicago sativa performance grown under Zn and Cd stress. At 600 mg/kg of Cd and Zn, a reduction of mycorrhization frequency by 3.6- and 2-fold, respectively, was observed without applying compost. The effect of AMF-Compost combination on alfalfa biomass production was enhanced in the absence and the presence of heavy metals. An improvement of relative water content by 1.7- and 1.5-fold was recorded in case AMF-Compost plant treatments grown under 600 mg/kg of Cd and Zn, respectively. The application of AMF-compost enhanced the stomatal conductance and total chlorophyll in alfalfa plants. Sugar contents were significantly increased in mycorrhized and treated plants with compost compared to the control, regardless of the applied Cd or Zn dose. Phenol content was significantly increased in plants amended with compost alone and treated by Cd. Regarding Cd and Zn accumulation, AMF-compost combination reduced the content of heavy metals accumulated in M. sativa.

Use of native plants and their associated bacteria rhizobiomes to remediate-restore Draa Sfar and Kettara mining sites, Morocco.

Soil and mine tailings are unreceptive to plant growth representing an imminent threat to the environment and resource sustainability. Using indigenous plants and their associated rhizobacteria to restore mining sites would be an eco-friendly solution to mitigate soil-metal toxicity. Soil prospection from Draa Sfar and Kettara mining sites in Morocco was carried out during different seasons for native plant sampling and rhizobacteria screening. The sites have been colonized by fifteen tolerant plant species having different capacities to accumulate Cu, Zn, and P in their shoots/root systems. In Draa Sfar mine, Suaeda vera J.F. Gmel., Sarcocornia fruticosa (L.) A.J. Scott., and Frankenia corymbosa Desf. accumulated mainly Cu (more than 90 mg kg-1), Atriplex halimus L. accumulated Zn (mg kg-1), and Frankenia corymbosa Desf. accumulated Pb (14 mg kg-1). As for Kettara mine, Aizoon canariense L. mainly accumulated Zn (270 mg kg-1), whereas Forsskalea tenacissima L. was the best shoot Cu accumulator with up to 50 mg kg-1, whereas Cu accumulation in roots was 21 mg kg-1. The bacterial screening revealed the strains' abilities to tolerate heavy metals up to 50 mg kg-1 Cu, 250 mg kg-1 Pb, and 150 mg kg-1 Zn. Isolated strains belonged mainly to Bacillaceae (73.33%) and Pseudomonadaceae (10%) and expressed different plant growth-promoting traits, alongside their antifungal activity. Results from this study will provide an insight into the ability of native plants and their associated rhizobacteria to serve as a basis for remediation-restoration strategies.

Restoring the plant productivity of heavy metal-contaminated soil using phosphate sludge, marble waste, and beneficial microorganisms.

Assisted natural remediation (ANR) has been highlighted as a promising, less expensive, and environmentally friendly solution to remediate soil contaminated with heavy metals. We tested the effects of three amendments (10% compost, C; 5 or 15% phosphate sludge, PS5 and PS15; and 5 or 15% marble waste, MW5 and MW15) in combination with microorganism inoculation (rhizobacteria consortium alone, mycorrhizae alone, and the two in-combination) on alfalfa in contaminated soil. Plant concentrations of Zn, Cu, and Pb were measured, along with proline and malondialdehyde production. The microbiological and physicochemical properties of the mining soil were evaluated. Application of the amendments allowed germination and promoted growth. Inoculation with the rhizobacteria consortium and/or mycorrhizae stimulated plant growth. PS and MW stimulated the production of proline. Inoculation of alfalfa with the rhizobacteria-mycorrhizae mixture and the application of MW allowed the safe cultivation of the legume, as shown by the low concentrations of metals in plant shoots. Zn and Pb concentrations were below the limits recommended for animal grazing and accumulated essentially in roots. Soil analyses showed the positive effect of the amendments on the soil physicochemical properties. All treatments increased soil pH (around 7), total organic carbon, and assimilable phosphorus content. Notably, an important decrease in soluble heavy metals concentrations was observed. Overall, our findings revealed that the applied treatments reduced the risk of metal-polluted soils limiting plant growth. The ANR has great potential for success in the restoration of polymetallic and acidic mining soils using the interaction between alfalfa, microorganisms, and organo-mineral amendments.

Unlocking the plant growth-promoting potential of yeast spp.: exploring species from the Moroccan extremophilic environment for enhanced plant growth and sustainable farming.

