Solubilization of zinc compounds by the diazotrophic, plant growth promoting bacterium Gluconacetobacter diazotrophicus

Augmentation of physiology and productivity, and reduction of lead accumulation in lettuce grown in lead contaminated soil by rhizobacteria-assisted rhizoengineeing

Microbial-assisted rhizoengineering is a promising biotechnology for improving crop productivity. In this study, lettuce roots were bacterized with two lead (Pb) tolerant rhizobacteria including Pseudomonas azotoformans ESR4 and P. poae ESR6, and a consortium consisted of ESR4 and ESR6 to increase productivity, physiology and antioxidants, and reduce Pb accumulation grown in Pb-contaminated soil i.e., 80 (Pb in native soil), 400 and 800 mg kg-1 Pb. In vitro studies showed that these strains and the consortium produced biofilms, synthesized indole-3-acetic acid and NH3, and solubilized phosphate challenging to 0, 100, 200 and 400 mg L-1 of Pb. In static conditions and 400 mg L-1 Pb, ESR4, ESR6 and the consortium adsorbed 317.0, 339.5 and 357.4 mg L-1 Pb, respectively, while 384.7, 380.7 and 373.2 mg L-1 Pb, respectively, in shaking conditions. Fourier transform infrared spectroscopy results revealed that several functional groups [Pb-S, M - O, O-M-O (M = metal ions), S-S, PO, CO, -NH, -NH2, C-C-O, and C-H] were involved in Pb adsorption. ESR4, ESR6 and the consortium-assisted rhizoengineering (i) increased leaf numbers and biomass production, (ii) reduced H2O2 production, malondialdehyde, electrolyte leakages, and transpiration rate, (iii) augmented photosynthetic pigments, photosynthetic rate, water use efficiency, total antioxidant capacity, total flavonoid content, total phenolic content, and minerals like Ca2+ and Mg2+ in comparison to non-rhizoengineering plants grown in Pb-contaminated soil. Principal component analysis revealed that higher pigment production and photosynthetic rate, improved water use efficiency and increased uptake of Ca2+ were interlinked to increased productivity by bacterial rhizoengineering of lettuce grown in different levels of Pb exposures. Surprisingly, Pb accumulation in lettuce roots and shoots was remarkably decreased by rhizoengineering than in non-rhizoengineering. Thus, these bacterial strains and this consortium could be utilized to improve productivity and reduce Pb accumulation in lettuce.

Unraveling the functional characteristics of endophytic bacterial diversity for plant growth promotion and enhanced secondary metabolite production in Pelargonium graveolens

The rich diversity of microbial endophytic communities associated with plants, often referred to as the second genome, serves as a compelling illustration of efficient co-evolution. This noteworthy partnership plays a pivotal role in sustaining plant well-being and enhancing plant adaptability across diverse habitats. Therefore, examining the diversity of endophytic microbes associated with their particular host plant is valuable for gaining insights into the vast spectrum of plant-microbe interactions. The present experiments aimed at investigating the bacterial endophytic diversity in both root and shoot tissues of Pelargonium graveolens, employing culture dependent and culture independent high-throughput metagenomics approach. A total of 614 and 620 operational taxonomic units (OTUs), encompassing 291 and 229 genera, were identified in the shoot and root tissues of P. graveolens, respectively. Furthermore, the subsequent classification of OTUs revealed 15 highly abundant phyla, with Proteobacteria dominating both root and shoot tissues. Notably, an exceptionally high abundance of Firmicutes phyla was observed in the shoot compared to the root. Additionally, 30 bacterial endophytes from the root, stem, petiole, and leaves were isolated and molecularly characterized, unveiling a consistent pattern of diversity distribution between the root and shoot of P. graveolens. Upon screening all isolates for plant growth promoting traits, Pseudomonas oryzihabitans was found to be positive for major biochemical test like nitrogen fixation, phosphate solubilization etc. and on inoculation resulted in about two-fold increase in content of essential oil accompanied by a significant rise in the geraniol and citronellol content. Diving deep into the genetic constitution of P. oryzihabitans unveiled a substantial number of genes directly and indirectly contributing to the endophyte's capability in colonizing host plants effectively. In summary, data obtained from metagenomics and culture dependent approaches including glass house trials suggest potential bacterial endophytes suitable for field applications for yield enhancement and in planta secondary metabolite enhancement investigations.

