A Malian native plant growth promoting Actinomycetes based biofertilizer improves maize growth and yield

Plant growth-promoting rhizobacteria: a potential bio-asset for restoration of degraded soil and crop productivity with sustainable emerging techniques

The rapid expansion of degraded soil puts pressure on agricultural crop yield while also increasing the likelihood of food scarcity in the near future at the global level. The degraded soil does not suit plants growth owing to the alteration in biogeochemical cycles of nutrients, soil microbial diversity, soil organic matter, and increasing concentration of heavy metals and organic chemicals. Therefore, it is imperative that a solution should be found for such emerging issues in order to establish a sustainable future. In this context, the importance of plant growth-promoting rhizobacteria (PGPR) for their ability to reduce plant stress has been recognized. A direct and indirect mechanism in plant growth promotion is facilitated by PGPR via phytostimulation, biofertilizers, and biocontrol activities. However, plant stress mediated by deteriorated soil at the field level is not entirely addressed by the implementation of PGPR at the field level. Thus, emerging methods such as CRISPR and nanotechnological approaches along with PGPR could manage degraded soil effectively. In the pursuit of the critical gaps in this respect, the present review discusses the recent advancement in PGPR action when used along with nanomaterials and CRISPR, impacting plant growth under degraded soil, thereby opening a new horizon for researchers in this field to mitigate the challenges of degraded soil.

Efficacy of citric acid chelate and Bacillus sp. in amelioration of cadmium and chromium toxicity in wheat

Heavy metals contamination in agricultural soil is a major issue having drastic effects on plants and human health. To solve this issue, we have formulated and tested a new approach of fusion of inorganic (citric acid chelate) and organic (Bacillus sp.) amelioration methods for heavy metals. The Bacillus sp. was heavy metal tolerant and showed plant growth-promoting characteristics including phosphate solubilization, siderophore production, hydrogen cyanide production, indole acetic acid production, and 1-Aminocyclopropane-1-carboxylate deaminase production. The analysis of data showed that plants receiving the combined application of citric acid (CA) chelate and Bacillus sp. mitigated heavy metal toxicity. They augmented the biomass production and amount of photosynthetic pigments in plant cells. They suppressed the negative effects of Cadmium (Cd) and Chromium (Cr) on plants' metabolic systems. A considerable increase was also observed in the activity of enzymatic and non-enzymatic antioxidants which reduced the damaging effects of reactive oxygen species and maintained internal structures of cells. The decrease in the content of Cr and Cd in wheat grains by the treatment of CA chelate and Bacillus sp. was 51%, and 27% respectively. The bioaccumulation of metals was also reduced to 49% (Cr) and 57% (Cd). This approach can be tested and applied in field conditions for soils with heavy metals contamination.

Establishment of actinobacteria-Satureja hortensis interactions under future climate CO2-enhanced crop productivity in drought environments of Saudi Arabia

Drought is a significant global constraint on agricultural production and food security. As a promising approach to improve plant growth and yield under challenging conditions, plant growth-promoting actinobacteria has attracted much interest. Further, elevated levels of atmospheric CO2 (eCO2) may promote the plant-actinobacteria interactions which could be effective to improve the plant growth for food production. Herein, we have investigated the impact of actinobacteria and/or CO2 on biomass production, photosynthesis, macronutrients, levels of organic acids, amino acids, and essential oils as well as antioxidant activities of Satureja hortensis under water-deficit conditions. Among different actinobacterial isolates evaluated for development of secondary metabolites and biological activities, Ac9 was highly capable of producing flavonoids, and it also showed high antioxidant and microbial activities. It markedly induced the plant growth, photosynthesis, and global metabolic improvement, under water-deficit conditions. Interestingly, treatment with Ac9 in combination with eCO2 substantially minimized drought stress-induced biomass and photosynthesis reductions in Satureja hortensis. Improved photosynthesis by Ac9 and/or eCO2 induced the primary and secondary metabolisms in drought-stressed plants. The levels of the majority of the detected organic acids, essential oil, and amino acids were further improved as a result of the synergistic action of Ac9 and eCO2, as compared to the individual treatments. Furthermore, Ac9 or eCO2 significantly improved the antioxidant activities in stressed plants; however, much more positive impact was obtained by their synchronous application. Thus, the current study suggests that actinobacterial treatment induces global metabolic changes in water-stressed Satureja hortensis, the effects that have been much more strengthened under eCO2.

