Plant Growth-Promoting Bacteria and Arbuscular Mycorrhizal Fungi

Argania spinosa is among the most important species of the Moroccan forest in terms of ecological, environmental, and socio-economic aspects. However, it faces a delicate balance between regeneration and degradation in its natural habitat. Hence, the efforts to preserve and regenerate argan forests are crucial for biodiversity, soil quality, and local livelihoods, yet they face challenges like overgrazing and climate change. Sustainable management practices, including reforestation and community engagement, are vital for mitigating degradation. Similarly, exploiting the argan tree’s rhizosphere can enhance soil quality by leveraging its rich microbial diversity. This approach not only improves crop growth but also maintains ecosystem balance, ultimately benefiting both agriculture and the environment. This enrichment can be achieved by different factors: mycorrhizae, plant extracts, algae extracts, and plant growth-promoting rhizobacteria (PGPR). The benefits provided by PGPR may include increased nutrient availability, phytohormone production, shoot, root development, protection against several plant pathogens, and disease reduction. In this study, the effect of rhizobacteria isolated from the Agran rhizosphere was evaluated on germination percentage and radicle length for Argania spinosa in vitro tests, growth, collar diameter, and branching number under greenhouse conditions. One hundred and twenty (120) bacteria were isolated from the argan rhizosphere and evaluated for their capacity for phosphate solubilization and indole acetic acid production. The results showed that 52 isolates could solubilize phosphorus, with the diameters of the solubilization halos varying from 0.56 ± 0.14 to 2.9 ± 0.08 cm. Among 52 isolates, 25 were found to be positive for indole acetic acid production. These 25 isolates were first tested on maize growth to select the most performant ones. The results showed that 14 isolates from 25 tested stimulated maize growth significantly, and 3 of them by 28% (CN005, CN006, and CN009) compared to the control. Eight isolates (CN005, CN006, CN004, CN007, CN008, CN009, CN010, and CN011) that showed plant growth of more than 19% were selected to evaluate their effect on argan germination rate and radicle length and were subjected to DNA extraction and conventional Sanger sequencing. The 8 selected isolates were identified as: Brevundimonas naejangsanensis sp2, Alcaligenes faecalis, Brevundimonas naejangsanensis sp3, Brevundimonas naejangsanensis sp4, Leucobacter aridicollis sp1, Leucobacter aridicollis sp2, Brevundimonas naejangsanensis sp1, and Staphylococcus saprophyticus. The results showed that Leucobacter aridicollis sp2 significantly increased the germination rate by 95.83%, and the radicle length with a value of 2.71 cm compared to the control (1.60 cm), followed by Brevundimonas naejangsanensis sp3 and Leucobacter aridicollis sp1 (2.42 cm and 2.11 cm, respectively). Under greenhouse conditions, the results showed that the height growth increased significantly for Leucobacter aridicollis sp1 (42.07%) and Leucobacter aridicollis sp2 (39.99%). The isolates Brevundimonas naejangsanensis sp3 and Leucobacter aridicollis sp1 increased the gain of collar diameter by 41.56 and 41.21%, respectively, followed by Leucobacter aridicollis sp2 and Staphyloccocus saprophyticus (38.68 and 22.79%). Leucobacter aridicollis sp1 increased the ramification number per plant to 12 compared to the control, which had 6 ramifications per plant. The use of these isolates represents a viable alternative in sustainable agriculture by improving the germination rate and root development of the argan tree, as well as its development, while increasing the availability of nutrients in the soil and consequently improving fertilization. Full article