In this study, we successfully isolated two distinct yeasts from Moroccan extreme environments. These yeasts were subjected to molecular characterization by analyzing their Internal Transcribed spacer (ITS) regions. Our research thoroughly characterizes plant growth-promoting abilities and their drought and salt stress tolerance. In a greenhouse assay, we examined the impact of selected yeasts on Medicago sativa's growth. Four treatments were employed: (i) control without inoculation (NI), (ii) inoculation with L1, (iii) inoculation with L2, and (iv) inoculation with the mixture L1 + L2. L1 isolated from Toubkal Mountain shared 99.83% sequence similarity to Rhodotorula mucilaginosa. Meanwhile, L2, thriving in the arid Merzouga desert, displayed a similar identity to Naganishia albida (99.84%). Yeast strains were tolerant to NaCl (2 M) and 60% PEG (polyethylene glycol P6000) in case of drought. Both strains could solubilize phsphorus, with L2 additionally demonstrating potassium solubilization. In addition, both strains produce indole acetic acid (up to 135 µl ml-1), have siderophore ability, and produce aminocyclopropane-1-carboxylic acid deaminase. Isolates L1 and L2, and their consortium showed that the single or combined strain inoculation of M. sativa improved plant growth, development, and nutrient assimilation. These findings pave the way for harnessing yeast-based solutions in agricultural practices, contributing to enhanced crop productivity and environmental sustainability.

Combined application of marble waste and beneficial microorganisms: toward a cost-effective approach for restoration of heavy metals contaminated sites.

Heavy metal (HM) pollution and the need to preserve the environment have gathered increasing scientific attention. The immobilization of HMs into less-soluble, less mobile, and less toxic forms in addition to the improvement of Medicago sativa L. growth and HMs accumulation were evaluated after the application of marble waste (MW) and/or beneficial PGP rhizobacteria and mycorrhizae to the mining soil compost. A greenhouse assay was conducted to elucidate the influence of both amendment and beneficial microorganisms. The application of marble waste to the soil-compost resulted in decreasing the bioavailability of metals (Cu, Zn, Pb, and Cd), thus ameliorating the installation of the vegetal cover for 6 months of culture. Cultivation of M. sativa under 5% MW-amended soil for 6 months increased the shoot dry weight by almost twofold, while the inoculation with rhizobacteria-mycorrhizae combined with the application of 15% MW resulted in an improvement of 3.5-fold in case of shoot dry weight. In addition, the application of marble waste amendment or their combination with metallo-resistant bacteria resulted in decreasing HM accumulation leading to HM content below the threshold recommended for animal grazing. Thus, the application of amendments and beneficial microorganisms appeared to guarantee the safe cultivation of alfalfa for 6 months of culture. The dual combination amendments and beneficial microorganisms showed the good potential to restore HM polluted soils and could stand as a novel approach for restoration of HM-contaminated soils.

Phosphate-Dependent Regulation of Growth and Stresses Management in Plants.

The importance of phosphorus in the regulation of plant growth function is well studied. However, the role of the inorganic phosphate (Pi) molecule in the mitigation of abiotic stresses such as drought, salinity, heavy metal, heat, and acid stresses are poorly understood. We revisited peer-reviewed articles on plant growth characteristics that are phosphorus (P)-dependently regulated under the sufficient-P and low/no-P starvation alone or either combined with one of the mentioned stress. We found that the photosynthesis rate and stomatal conductance decreased under Pi-starved conditions. The total chlorophyll contents were increased in the P-deficient plants, owing to the lack of Pi molecules to sustain the photosynthesis functioning, particularly, the Rubisco and fructose-1,6-bisphosphatase function. The dry biomass of shoots, roots, and P concentrations were significantly reduced under Pi starvation with marketable effects in the cereal than in the legumes. To mitigate P stress, plants activate alternative regulatory pathways, the Pi-dependent glycolysis, and mitochondrial respiration in the cytoplasm. Plants grown under well-Pi supplementation of drought stress exhibited higher dry biomass of shoots than the no-P treated ones. The Pi supply to plants grown under heavy metals stress reduced the metal concentrations in the leaves for the cadmium (Cd) and lead (Pb), but could not prevent them from absorbing heavy metals from soils. To detoxify from heavy metal stress, plants enhance the catalase and ascorbate peroxidase activity that prevents lipid peroxidation in the leaves. The HvPIP and PHO1 genes were over-expressed under both Pi starvation alone and Pi plus drought, or Pi plus salinity stress combination, implying their key roles to mediate the stress mitigations. Agronomy Pi-based interventions to increase Pi at the on-farm levels were discussed. Revisiting the roles of P in growth and its better management in agricultural lands or where P is supplemented as fertilizer could help the plants to survive under abiotic stresses.

Biofertilizers as Strategies to Improve Photosynthetic Apparatus, Growth, and Drought Stress Tolerance in the Date Palm.

