Bacillus subtilis Inoculation Improves Nutrient Uptake and Physiological Activity in Sugarcane under Drought Stress

Endophytic bacilli from Cyamopsis tetragonoloba (L.) Taub. induces plant growth and drought tolerance

Cyamopsis tetragonoloba (L.) Taub. (guar) is a commercially important crop known for its galactomannan content in seeds. Drought stress is a significant global concern that compromises the productivity of major legumes including guar. The endophytic microbes associated with plants play a significant role in enhancing plant growth and modulating the impact of abiotic stress(s). The present study involved the isolation of 73 endophytic bacteria from the guar seeds of drought-tolerant (RGC-1002 and RGC-1066) and sensitive (Sarada and Varsha) varieties. Based on multiple PGP attributes and drought tolerance, at 50% PEG6000 w/v, 11 efficient isolates were selected and identified through 16S rRNA gene sequencing. Isolates belonging to ten different species of bacilli including Cytobacillus oceanisediminis, Mesobacillus fermenti, Peribacillus simplex from sensitive and Bacillus zanthoxyli, B. safensis, B. velezensis, B. altitudinis, B. licheniformis, B. tequilensis, and B. paralicheniformis isolated from tolerant varieties. A greenhouse experiment with a drought-sensitive guar variety demonstrated that inoculation of selected isolates showed comparatively better plant growth, higher relative water content (RWC), decreased carbon isotope discrimination ratio (Δ13C), increased chlorophyll, carotenoids, anthocyanin, and proline content, decreased malondialdehyde (MDA) and modulated defense enzymes as compared to their uninoculated controls. Tolerant variety isolates B. tequilensis NBRI14G and B. safensis NBRI10R showed the most promising results in improving plant growth and also drought stress tolerance in guar plants. This study represents for the first time that seed endophytic bacterial strains from guar can be utilized to develop the formulation for improving the productivity of guar under drought-stress conditions.

Parameter evaluation for developing phosphate-solubilizing Bacillus inoculants

Bacterial inoculants have been used in agriculture to improve plant performance. However, laboratory and field requirements must be completed before a candidate can be employed as an inoculant. Therefore, this study aimed to evaluate the parameters for inoculant formulation and the potential of Bacillus subtilis (B70) and B. pumilus (B32) to improve phosphorus availability in maize (Zea mays L.) crops. In vitro experiments assessed the bacterial ability to solubilize and mineralize phosphate, their adherence to roots, and shelf life in cassava starch (CS), carboxymethyl cellulose (CMC), peat, and activated charcoal (AC) stored at 4 °C and room temperature for 6 months. A field experiment evaluated the effectiveness of strains to increase the P availability to plants growing with rock phosphate (RP) and a mixture of RP and triple superphosphate (TS) and their contribution to improving maize yield and P accumulation in grains. The B70 was outstanding in solubilizing RP and phytate mineralization and more stable in carriers and storage conditions than B32. However, root adherence was more noticeable in B32. Among carriers, AC was the most effective for preserving viable cell counts, closely similar to those of the initial inoculum of both strains. Maize productivity using the mixture RPTS was similar for B70 and B32. The best combination was B70 with RP, which improved the maize yield (6532 kg ha-1) and P accumulation in grains (15.95 kg ha-1). Our results indicated that the inoculant formulation with AC carrier and B70 is a feasible strategy for improving phosphorus mobilization in the soil and maize productivity.