Drought affects several plant physiological characteristics such as photosynthesis, carbon metabolism, and chlorophyll content, causing hormonal and nutritional imbalances and reducing nutrient uptake and transport, which inhibit growth and development. The use of bioinoculants based on plant growth-promoting microorganisms such as plant growth-promoting rhizobacteria (PGPR), yeasts, and arbuscular mycorrhizal fungi (AMF) has been proposed as an alternative to help plants tolerate drought. However, most studies have been based on the use of a single type of microorganism, while consortia studies have been scarcely performed. Therefore, the aim of this study was to evaluate different combinations of three PGPR, three AMF, and three yeasts with plant growth-promoting attributes to improve the biochemical, nutritional, and physiological behavior of strawberry plants growing under severe drought. The results showed that the growth and physiological attributes of the non-inoculated plants were significantly reduced by drought. In contrast, plants inoculated with the association of the fungus Claroideoglomus claroideum, the yeast Naganishia albida, and the rhizobacterium Burkholderia caledonica showed a stronger improvement in tolerance to drought. High biomass, relative water content, fruit number, photosynthetic rate, transpiration, stomatal conductance, quantum yield of photosystem II, N concentration, P concentration, K concentration, antioxidant activities, and chlorophyll contents were significantly improved in inoculated plants by up to 16.6%, 12.4%, 81.2%, 80%, 79.4%, 71.0%, 17.8%, 8.3%, 6.6%, 57.3%, 41%, and 22.5%, respectively, compared to stressed non-inoculated plants. Moreover, decreased malondialdehyde levels by up to 32% were registered. Our results demonstrate the feasibility of maximizing the effects of inoculation with beneficial rhizosphere microorganisms based on the prospect of more efficient combinations among different microbial groups, which is of interest to develop bioinoculants oriented to increase the growth of specific plant species in a global scenario of increasing drought stress. Full article

Soilborne pathogens reduce 60% of the yield of onion crops. A common fungal pathogen causing wilt disease and severe losses is Fusarium basal rot (FBR). In this study, the combination of Arbuscular Mycorrhizal Fungi (AMF) with Trichoderma harzianum was investigated against FBR. Onion samples were collected from the Ankara–Polatlı region. Among the isolates, isolate S6 was identified as F. oxysporum f. sp. cepae (FOC) using morphological and molecular methods and pathogenicity tests. Different combinations of AMF (Funneliformis mosseae pure strain and the commercial AMF) and T. harzianum were inoculated on susceptible onion cultivars (Seç, Gence, and Şampiyon). The effects of the treatments on FOC biocontrol were studied under growth chamber conditions. The results showed that Şampiyon was the most resistant, while Gence was the most susceptible to basal rot disease. Different colonization rates (8.91–24%), spore densities (16.4–50.4 spore/10 g soil), and the extent to which a plant needs mycorrhizal conditions to grow to its maximum potential (i.e., mycorrhizal dependencies—18.3–51.9%) were recorded by treatment. Both single and combined applications of AMF and Trichoderma applications suppressed FOC. Suppressive effects were more pronounced when the F. mosseae pure strain was used alone (when F. mosseae was used, disease severity decreased from 90 to 68%, p < 0.05). The F. mosseae pure strain also showed the best plant growth promotion and phosphorus content release. The results indicate an interesting potential use of F. mosseae and the combination of AMF with T. harzianum in the management of FOC in onions. Full article

Nodule endophytes and associated bacteria are non-symbiotic bacteria that colonize legume nodules. They accompany nodulating rhizobia and can form beneficial associations, as some of them are plant growth-promoting rhizobacteria (PGPR) that are able to promote germination and plant growth and increase tolerance to biotic and abiotic stress. White lupin (Lupinus albus) is a legume crop that is gaining relevance as a suitable alternative to soybean as a plant protein source. Eleven nodule-associated bacteria were isolated from white lupin nodules grown in a Tunisian soil. They belonged to the genera Rhizobium, Ensifer, Pseudomonas and Bacillus. Their plant growth-promoting (PGP) and enzymatic activities were tested in vitro. Strains Pseudomonas sp., L1 and L12, displayed most PGP activities tested, and were selected for in planta assays. Inoculation with strains L1 or L12 increased seed germination and had the same positive effects on all plant growth parameters as did inoculation with symbiotic Bradyrhizobium canariense, with no significant differences among treatments. Inoculation with efficient nitrogen-fixing rhizobia must compete with rhizobia present in the soil that sometimes nodulate efficiently but fix nitrogen poorly, leading to a low response to inoculation. In such cases, inoculation with highly effective PGPR might represent a feasible alternative to boost crop productivity. Full article