Rainfall regimes are expected to shift on a regional scale as the water cycle intensifies in a warmer climate, resulting in greater extremes in dry versus wet conditions. Such changes are having a strong impact on the agro-physiological functioning of plants that scale up to influence interactions between plants and microorganisms and hence ecosystems. In (semi)-arid ecosystems, the date palm (Phoenix dactylifera L.) -an irreplaceable tree- plays important socio-economic roles. In the current study, we implemeted an adapted management program to improve date palm development and its tolerance to water deficit by using single or multiple combinations of exotic and native arbuscular mycorrhizal fungi (AMF1 and AMF2 respectively), and/or selected consortia of plant growth-promoting rhizobacteria (PGPR: B1 and B2), and/or composts from grasses and green waste (C1 and C2, respectively). We analyzed the potential for physiological functioning (photosynthesis, water status, osmolytes, mineral nutrition) to evolve in response to drought since this will be a key indicator of plant resilience in future environments. As result, under water deficit, the selected biofertilizers enhanced plant growth, leaf water potential, and electrical conductivity parameters. Further, the dual-inoculation of AMF/PGPR amended with composts alone or in combination boosted the biomass under water deficit conditions to a greater extent than in non-inoculated and/or non-amended plants. Both single and dual biofertilizers improved physiological parameters by elevating stomatal conductance, photosynthetic pigments (chlorophyll and carotenoids content), and photosynthetic efficiency. The dual inoculation and compost significantly enhanced, especially under drought stress, the concentrations of sugar and protein content, and antioxidant enzymes (polyphenoloxidase and peroxidase) activities as a defense strategy as compared with controls. Under water stress, we demonstrated that phosphorus was improved in the inoculated and amended plants alone or in combination in leaves (AMF2: 807%, AMF1+B2: 657%, AMF2+C1+B2: 500%, AMF2+C2: 478%, AMF1: 423%) and soil (AMF2: 397%, AMF1+B2: 322%, AMF2+C1+B2: 303%, AMF1: 190%, C1: 188%) in comparison with controls under severe water stress conditions. We summarize the extent to which the dual and multiple combinations of microorganisms can overcome challenges related to drought by enhancing plant physiological responses.

Use of Rhizobacteria and Mycorrhizae Consortium in the Open Field as a Strategy for Improving Crop Nutrition, Productivity and Soil Fertility.

Plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) are known for their beneficial effects. In recent years, more attention has been paid to their use as biofertilizers to reduce the use of chemical fertilizers causing significant damage to the environment. To have high plant yields, biofertilizers may not be able to sustain plant demands and could be used in combination with chemical fertilizers. However, the application of biofertilizers in the field such as rhizobacteria and AMF are understudied and powerfully needed. In this context, this study aims to evaluate the effect of inoculation with rhizobacteria and AMF and their potential to stimulate two of the most economically important crops in Mediterranean semi-arid areas (Vicia faba L. and Triticum durum L.). The effect of inoculation was studied in field experiment with six treatments: (i) the control without inoculation (C), (ii) PGPR alone (PG), (iii) rhizobia alone (R), (iv) the mixture of PGPR and rhizobia (PR), (v) AMF alone (M), and (vi) the mixture of PGPR, rhizobia and AMF (PRM). The inoculation with the consortium of PGPR-rhizobia-AMF (PRM) induced the greatest effect. This inoculation improved the growth parameters (dry weight of shoots and roots) of faba bean and wheat. An improvement of 130, 200, and 78% was observed in V. faba shoot and root dry weight, and the number of leaves, respectively. Similarly, shoot and root dry weight and number of leaves of T. durum were enhanced by 293, 258, and 87%, respectively. The inoculation improved the productivity of studied plants presented by the number and weight of bean pods (270 × 104 ha-1 and 30737.5 kg.ha-1) and wheat spikes (440 × 104 ha-1 and 10560 kg.ha-1). In addition, the mineral analyses showed that the inoculation with PGPR-rhizobia-mycorrhizae improved N, P, Ca, K, and Na shoots contents, as well as the contents of sugar and proteins. Finally, we revealed the positive impact of the tested biofertilizers and the interest of adoption of innovative practices improving crops productivity and soil fertility.

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.

Safe Cultivation of Medicago sativa in Metal-Polluted Soils from Semi-Arid Regions Assisted by Heat- and Metallo-Resistant PGPR.

Soil contamination with heavy metals is a constraint for plant establishment and development for which phytoremediation may be a solution, since rhizobacteria may alleviate plant stress under these conditions. A greenhouse experiment was conducted to elucidate the effect of toxic metals on growth, the activities of ROS (reactive oxygen species)-scavenging enzymes, and gene expression of Medicago sativa grown under different metal and/or inoculation treatments. The results showed that, besides reducing biomass, heavy metals negatively affected physiological parameters such as chlorophyll fluorescence and gas exchange, while increasing ROS-scavenging enzyme activities. Inoculation of M. sativa with a bacterial consortium of heat- and metallo-resistant bacteria alleviated metal stress, as deduced from the improvement of growth, lower levels of antioxidant enzymes, and increased physiological parameters. The bacteria were able to effectively colonize and form biofilms onto the roots of plants cultivated in the presence of metals, as observed by scanning electron microscopy. Results also evidenced the important role of glutathione reductase (GR), phytochelatin synthase (PCS), and metal transporter NRAMP1 genes as pathways for metal stress management, whereas the gene coding for cytochrome P450 (CP450) seemed to be regulated by the presence of the bacteria. These outcomes showed that the interaction of metal-resistant rhizobacteria/legumes can be used as an instrument to remediate metal-contaminated soils, while cultivation of inoculated legumes on these soils is still safe for animal grazing, since inoculation with bacteria diminished the concentrations of heavy metals accumulated in the aboveground parts of the plants to below toxic levels.