Alleviation of water-deficit stress in turmeric plant (Curcuma longa L.) using phosphate solubilizing rhizo-microbes inoculation

The objective of this study was to assess the effects of phosphate solubilizing rhizo-microbes inoculants on nutrient balance, physiological adaptation, growth characteristics, and rhizome yield traits as well as curcuminoids yield at the secondary-rhizome initiation stage of turmeric plants, subsequently subjected to water-deficit (WD) stress. Phosphorus contents in the leaf tissues of Talaromyces aff. macrosporus and Burkholderia sp. (Bruk) inoculated plants peaked at 0.33 and 0.29 mg g-1 DW, respectively, under well-watered (WW) conditions; however, phosphorus contents declined when subjected to WD conditions (p ≤ 0.05). Similarly, potassium and calcium contents reached their maximum values at 5.33 and 3.47 mg g-1 DW, respectively, in Burk inoculated plants under WW conditions, which contributed to sustained rhizome fresh weight even when exposed to WD conditions (p ≤ 0.05). There was an increase in free proline content in T. aff. macrosporus and Burk inoculated plants under WD conditions, which played a crucial role in controlling leaf osmotic potential, thereby stabilizing leaf greenness and maximum quantum yield of PSII. As indicators of drought stress, there were noticeable restrictions in stomatal gas exchange parameters, including net photosynthetic rate, stomatal conductance, and transpiration rate, accompanied by an increase in leaf temperature. These changes resulted in reduced total soluble sugar levels. Interestingly, total curcuminoids and curcuminoids yield in Burk inoculated plants under WD conditions were retained, especially in relation to rhizome biomass. Burk inoculation in turmeric plants is recommended as a promising technique as it alleviates water-deficit stress, sustains rhizome biomass, and stabilizes curcuminoids yield.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-03922-x.

Exogenous application of sulfur-rich thiourea (STU) to alleviate the adverse effects of cobalt stress in wheat

Heavy metal stress affects crop growth and yields as wheat (Triticum aestivum L.) growth and development are negatively affected under heavy metal stress. The study examined the effect of cobalt chloride (CoCl2) stress on wheat growth and development. To alleviate this problem, a pot experiment was done to analyze the role of sulfur-rich thiourea (STU) in accelerating the defense system of wheat plants against cobalt toxicity. The experimental treatments were, i) Heavy metal stress (a) control and (b) Cobalt stress (300 µM), ii) STU foliar applications; (a) control and (b) 500 µM single dose was applied after seven days of stress, and iii) Wheat varieties (a) FSD-2008 and (b) Zincol-2016. The results revealed that cobalt stress decreased chlorophyll a by 10%, chlorophyll b by 16%, and carotenoids by 5% while foliar application of STU increased these photosynthetic pigments by 16%, 15%, and 15% respectively under stress conditions as in contrast to control. In addition, cobalt stress enhances hydrogen peroxide production by 11% and malondialdehyde (MDA) by 10%. In comparison, STU applications at 500 µM reduced the production of these reactive oxygen species by 5% and by 20% by up-regulating the activities of antioxidants. Results have revealed that the activities of SOD improved by 29%, POD by 25%, and CAT by 28% under Cobalt stress. Furthermore, the foliar application of STU significantly increased the accumulation of osmoprotectants as TSS was increased by 23% and proline was increased by 24% under cobalt stress. Among wheat varieties, FSD-2008 showed better adaptation under Cobalt stress by showing enhanced photosynthetic pigments and antioxidant activities compared to Zincol-2016. In conclusion, the foliar-applied STU can alleviate the negative impacts of Cobalt stress by improving plant physiological attributes and upregulating the antioxidant defense system in wheat.

Recent advances in Bacillus-mediated plant growth enhancement: a paradigm shift in redefining crop resilience