Phytoremediation, an environmentally friendly and sustainable approach for addressing Cu-contaminated environments, remains underutilized in mine tailings. Arbuscular mycorrhizal fungi (AMF) play a vital role in reducing Cu levels in plants through various mechanisms, including glomalin stabilization, immobilization within fungal structures, and enhancing plant tolerance to oxidative stress. Yeasts also contribute to plant growth and metal tolerance by producing phytohormones, solubilizing phosphates, generating exopolysaccharides, and facilitating AMF colonization. This study aimed to assess the impact of AMF and yeast inoculation on the growth and antioxidant response of Oenothera picensis plants growing in Cu mine tailings amended with compost. Plants were either non-inoculated (NY) or inoculated with Meyerozyma guilliermondii (MG), Rhodotorula mucilaginosa (RM), or a combination of both (MIX). Plants were also inoculated with Claroideoglomus claroideum (CC), while others remained non-AMF inoculated (NM). The results indicated significantly higher shoot biomass in the MG-NM treatment, showing a 3.4-fold increase compared to the NY-NM treatment. The MG-CC treatment exhibited the most substantial increase in root biomass, reaching 5-fold that in the NY-NM treatment. Co-inoculation of AMF and yeast influenced antioxidant activity, particularly catalase and ascorbate peroxidase. Furthermore, AMF and yeast inoculation individually led to a 2-fold decrease in total phenols in the roots. Yeast inoculation notably reduced non-enzymatic antioxidant activity in the ABTS and CUPRAC assays. Both AMF and yeast inoculation promoted the production of photosynthetic pigments, further emphasizing their importance in phytoremediation programs for mine tailings. Full article

According to the EU, the global consumption of biomass, fossil fuels, metals, and minerals is expected to double by 2050, while waste will increase by 70%. In this context, the Circular Economy Action Plan (CEAP) intends to integrate development and sustainability. In this regard, tailored biofertilizers based on plant growth-promoting bacteria (PGPB) can improve plant yield with fewer inputs. In our project, an autochthonous halophyte of the Andalusian marshes, namely Mesembryanthemum crystallinum, was selected for its interest as a source of pharmaceuticals and nutraceuticals. The aim of this work was to use a culturomics approach for the isolation of specific PGPB and endophytes able to promote plant growth and, eventually, modulate the metabolome of the plant. For this purpose, a specific culture medium based on M. crystallinum biomass, called Mesem Agar (MA), was elaborated. Bacteria of three compartments (rhizosphere soil, root endophytes, and shoot endophytes) were isolated on standard tryptone soy agar (TSA) and MA in order to obtain two independent collections. A higher number of bacteria were isolated on TSA than in MA (47 vs. 37). All the bacteria were identified, and although some of them were isolated in both media (Pseudomonas, Bacillus, Priestia, Rosellomorea, etc.), either medium allowed the isolation of specific members of the M. crystallinum microbiome such as Leclercia, Curtobacterium, Pantoea, Lysinibacillus, Mesobacillus, Glutamicibacter, etc. Plant growth-promoting properties and extracellular degrading activities of all the strains were determined, and distinct patterns were found in both media. The three best bacteria of each collection were selected in order to produce two different consortia, whose effects on seed germination, root colonization, plant growth and physiology, and metabolomics were analyzed. Additionally, the results of the plant metabolome revealed a differential accumulation of several primary and secondary metabolites with pharmaceutical properties. Overall, the results demonstrated the feasibility of using “low cost media” based on plant biomass to carry out a culturomics approach in order to isolate the most suitable bacteria for biofertilizers. In this way, a circular model is established in which bacteria help plants to grow, and, in turn, a medium based on plant wastes supports bacterial growth at low prices, which is the reason why this approach can be considered within the model of “circular agronomy”. Full article

The search for methanotrophs as plant-growth-promoting rhizobacteria (PGPR) presents an important contribution to mitigating the impact of global warming by restoring the natural soil potential for consuming methane while benefiting plants during droughts. Our in silico simulations suggest that water, produced as a byproduct of methane oxidation, can satisfy the cell growth requirement. In addition to water, methanotrophs can produce metabolites that stimulate plant growth. Considering this, we proposed that applying methanotrophs as PGPR can alleviate the effect of droughts on crops, while stimulating atmospheric methane consumption. In this work, we isolated a series of methanotrophic communities from the rhizospheres of different crops, including Italian sweet pepper and zucchini, using an atmosphere enriched with pure methane gas, to determine their potential for alleviating drought stress in wheat plants. Subsequently, 23 strains of nonmethanotrophic bacteria present in the methanotrophic communities were isolated and characterized. We then analyzed the contribution of the methane-consuming consortia to the improvement of plant growth under drought conditions, showing that some communities contributed to increases in the wheat plants’ lengths and weights, with statistically significant differences according to ANOVA models. Furthermore, we found that the presence of methane gas can further stimulate the plant–microbe interactions, resulting in larger plants and higher drought tolerance. Full article