Zinc solubilization and organic acid production by the entomopathogenic fungus, Metarhizium pingshaense sheds light on its key ecological role in the environment

We report for the first-time higher zinc (Zn) solubilization efficiency and plant growth promotion by an entomopathogenic fungus (EPF), Metarhizium pingshaense IISR-EPF-14, which was earlier isolated from Conogethes punctiferalis, a pest of global importance. The Zn solubilizing efficiency of the fungus varied depending on the type of insoluble source of Zn used, which was observed to be 1.6 times higher in Zn3(PO4)2-amended media compared to ZnO media. In liquid media, there was a 6.2-fold increase in available Zn in ZnO-amended media, whereas a 20.2-fold increase in available Zn was recorded in Zn3(PO4)2 medium. We ascribe the production of various organic acids such as gluconic, keto-gluconic, oxalic, tartaric, malonic, succinic and formic acids, which in general, interact with insoluble Zn sources and make them soluble by forming metal cations and displacing anions as the major mechanism for Zn solubilization by M. pingshaense. However, the type and amount of organic acid produced in the media varied depending on the source of Zn used and the incubation period. Application of the fungus alone and in combination with insoluble Zn sources enhanced various plant growth parameters in rice and cardamom plants. Moreover, the uptake of Zn in rice plants was enhanced up to ~2.5-fold by fungal application. The fungus also exhibited various other plant growth-promoting traits, such as production of Indole-3-acetic acid, ammonia, siderophores, solubilization of mineral phosphate, and production of hydrolytic enzymes such as α-amylase, protease, and pectinase. Hence, apart from its use as a biological control agent, M. pingshaense has the potential to be used as a bio-fortifier to enhance the solubilization and uptake of Zn from nutrient poor soils under field conditions. Our findings shed light on the broader ecological role played by this fungus and widen its scope for utilization in sustainable agriculture.

Salt tolerant Pseudomonas taiwanensis PWR-1 in combination with a reduced dose of mineral fertilizers improves the nutritional and antioxidant properties of wheatgrass grown in saline soil

Salt-tolerant plant growth promoting rhizobacteria (ST-PGPR) are known to ameliorate salt stress in plants by various mechanisms. The current study aims to investigate the role of an ST-PGPR strain Pseudomonas taiwanensis PWR-1 applied along with a reduced dose of mineral fertilizers (N, P, and K) in the improvement of the antioxidant and nutritional properties of wheatgrass (Triticum aestivum L.) grown in saline soil. Application of P. taiwanensis PWR-1 along with 50% of the recommended dose of mineral fertilizers resulted in a significant improvement of growth parameters including shoot length (22.79%), root length (20.38%), fresh weight (13.15%), dry weight (92.34%), vigor index (13.36%), and relative water content (48.24%). The combined application of PWR-1 and mineral fertilizers increased the production of osmoprotectants (proline, total soluble sugars, glycine betaine), antioxidants (SOD, POD, APX, CAT, PPO, and reduced glutathione), and free radical scavengers (DPPH and H2O2) in wheatgrass. Furthermore, the concentration of micronutrients (Zn and Fe), macronutrients (N, and P), and vitamins (B1 and E) also increased in the above treatment. Oxidative stress markers (malondialdehyde and electrolyte leakage) and Na+ accumulation were significantly reduced whilst K+ content increased in the shoot, which helped in maintaining the K+/Na+ ratio in wheatgrass under saline conditions. The results indicated that the application of ST-PGPR could not only reduce the dosage of mineral fertilizers but might be useful for improving the nutritional and antioxidant properties of medicinal crops such as wheatgrass under salt-stress conditions. Implementing this approach could result in the reduction of chemical usage, while also facilitating enhanced uptake of micronutrients in crops, particularly in regions affected by salinity.

Selenium hyperaccumulator plant Cardamine enshiensis: from discovery to application

Selenium (Se) is an essential trace element for animals and humans. Se biofortification and Se functional agriculture are emerging strategies to satisfy the needs of people who are deficient in Se. With 200 km2 of Se-excess area, Enshi is known as the "world capital of Se." Cardamine enshiensis (C. enshiensis) is a Se hyperaccumulation plant discovered in the Se mine drainage area of Enshi. It is edible and has been approved by National Health Commission of the People's Republic of China as a new source of food, and the annual output value of the Se-rich industry in Enshi City exceeds 60 billion RMB. This review will mainly focus on the discovery and mechanism underlying Se tolerance and Se hyperaccumulation in C. enshiensis and highlight its potential utilization in Se biofortification agriculture, graziery, and human health.