The Bacillus genus has emerged as an important player in modern agriculture, revolutionizing plant growth promotion through recent advances. This review provides a comprehensive overview of the critical role Bacillus species play in boosting plant growth and agricultural sustainability. Bacillus genus bacteria benefit plants in a variety of ways, according to new research. Nitrogen fixation, phosphate solubilization, siderophore production, and the production of growth hormones are examples of these. Bacillus species are also well-known for their ability to act as biocontrol agents, reducing phytopathogens and protecting plants from disease. Molecular biology advances have increased our understanding of the complex interplay between Bacillus species and plants, shedding light on the genetic and metabolic underpinnings of these interactions. Furthermore, novel biotechnology techniques have enabled the development of Bacillus-based biofertilizers and biopesticides, providing sustainable alternatives to conventional chemical inputs. Apart from this, the combination of biochar and Bacillus species in current biotechnology is critical for improving soil fertility and encouraging sustainable agriculture through enhanced nutrient retention and plant growth. This review also emphasizes the Bacillus genus bacteria's ability to alleviate environmental abiotic stresses such as drought and salinity, hence contributing to climate-resilient agriculture. Moreover, the authors discuss the challenges and prospects associated with the practical application of Bacillus-based solutions in the field. Finally, recent advances in Bacillus-mediated plant growth promotion highlight their critical significance in sustainable agriculture. Understanding these improvements is critical for realizing the full potential of Bacillus genus microorganisms to address current global food production concerns.

Molecular and biochemical responses of sesame (Sesame indicum L.) to rhizobacteria inoculation under water deficit

Introduction

Water scarcity is a challenge for sesame cultivation under rainfed conditions. In this scenario, a potential strategy to alleviate the water deficit is the application of plant growth-promoting bacteria. The objective of this study was to analyze the interaction of rhizobacteria with sesame cultivation under water deficit conditions.

Methods

An experiment was conducted in pots in a greenhouse using the BRS Morena sesame cultivar. The experimental design was completely randomized in a factorial scheme: 2 (irrigation regimes - daily irrigation and water deficit by suspending irrigation until 90% stomatal closure) x 6 (treatments with nitrogen or inoculants), with 5 replications. The types of fertilization were characterized by the addition of nitrogen (ammonium sulfate; 21% N), inoculants based on Bacillus spp. (pant001, ESA 13, and ESA 402), Agrobacterium sp. (ESA 441), and without nitrogen (control). On the fifth day after the suspension of irrigation, plant material was collected for gene expression analysis (DREB1 and HDZ7), activities of antioxidant enzymes (superoxide dismutase and catalase), relative proline content, and photosynthetic pigments. At the end of the crop cycle (about 85 days), production characteristics (root dry matter, aboveground dry matter, number of capsules, and thousand seed weight), as well as leaf nitrogen (N) and phosphorus (P) content, were evaluated.

Results and Discussion

There was a positive effect on both production and biochemical characteristics (proline, superoxide dismutase, catalase, and photosynthetic pigments). Regarding gene expression, most of the inoculated treatments exhibited increased expression of the DREB1 and HDZ7 genes. These biological indicators demonstrate the potential of rhizobacteria for application in sesame cultivation, providing nutritional supply and reducing the effects of water deficit.

Enhancing Water Status and Nutrient Uptake in Drought-Stressed Lettuce Plants (Lactuca sativa L.) via Inoculation with Different Bacillus spp. Isolated from the Atacama Desert

Drought is a major challenge for agriculture worldwide, being one of the main causes of losses in plant production. Various studies reported that some soil's bacteria can improve plant tolerance to environmental stresses by the enhancement of water and nutrient uptake by plants. The Atacama Desert in Chile, the driest place on earth, harbors a largely unexplored microbial richness. This study aimed to evaluate the ability of various Bacillus sp. from the hyper arid Atacama Desert in the improvement in tolerance to drought stress in lettuce (Lactuca sativa L. var. capitata, cv. "Super Milanesa") plants. Seven strains of Bacillus spp. were isolated from the rhizosphere of the Chilean endemic plants Metharme lanata and Nolana jaffuelii, and then identified using the 16s rRNA gene. Indole acetic acid (IAA) production, phosphate solubilization, nitrogen fixation, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity were assessed. Lettuce plants were inoculated with Bacillus spp. strains and subjected to two different irrigation conditions (95% and 45% of field capacity) and their biomass, net photosynthesis, relative water content, photosynthetic pigments, nitrogen and phosphorus uptake, oxidative damage, proline production, and phenolic compounds were evaluated. The results indicated that plants inoculated with B. atrophaeus, B. ginsengihumi, and B. tequilensis demonstrated the highest growth under drought conditions compared to non-inoculated plants. Treatments increased biomass production and were strongly associated with enhanced N-uptake, water status, chlorophyll content, and photosynthetic activity. Our results show that specific Bacillus species from the Atacama Desert enhance drought stress tolerance in lettuce plants by promoting several beneficial plant traits that facilitate water absorption and nutrient uptake, which support the use of this unexplored and unexploited natural resource as potent bioinoculants to improve plant production under increasing drought conditions.