Phytohormones play a crucial role in regulating growth, productivity, and development while also aiding in the response to diverse environmental changes, encompassing both biotic and abiotic factors. Phytohormone levels in soil and plant tissues are influenced by specific soil bacteria, leading to direct effects on plant growth, development, and stress tolerance. Specific plant growth-promoting bacteria can either synthesize or degrade specific plant phytohormones. Moreover, a wide range of volatile organic compounds synthesized by plant growth-promoting bacteria have been found to influence the expression of phytohormones. Bacteria–plant interactions become more significant under conditions of abiotic stress such as saline soils, drought, and heavy metal pollution. Phytohormones function in a synergistic or antagonistic manner rather than in isolation. The study of plant growth-promoting bacteria involves a range of approaches, such as identifying singular substances or hormones, comparing mutant and non-mutant bacterial strains, screening for individual gene presence, and utilizing omics approaches for analysis. Each approach uncovers the concealed aspects concerning the effects of plant growth-promoting bacteria on plants. Publications that prioritize the comprehensive examination of the private aspects of PGPB and cultivated plant interactions are of utmost significance and crucial for advancing the practical application of microbial biofertilizers. This review explores the potential of PGPB–plant interactions in promoting sustainable agriculture. We summarize the interactions, focusing on the mechanisms through which plant growth-promoting bacteria have a beneficial effect on plant growth and development via phytohormones, with particular emphasis on detecting the synthesis of phytohormones by plant growth-promoting bacteria. Full article

The application of biostimulants has been proven to be an advantageous tool and an appropriate form of management towards the effective use of natural resources, food security, and the beneficial effects on plant growth and yield. Plant-growth-promoting rhizobacteria (PGPR) are microbes connected with plant roots that can increase plant growth by different methods such as producing plant hormones and molecules to improve plant growth or providing increased mineral nutrition. They can colonize all ecological niches of roots to all stages of crop development, and they can affect plant growth and development directly by modulating plant hormone levels and enhancing nutrient acquisition such as of potassium, phosphorus, nitrogen, and essential minerals, or indirectly via reducing the inhibitory impacts of different pathogens in the forms of biocontrol parameters. Many plant-associated species such as Pseudomonas, Acinetobacter, Streptomyces, Serratia, Arthrobacter, and Rhodococcus can increase plant growth by improving plant disease resistance, synthesizing growth-stimulating plant hormones, and suppressing pathogenic microorganisms. The application of biostimulants is both an environmentally friendly practice and a promising method that can enhance the sustainability of horticultural and agricultural production systems as well as promote the quantity and quality of foods. They can also reduce the global dependence on hazardous agricultural chemicals. Science Direct, Google Scholar, Springer Link, CAB Direct, Scopus, Springer Link, Taylor and Francis, Web of Science, and Wiley Online Library were checked, and the search was conducted on all manuscript sections in accordance with the terms Acinetobacter, Arthrobacter, Enterobacter, Ochrobactrum, Pseudomonas, Rhodococcus, Serratia, Streptomyces, Biostimulants, Plant growth promoting rhizobactera, and Stenotrophomonas. The aim of this manuscript is to survey the effects of plant-growth-promoting rhizobacteria by presenting case studies and successful paradigms in various agricultural and horticultural crops. Full article

This review covers the literature data on plant growth-promoting bacteria in soil, which can fix atmospheric nitrogen, solubilize phosphates, produce and secrete siderophores, and may exhibit several different behaviors simultaneously. We discuss perspectives for creating bacterial consortia and introducing them into the soil to increase crop productivity in agrosystems. The application of rhizosphere bacteria—which are capable of fixing nitrogen, solubilizing organic and inorganic phosphates, and secreting siderophores, as well as their consortia—has been demonstrated to meet the objectives of sustainable agriculture, such as increasing soil fertility and crop yields. The combining of plant growth-promoting bacteria with mineral fertilizers is a crucial trend that allows for a reduction in fertilizer use and is beneficial for crop production. Full article