Improving the Growth and Productivity of Macrotyloma uniflorum Medicinal Plant by the Co-inoculation of P, Zn and K-Solubilizing Fungi Under Field Conditions

The presence of small amount of soluble forms of Phosphorus (P), Potassium (K) and Zinc (Zn) in most soils is one of the limiting factors for agronomic crop production. The current study focuses on Macrotyloma uniflorum (horse gram or gahat), the most commonly cultivated crop in Uttarakhand. The current initiative and study were started, because there is a little information available on the impact of co-inoculation of beneficial fungi on crops in agricultural fields. Aspergillus niger K7 and Penicillium chrysogenum K4 were isolated and selected for the study on the basis of in vitro P, K and Zn-solubilizing activity. The solubilizing efficiency of K4 strain was 140% and K7 was 173.9% for P. However, the solubilizing efficiencies of K4 and K7 were 160% and 138.46% for Zn and 160% and 466% for K, respectively. The field trials were performed for two consecutive years, and growth and yield related parameters were measured for evaluation of the effect of P, K and Zn-solubilizing fungal strains on the crop. All the treatments showed a significant (P < 0.05) increase in growth and yield of M. uniflorum plants over uninoculated control; however, the best treatment was found to be soil inoculated with P. chrysogenum K4 + A. niger K7 in which the yield was enhanced by 71% over control. Thus, the co-inoculation of K4 and K7 strains showed a great potential to improve the growth and yield of plants. Both the fungal strains simultaneously solubilized three important nutritional elements in soil, which is a rare trait. Moreover, the capacity of these fungal strains to enhance the plant root nodulation and microbial count in soil makes the co-inoculation practice quite beneficial for sustainable agriculture.

Plant growth-promoting traits of culturable seed microbiome of citrus species from Purvanchal Himalaya

Despite Northeastern India being "Treasure House of Citrus Genetic Wealth," genetic erosion of citrus diversity poses severe concern with a corresponding loss in seed microbial diversity. The seed microbiome of citrus species unique to the Purvanchal Himalaya is seldom explored for their use in sustainable orchard management. Isolation and characterization of culturable seed microbiomes of eight citrus species, namely, Citrus reticulata Blanco, C. grandis (L.) Osbeck, C. latipes Tanaka, C. megaloxycarpa Lushaigton, C. jambhiri Lush, C. sinensis (L.) Osbeck, C. macroptera Montr, and C. indica Tanaka collected from NE India were carried out. The isolates were then screened for an array of plant growth-promoting (PGP) traits [indole acetic acid (IAA) production, N₂ fixation, phosphate and zinc complex dissolution, siderophores, and Hydrogen Cyanide (HCN) production]. The pure culture isolates of seed microbiomes were capable of dissolving insoluble Ca₃(PO₄)₂ (1.31-4.84 µg Pi ml^-1 h^-1), Zn₃(PO₄)₂ (2.44-3.16 µg Pi ml^-1 h^-1), AlPO₄ (1.74-3.61 µg Pi ml^-1 h^-1), and FePO₄ (1.54-4.61µg Pi ml^-1 h^-1), mineralized phytate (12.17-18.00 µg Pi ml^-1 h^-1) and produced IAA-like substances (4.8-187.29 µg ml^-1 h^-1). A few isolates of the seed microbiome were also able to fix nitrogen, secrete siderophore-like compounds and HCN, and dissolve ZnSO₄ and ZnO. The 16S ribosomal Ribonucleic Acid (rRNA)-based taxonomic findings revealed that Bacillus was the most dominant genus among the isolates across citrus species. Isolates CG2-1, CME6-1, CME6-4, CME6-5, CME6-9, CJ7-1, CMA10-1, CI11-3, and CI11-4 were identified as promising bioinoculants for development of microbial consortium having multifaceted PGP traits for nutritional benefits of nitrogen, phosphorus and zinc, and IAA hormonal benefits to citrus crops for better fitness in acid soils.

Finger Millet (Eleusine coracana) Plant-Endophyte Dynamics: Plant Growth, Nutrient Uptake, and Zinc Biofortification

Endophytic fungi and bacteria were isolated from finger millet and their effects on finger millet growth parameters and zinc and NPK contents in grains were studied. Out of 70 fungal and 112 bacterial endophytes, the two best fungal and bacterial isolates were selected on the basis of zinc solubilization and plant-growth-promoting attributes. The fungal isolates identified were Aspergillus terreus and Lecanicillium sp., and the bacterial isolates were Pseudomonas bijieensis and Priestia megaterium. The endophytic zinc, NPK mobilization, and plant-growth-promoting efficacy were determined in a pot experiment with zinc carbonate as the zinc source. Endophytic-primed plants showed enhanced shoot and root lengths compared to the unprimed control. Endophytes increased the zinc content in grains by between 12.12% and 18.80% compared to control plants. Endophytes also augmented the NPK concentrations in seeds compared to control plants and exhibited stability in a diverse range of pHs, temperatures, and NaCl concentrations, and exhibited growth on various carbohydrate and nitrogen sources. This is the first study reporting the interaction of Aspergillus terreus, Lecanicillium sp., Pseudomonas bijieensis, and Priestia megaterium with finger millet for grain Zn biofortification and NPK concentration enhancement. This study indicated that zinc-dissolving endophytes possess the potential for enhancing the zinc and NPK content in grains in addition to the plant-growth-promoting attributes.