Yield, nutrition, and leaf gas exchange of lettuce plants in a hydroponic system in response to Bacillus subtilis inoculation

Inoculation with Bacillus subtilis is a promising approach to increase plant yield and nutrient acquisition. In this context, this study aimed to estimate the B. subtilis concentration that increases yield, gas exchange, and nutrition of lettuce plants in a hydroponic system. The research was carried out in a greenhouse in Ilha Solteira, Brazil. A randomized block design with five replications was adopted. The treatments consisted of B. subtilis concentrations in nutrient solution [0 mL "non-inoculated", 7.8 × 103, 15.6 × 103, 31.2 × 103, and 62.4 × 103 colony forming units (CFU) mL-1 of nutrient solution]. There was an increase of 20% and 19% in number of leaves and 22% and 25% in shoot fresh mass with B. subtilis concentrations of 15.6 × 103 and 31.2 × 103 CFU mL-1 as compared to the non-inoculated plants, respectively. Also, B. subtilis concentration at 31.2 × 103 CFU mL-1 increased net photosynthesis rate by 95%, intercellular CO2 concentration by 30%, and water use efficiency by 67% as compared to the non-inoculated treatments. The concentration of 7.8 × 103 CFU mL-1 improved shoot accumulation of Ca, Mg, and S by 109%, 74%, and 69%, when compared with non-inoculated plants, respectively. Inoculation with B. subtilis at 15.6 × 103 CFU mL-1 provided the highest fresh leaves yield while inoculation at 15.6 × 103 and 31.2 × 103 CFU mL-1 increased shoot fresh mass and number of leaves. Concentrations of 7.8 × 103 and 15.6 × 103 increased shoot K accumulation. The concentrations of 7.8 × 103, 15.6 × 103, and 31.2 × 103 CFU mL-1 increased shoot N accumulation in hydroponic lettuce plants.

Effect of Priestia endophytica on the metabolites accumulation in chicory and lettuce plants cultivated in vitro

The influence of the culture medium without bacterial cells, obtained after the cultivation of endophytic bacteria Priestia endophytica UCM B-5715, on the growth and synthesis of some metabolites in lettuce and chicory seedlings under in vitro conditions was studied. Bacteria were cultivated in liquid LB medium at 37 ºC for 24 h with periodic stirring. The culture fluid was separated from the cell biomass. For preparing the test solution, the supernatant was sterilized by filtration through a filter with a pore diameter of 0.2 µm (Sartorius, Minisart) and diluted with sterile distilled water. The 20% culture fluid (30 µl/plant) was applied to 3-day-old seedlings. In 28 days root and shoot weights of treated chicory plants were 54.3 ± 6.9 and 260.0 ± 20.2 mg, respectively (8.0 ± 0.7 and 91.4 ± 7.0 mg for the control plants). Total flavonoid content and antioxidant activity increased only in chicory plants after the addition of the test solution. Significant changes in the metabolism of treated plants were detected. In the treated lettuce plants asparagine content increased compared to the control (90 vs 22 µg/g, p < 0.1). The median content of fructose was also higher in treated lettuce and chicory plants (1469 vs 73 µg/g and 2278 vs 1051 µg/g). Therefore, the use of culture fluid obtained after the cultivation of P. endophytica UСM B-5715 stimulated the growth of lettuce and chicory plants, affecting the synthesis of some compounds in single-treated plants. These results indicate the potential of compounds excreted during bacterial growth to create natural growth stimulators.

