An Actinobacterium Strain From Soil of Cerrado Promotes Phosphorus Solubilization and Plant Growth in Soybean Plants

Streptomyces N2A, an endophytic actinobacteria that promotes soybean growth and increases yield and seed quality under field conditions

Sustainable agriculture based on the use of soil-beneficial microbes such as plant growth-promoting rhizobacteria (PGPR) and biocontrol agents (BCA) is gaining great consideration to reduce the use of agrochemicals for crop production. With this aim, in this study, a total of 78 actinobacteria were isolated from the rhizosphere and endosphere of soybean roots. Based on in vitro compatibility with Bradyrhizobium japonicum, the ability to produce phytohormones, siderophores, exo-enzymes, antifungal compounds and phosphate solubilization (PGPR traits), two endophytic strains, named N2A and N9, were selected to evaluate their effects on plant growth and development at greenhouse and field conditions. Greenhouse trials showed significantly promoted seedling emergence compared to control and the conventional fungicide treatment. Analysis of growth and development associated parameters at reproductive stages and maturity at greenhouse, but also and most importantly, in field experiments showed significant improvements. Plant biomass, node number, pod number, and consequently yield, were higher in plants previously treated with N2A and co-inoculated with B. japonicum compared to the conventional seed treatment. Furthermore, a significant increase in health status and vigor was observed for seeds harvested from the N2A-treated plants in relation to seeds obtained from the conventional treatment. Thus, we demonstrated that Streptomyces sp. N2A can replace traditional chemical fungicides to protect the seed during germination, allowing good implantation, but also, stimulating the growth and development of soybean crop increasing yield and seed quality at field conditions. Altogether, this supports the potential use of Streptomyces N2A as a PGPR for soybean crop production more efficiently and sustainably.

Study on the mechanism of salt relief and growth promotion of Enterobacter cloacae on cotton

AIMS

In-depth studies on plant ion uptake and plant growth-promoting rhizobacteria (PGPR) at the molecular level will help to further reveal the effects of PGPR on plants and their interaction mechanisms under salt stress.

METHODS

Cotton was inoculated with a PGPR-Enterobacter cloacae Rs-35, and the ion uptake capacity, membrane transporter protein activity, and expression of key genes were determined under salt stress. Changes in the endogenous hormone content of cotton were also determined. Further, the genome-wide metabolic pathway annotation of E. cloacae Rs-35 and its differential enrichment pathway analysis of multi-omics under salinity environments were performed.

RESULTS

In a pot experiment of saline-alkali soil, E. cloacae Rs-35-treated cotton significantly increased its uptake of K+ and Ca2+ and decreased uptake of Na+, elevated the activity of the H+-ATPase, and increased the sensitivity of the Na+/H+ reverse transporter protein on the vesicle membrane. Meanwhile, inoculation with E. cloacae Rs-35 could promote cotton to maintain the indole-3-acetic acid (IAA) content under salt stress. Genome-wide annotation showed that E. cloacae Rs-35 was respectively annotated to 31, 38, and 130 related genes in osmotic stress, phytohormone and organic acid metabolism, and ion uptake metabolic pathway. Multi-omics differences analysis showed that E. cloacae Rs-35 were enriched to tryptophan metabolism, multiple amino acid biosynthesis, carbon and glucose synthesis, and oxidative phosphorylation metabolic pathways at the transcriptome, proteome, and metabolome.

CONCLUSION

E. cloacae Rs-35 can promote cotton balance cell ion concentration, stabilize intracellular IAA changes, stimulate induction of systemic tolerance, and promote the growth of cotton plants under salt stress.

The Hyphosphere of Leaf-Cutting Ant Cultivars Is Enriched with Helper Bacteria

Bacteria can live in a variety of interkingdom communities playing key ecological roles. The microbiome of leaf-cutting attine ant colonies are a remarkable example of such communities, as they support ants' metabolic processes and the maintenance of ant-fungus gardens. Studies on this topic have explored the bacterial community of the whole fungus garden, without discerning bacterial groups associated with the nutrient storage structures (gongylidia) of ant fungal cultivars. Here we studied bacteria isolated from the surface of gongylidia in the cultivars of Atta sexdens and Acromyrmex coronatus, to assess whether the bacterial community influences the biology of the fungus. A total of 10 bacterial strains were isolated from gongylidia (Bacillus sp., Lysinibacillus sp., Niallia sp., Staphylococcus sp., Paenibacillus sp., Pantoea sp., Staphylococcus sp., and one Actinobacteria). Some bacterial isolates increased gongylidia production and fungal biomass while others had inhibitory effects. Eight bacterial strains were confirmed to form biofilm-like structures on the fungal cultivar hyphae. They also showed auxiliary metabolic functions useful for the development of the fungal garden such as phosphate solubilization, siderophore production, cellulose and chitin degradation, and antifungal activity against antagonists of the fungal cultivar. Bacteria-bacteria interaction assays revealed heterogeneous behaviors including synergism and competition, which might contribute to regulate the community structure inside the garden. Our results suggest that bacteria and the ant fungal cultivar interact directly, across a continuum of positive and negative interactions within the community. These complex relationships could ultimately contribute to the stability of the ant-fungus mutualism.