Cold-tolerant phosphate-solubilizing Pseudomonas strains promote wheat growth and yield by improving soil phosphorous (P) nutrition status

It is well-known that phosphate-solubilizing bacteria (PSB) promote crop growth and yield. The information regarding characterization of PSB isolated from agroforestry systems and their impact on wheat crops under field conditions is rarely known. In the present study, we aim to develop psychrotroph-based P biofertilizers, and for that, four PSB strains (Pseudomonas sp. L3, Pseudomonas sp. P2, Streptomyces sp. T3, and Streptococcus sp. T4) previously isolated from three different agroforestry zones and already screened for wheat growth under pot trial conditions were evaluated on wheat crop under field conditions. Two field experiments were employed; set 1 includes PSB + recommended dose of fertilizers (RDF) and set 2 includes PSB – RDF. In both field experiments, the response of the PSB-treated wheat crop was significantly higher compared to the uninoculated control. In field set 1, an increase of 22% in grain yield (GY), 16% in biological yield (BY), and 10% in grain per spike (GPS) was observed in consortia (CNS, L3 + P2) treatment, followed by L3 and P2 treatments. Inoculation of PSB mitigates soil P deficiency as it positively influences soil alkaline phosphatase (AP) and soil acid phosphatase (AcP) activity which positively correlated with grain NPK %. The highest grain NPK % was reported in CNS-treated wheat with RDF (N–0.26%, P–0.18%, and K-1.66%) and without RDF (N-0.27, P-0.26, and K-1.46%), respectively. All parameters, including soil enzyme activities, plant agronomic data, and yield data were analyzed by principal component analysis (PCA), resulting in the selection of two PSB strains. The conditions for optimal P solubilization, in L3 (temperature-18.46, pH–5.2, and glucose concentration–0.8%) and P2 (temperature-17°C, pH–5.0, and glucose concentration–0.89%), were obtained through response surface methodology (RSM) modeling. The P solubilizing potential of selected strains at &lt;20°C makes them a suitable candidate for the development of psychrotroph-based P biofertilizers. Low-temperature P solubilization of the PSB strains from agroforestry systems makes them potential biofertilizers for winter crops.

Biofortification: A long-term solution to improve global health- a review

Biofortification is a revolutionary technique for improving plant nutrition and alleviating human micronutrient deficiency. Fertilizers can help increase crop yield and growth, but applying too much fertilizer can be a problem because it leads to the release of greenhouse gases and eutrophication. One of the major global hazards that affects more than two million people globally is the decreased availability of micronutrients in food crops, which results in micronutrient deficiencies or "hidden hunger" in people. Micronutrients, like macronutrients, perform a variety of roles in plant and human nutrition. This review has highlighted the importance of micronutrients as well as their advantages. The uneven distribution of micronutrients in geological areas is not the only factor responsible for micronutrient deficiencies, other parameters including soil moisture, temperature, texture of the soil, and soil pH significantly affects the micronutrient concentration and their availability in the soil. To overcome this, different biofortification approaches are assessed in the review in which microbes mediated, Agronomic approaches, Plant breeding, and transgenic approaches are discussed. Hidden hunger can result in risky health conditions and diseases such as cancer, cardiovascular disease, osteoporosis, neurological disorders, and many more. Microbes-mediated biofortification is a novel and promising solution for the bioavailability of nutrients to plants in order to address these problems. Biofortification is cost effective, feasible, and environmentally sustainable. Bio-fortified crops boost our immunity, which helps us to combat these deadly viruses. The studies we discussed in this review have demonstrated that they can aid in the alleviation of hidden hunger.