The Effects of Seed Pretreatment with Endophytic Bacteria Bacillus subtilis on the Water Balance of Spring and Winter Wheat Seedlings under Short-Time Water Deficit

We investigated the effect of pre-sowing seed treatment with endophytic Bacillus subtilis 10-4 (B. subtilis) on spring and winter wheat (Triticum aestivum L.; cultivars Ekada-70 (Ek) and Scepter (Sc), respectively) growth and tolerance under 1-24 h of drought stress, modulated by 12% polyethylene glycol 6000 (PEG). The results showed that drought decreased transpiration intensity (TI), root relative water content (RWC), osmotic potential (Ψ_π) of cell sap, and induced proline accumulation and electrolyte leakage (EL) in both wheat cultivars. It was revealed that Sc was more responsive to PEG and B. subtilis treatments than Ek. Under drought, Ek did not significantly change root length, shoot height, or dry biomass. The pretreatment of wheat plants with B. subtilis performed significantly better under drought conditions through the enhanced TI, RWC, and Ψ_π of the cell sap in comparison with the plants treated with 12% PEG alone. B. subtilis also reduced stress-caused EL, especially in the Sc cultivar. Under water deficit wheat seedlings, pretreated with B. subtilis, have a higher proline accumulation in comparison to untreated stressed plants. Taken together, our results demonstrate the crucial role of endophytic B. subtilis in ameliorating the adverse effects of water stress on the water balance of both winter and spring wheat cultivars.

Integrated Microbiome and Metabolomic Analysis Reveal Responses of Rhizosphere Bacterial Communities and Root exudate Composition to Drought and Genotype in Rice (Oryza sativa L.)

BACKGROUND

As climate change events become more frequent, drought is an increasing threat to agricultural production and food security. Crop rhizosphere microbiome and root exudates are critical regulators for drought adaptation, yet our understanding on the rhizosphere bacterial communities and root exudate composition as affected by drought stress is far from complete. In this study, we performed 16S rRNA gene amplicon sequencing and widely targeted metabolomic analysis of rhizosphere soil and root exudates from two contrasting rice genotypes (Nipponbare and Luodao 998) exposed to drought stress.

RESULTS

A reduction in plant phenotypes was observed under drought, and the inhibition was greater for roots than for shoots. Additionally, drought exerted a negligible effect on the alpha diversity of rhizosphere bacterial communities, but obviously altered their composition. In particular, drought led to a significant enrichment of Actinobacteria but a decrease in Firmicutes. We also found that abscisic acid in root exudates was clearly higher under drought, whereas lower jasmonic acid and L-cystine concentrations. As for plant genotypes, variations in plant traits of the drought-tolerant genotype Luodao 998 after drought were smaller than those of Nipponbare. Interestingly, drought triggered an increase in Bacillus, as well as an upregulation of most organic acids and a downregulation of all amino acids in Luodao 998. Notably, both Procrustes analysis and Mantel test demonstrated that rhizosphere microbiome and root exudate metabolomic profiles were highly correlated. A number of differentially abundant genera responded to drought and genotype, including Streptomyces, Bacillus and some members of Actinobacteria, were significantly associated with organic acid and amino acid contents in root exudates. Further soil incubation experiments showed that Streptomyces was regulated by abscisic acid and jasmonic acid under drought.

CONCLUSIONS

Our results reveal that both drought and genotype drive changes in the compositions of rice rhizosphere bacterial communities and root exudates under the greenhouse condition, and that organic acid exudation and suppression of amino acid exudation to select specific rhizosphere bacterial communities may be an important strategy for rice to cope with drought. These findings have important implications for improving the adaptability of rice to drought from the perspective of plant-microbe interactions.