A Bioactive Fraction from Streptomyces sp. Enhances Maize Tolerance against Drought Stress

Drought stress is threatening the growth and productivity of many economical crops. Therefore, it is necessary to establish innovative and efficient approaches for improving crop growth and productivity. Here we investigated the potentials of the cell-free extract of Actinobacteria (Ac) isolated from a semi-arid habitat (Al-Jouf region, Saudi Arabia) to recover the reduction in maize growth and improve the physiological stress tolerance induced by drought. Three Ac isolates were screened for production of secondary metabolites, antioxidant and antimicrobial activities. The isolate Ac3 revealed the highest levels of flavonoids, antioxidant and antimicrobial activities in addition to having abilities to produce siderophores and phytohormones. Based on seed germination experiment, the selected bioactive fraction of Ac3 cell-free extract (F2.7, containing mainly isoquercetin), increased the growth and photosynthesis rate under drought stress. Moreover, F2.7 application significantly alleviated drought stress-induced increases in H₂O₂, lipid peroxidation (MDA) and protein oxidation (protein carbonyls). It also increased total antioxidant power and molecular antioxidant levels (total ascorbate, glutathione and tocopherols). F2.7 improved the primary metabolism of stressed maize plants; for example, it increased in several individuals of soluble carbohydrates, organic acids, amino acids, and fatty acids. Interestingly, to reduce stress impact, F2.7 accumulated some compatible solutes including total soluble sugars, sucrose and proline. Hence, this comprehensive assessment recommends the potentials of actinobacterial cell-free extract as an alternative ecofriendly approach to improve crop growth and quality under water deficit conditions.

Drought tolerant Ochrobactrum sp. inoculation performs multiple roles in maintaining the homeostasis in Zea mays L. subjected to deficit water stress

Plant growth-promoting rhizobacteria (PGPR) improve plant health under various biotic and abiotic stresses. However, the underlying mechanisms of the protective effects of PGPR in deficit water stress (WS) remain less explored. This study aimed to characterize the role of Ochrobactrum sp. NBRISH6 inoculation on maize (Zea mays "Maharaja") under WS conditions using multiple approaches such as physiological, anatomical, metabolic, and molecular. The effect of NBRISH6 inoculation using maize as a host plant was characterized under greenhouse conditions in deficit water stress. Results from this study demonstrated that NBRISH6 significantly lowered the expression of genes involved in the abscisic acid cycle, deficit water stress-response, osmotic stress, and antioxidant enzyme activity (superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and polyphenol oxidase). Phytohormones, i.e. indole acetic acid (IAA) and salicylic acid (SA) levels, intercellular CO₂ concentration, metabolites such as simple sugars, amino acids, aliphatic hydrocarbons, and the number of shrunken pith cells modulated in maize roots inoculated with NBRISH6. The NBRISH6 inoculation also improved the plant vegetative properties (root length, 33.80%; shoot length, 20.68%; root dry weight, 39.21%; shoot dry weight, 61.95%), shoot nutrients, xylem cells, root hairs, vapor pressure deficit (75%), intrinsic water-use efficiency (41.67%), photosynthesis rate (83.33%), and total chlorophyll (16.15%) as compared to the respective stress controls. This study provides valuable insights into mechanistic functions of PGPR in WS amelioration and promoting plant physiological response.

Deep tillage combined with biofertilizer following soil fumigation improved chrysanthemum growth by regulating the soil microbiome