Improving the efficiency of phosphate rocks combined with phosphate solubilizing Actinomycetota to increase wheat growth under alkaline and acidic soils

Low availability of phosphorus (P) in both acidic and alkaline soils is a major problem for sustainable improvement in wheat crops yield. Optimization of crops productivity can be achieved by increasing the bioavailability of P by phosphate solubilizing Actinomycetota (PSA). However, their effectiveness may vary with changing agro-climatic conditions. In this regard, a greenhouse experiment was conducted to assess the interaction inoculation of five potential PSA (P16-P18-BC3-BC10 and BC11) and RPs (RP1- RP2-RP3 and RP4) on the growth and yield of wheat crop in unsterilized P- deficient alkaline and acidic soils. Their performance was compared with single super phosphate (TSP) and reactive RP (BG4). The in-vitro tests showed that all PSA colonize wheat root and form a strong biofilm except Streptomyces anulatus strain P16. Our findings revealed that all PSA significantly improve the shoot/root dry weights, spike biomass, chlorophyll contents as well as nutrients uptake in plants fertilized with RP3 and RP4. However, the combined application of Nocardiopsis alba BC11 along with RP4 in alkaline soil, was effective in optimizing wheat yield attributes and improve the yield biomass up to 19.7% as compared to the triple superphosphate (TSP). This study supports the view that the inoculation with Nocardiopsis alba BC11 has a broad RP solubilization and could alleviate the agricultural losses due to P limitation in acidic and alkaline soils.

Diversity, mechanisms and beneficial features of phosphate-solubilizing Streptomyces in sustainable agriculture: A review

Currently, the use of phosphate (P) biofertilizers among many bioformulations has attracted a large amount of interest for sustainable agriculture. By acting as growth promoters, members of the Streptomyces genus can positively interact with plants. Several studies have shown the great potential of this bacterial group in supplementing P in a soluble, plant-available form by several mechanisms. Furthermore, some P-solubilizing Streptomyces (PSS) species are known as plant growth-promoting rhizobacteria that are able to promote plant growth through other means, such as increasing the availability of soil nutrients and producing a wide range of antibiotics, phytohormones, bioactive compounds, and secondary metabolites other than antimicrobial compounds. Therefore, the use of PSS with multiple plant growth-promoting activities as an alternative strategy appears to limit the negative impacts of chemical fertilizers in agricultural practices on environmental and human health, and the potential effects of these PSS on enhancing plant fitness and crop yields have been explored. However, compared with studies on the use of other gram-positive bacteria, studies on the use of Streptomyces as P solubilizers are still lacking, and their results are unclear. Although PSS have been reported as potential bioinoculants in both greenhouse and field experiments, no PSS-based biofertilizers have been commercialized to date. In this regard, this review provides an overview mainly of the P solubilization activity of Streptomyces species, including their use as P biofertilizers in competitive agronomic practices and the mechanisms through which they release P by solubilization/mineralization, for both increasing P use efficiency in the soil and plant growth. This review further highlights and discusses the beneficial association of PSS with plants in detail with the latest developments and research to expand the knowledge concerning the use of PSS as P biofertilizers for field applications by exploiting their numerous advantages in improving crop production to meet global food demands.

Characterization and selection of endophytic actinobacteria for growth and disease management of Tea (Camellia sinensis L.)

Endophytic microbes are vital for nutrient solubilization and uptake, growth, and survival of plants. Here, 88 endophytic actinobacteria (EnA) associated with five tea clones were isolated, assessed for their diversity, plant growth promoting (PGP), and biocontrol traits, and then used as an inoculant for PGP and disease control in host and non-host plants. Polyphasic methods, including phenotypic and genotypic characteristics led to their identification as Streptomyces, Microbacterium, Curtobacterium, Janibacter, Rhodococcus, Nocardia, Gordonia, Nocardiopsis, and Kribbella. Out of 88 isolates, 35 (39.77%) showed antagonistic activity in vitro against major fungal pathogens, viz. Fusarium oxysporum, Rhizoctonia solani, Exobasidium vexans, Poria hypobrunnea, Phellinus lamaensis, and Nigrospora sphaerica. Regarding PGP activities, the percentage of isolates that produced indole acetic acid, siderophore, and ammonia, as well as P-solubilisation and nitrogen fixation, were 67.05, 75, 80.68, 27.27, 57.95, respectively. A total of 51 and 42 isolates showed chitinase and 1-aminocyclopropane-1-carboxylic acid deaminase activity, respectively. Further, two potent Streptomyces strains KA12 and MA34, selected based on the bonitur scale, were screened for biofilm formation ability and tested in vivo under nursery conditions. Confocal laser scanning microscopy and the crystal violet staining technique revealed that these Streptomyces strains can form biofilms, indicating the potential for plant colonization. In the nursery experiment, they significantly enhanced the shoot and root biomass, shoot and root length, and leaf number in host tea plants. Additionally, treatment of tomato seeds by KA12 suppressed the growth of fungal pathogen Fusarium oxysporum, increased seed germination, and improved root architecture, demonstrating its ability to be used as a seed biopriming agent. Our results confirm the potential of tea endophytic actinobacterial strains with multifarious beneficial traits to enhance plant growth and suppress fungal pathogens, which may be used as bioinoculant for sustainable agriculture.