Sustained monoculture often leads to the inhibition of plant growth, the decrease of the soil microbial diversity, and changes in soil microbial community composition, particularly to the accumulation of soil‐borne pathogens. In this study, we conducted field experiments to investigate the practical effects of tilling the soil down to a depth of 40 cm (40dp) in combination with dazomet (D) soil fumigation and/or the application of a bio‐organic fertilizer (B) on chrysanthemum growth, with a focus on the potential mechanisms underlying the responses of the soil microbiome. The growth indices of chrysanthemum were significantly (p < .05) increased in the DB + 40dp treatment compared to that in other treatments. The weighted and unweighted UniFrac distances in the principal coordinate analysis (PCoA) revealed that soil bacterial and fungal community compositions were separated according to the treatments. The abundance of genera potentially expressing growth promotion, such as Pseudomonas and Bacillus, was increased in the DB + 40dp treatment. In addition, the combined DB + 40dp treatment enhanced the activities of catalase, urease, sucrase, and β‐d‐glucosidase, and significantly increased the levels of available nitrogen, phosphorus, and potassium in the soil. The redundancy analysis (RDA) implied that the composition of the microbiome was correlated to soil enzymatic activities and soil potassium availability in the rhizosphere soil of chrysanthemum plants. Our findings suggest that the DB + 40dp treatment is a better strategy for improving chrysanthemum growth and regulating the rhizosphere microbiome in monoculture soils than the methods presently employed by commercial chrysanthemum producers.

Priming maize seeds with cyanobacteria enhances seed vigour and plant growth in elite maize inbreds

Biofilm formation of a nitrogen-fixing cyanobacterium Anabaena torulosa with a beneficial fungus Trichoderma viride (An-Tr) was examined under laboratory conditions. A gradual enhancement in growth over A. torulosa alone was recorded in the biofilm, with 15-20% higher values in nitrogen fixation, IAA and exopolysaccharide production illustrating the synergism among the partners in the biofilm. To investigate the role of such biofilms in priming seed attributes, mesocosm studies using primed seeds of two maize inbred lines (V6, V7) were undertaken. Beneficial effects of biofilm (An-Tr) were recorded, as compared to uninoculated treatment and cyanobacterial consortium (Anabaena-Nostoc; BF 1-4) at both stages (7 and 21 DAS, days after sowing) with a significant increase of more than 20% in seedling attributes, along with 5-15% increment in seed enzyme activities. More than three- to fivefold higher values in nitrogen fixation and C-N mobilizing enzyme activities, and significant increases in leaf chlorophyll, proteins and PEP carboxylase activity were observed with V7-An-Tr biofilm. Cyanobacterial inoculation brought about distinct changes in the soil phospholipid fatty acid profiles (PLFA); particularly, significant changes in those representing eukaryotes and anaerobic bacteria. Principal component analyses illustrated the significant role of dehydrogenase activity and microbial biomass carbon and distinct elicited effects on soil microbial communities, as evidenced by the PLFA. This investigation highlighted the promise of cyanobacteria as valuable priming options to improve mobilization of nutrients at seed stage, modulating the abundance and activities of various soil microbial communities, thereby, enhanced plant growth and vigour of maize plants.

Actinobacterium isolated from a semi-arid environment improves the drought tolerance in maize (Zea mays L.)

Drought represents a major constraint for agricultural productivity and food security worldwide. Plant growth promoting actinobacteria have attracted the attention as a promising approach to enhance plant growth and yield under stressful conditions. In this regard, bioprospecting in arid and semi-arid environments could reveal uncommon bacteria with improved biological activities. In the present study, the ability of actinobacteria isolated from a semi-arid environment (Saudi Arabia) to mitigate the negative impact of drought on growth and physiology of maize, a drought-sensitive crop, has been investigated. Among the different actinobacterial isolates screened for secondary metabolites production and biological activities, isolate Ac5 showed high ability of flavonoid, phytohormones and siderophores production. Moreover, Ac5 improved the growth and photosynthesis and induced a global metabolic change in the bacterized plants under water-deficit conditions. Interestingly, Ac5 treatment significantly mitigated the detrimental effects of drought stress on maize. Reduced H₂O₂ accumulation and lipid peroxidation accompanied with higher levels of molecular antioxidants (total ascorbate, glutathione, tocopherols, phenolic acids and flavonoids) were observed in the bacterized plants. From the osmoregulation point of view, drought-stressed bacterized maize accumulated higher levels of compatible solutes, such as sucrose, total soluble sugars, proline, arginine and glycine betaine, as compared with the non-bacterized plants. Therefore, this study highlights the comprehensive impact of actinobacteria on the global plant metabolism and suggests the potential utilization of actinobacteria isolated from semi-arid environments to mitigate the negative impact of drought on crop plants.