Zinc-Solubilizing Streptomyces spp. as Bioinoculants for Promoting the Growth of Soybean (Glycine max (L.) Merrill)

Zinc-solubilizing bacteria can convert the insoluble form of zinc into soluble forms available to plants. This study was conducted to isolate and screen zinc-solubilizing actinobacteria from rhizosphere soils and to assess their effect on vegetable soybean growth. In total, 200 actinobacteria strains belonging to 10 genera were isolated from rhizosphere soil samples. Among these isolates, four showed zinc solubilization with solubilizing index values ranging from 3.11 to 3.78 on Bunt and Rovira agar supplemented with 0.1% zinc oxide. For the quantitative assay, in broth culture, strains CME34 and EX51 solubilized maximum available zinc contents of 529.71 and 243.58 μg/ml. Furthermore, indole-3-acetic acid (IAA) and ammonia were produced by these two strains, the strain CME34 produced the highest amount of IAA 4.62 μg/ml and the strain EX51 produced the highest amount of ammonia 361.04 μg/ml. In addition, the phosphate-solubilizing abilities in Pikovskaya's medium of CME34 and EX51 were 64.67 and 115.67 μg/ml. Based on morphological and biochemical characterization and 16S rDNA sequencing, the strains CME34 and EX51 were closely related to the genus Streptomyces. In a greenhouse experiment, single-strain inoculation of Streptomyces sp. CME34 or EX51 significantly increased the shoot length, root length, plant dry weight, number of pods per plant and number of seeds per plant of vegetable soybean plants compared to the uninoculated control. These findings facilitated the conclusion that the two Streptomyces strains have potential as zinc solubilizers and can be suggested as bioinoculants to promote the growth and yield of soybean.

Increase of P and Cd bioavailability in the rhizosphere by endophytes promoted phytoremediation efficiency of Phytolacca acinosa

The information about the spatial distribution of bioavailable phosphorus (P) and heavy metal (HM) in the rhizosphere could aid in the precise phytoremediation regulation. In this study, a rhizobox system was adapted to study soil-root interactions and used to access the endophyte inoculation variation on bioavailable P and cadmium (Cd) spatial distribution during phytoremediation of Cd contaminated soils. Results showed that endophyte PE31 Bacillus cereus inoculation enhanced Cd uptake of P. acinosa by 52.70% and 46.73% in low and high Cd contaminated soils, increasing the phytoremediation hotspot area from 45.78% and 15.29% to 60.97% and 21.80%, respectively. Available P and Cd significantly diminished because root activities depleted large amounts of bioavailable P and Cd concentrations. However, PE31 increased bioavailable P and Cd concentration in the rhizosphere soil. The bioavailable P enhancement in the rhizosphere was positively correlated to plant growth and Cd accumulation. Overall, endophyte inoculation compensated the diminution of bioavailable P and Cd in the rhizosphere to improve plant biomass and HM absorption, and thus promote phytoremediation efficiency. This study helped to better understand bioavailable P and Cd spatial distribution under endophyte inoculation, which could provide effective management strategies for the precise regulation of phytoremediation.

2,4-Diacetylphloroglucinol producing Pseudomonas fluorescens JM-1 for management of ear rot disease caused by Fusarium moniliforme in Zea mays L

Maize (Zea mays L.) is a major cereal crop grown in a large number of countries. Loss in maize yield due to biotic stresses including fungal phytopathogens is a matter of immense concern. Control measures applied for eradication of fungal phytopathogens in maize are not up to the mark and more often involve harsh chemical(s)/pesticide(s) that cause deleterious effects both in humans and soil biota. Greener alternatives, such as the use of rhizosphere microbes in the form of bioinoculants, have proven to be very successful in terms of enhancing crop yield and suppressing fungal phytopathogens. In the present study, fluorescent pseudomonads were isolated from the maize rhizosphere and monitored for their plant growth-promoting (PGP) and biocontrol activities against Fusarium moniliforme. Based on various PGP traits and biocontrol potential, isolate JM-1 was found to be most effective and as per 16S rRNA gene sequencing analysis was identified as Pseudomonas fluorescens. Further experiments showed that the biocontrol potential of JM-1 against ear rot fungus involved the production of antifungal compound 2,4-diacetylphloroglucinol (DAPG). When examined for antagonistic interaction under scanning electron microscopy (SEM), structural abnormality, hyphal lysis, and deformity in fungal mycelium were observed. In the pot experiment, application of talc-based JM-1 containing bioformulation (in pot trials) showed significant enhancement in maize growth parameters (including the seed number and weight) in comparison to control even in presence of the phytopathogen. Ear fresh weight, dry weight, number of seeds per plant, and 100-grain weight were found to increase significantly by 34, 34, 52, and 18% respectively, in comparison to control. P. fluorescens JM-1 can therefore be used as a bioinoculant for ear rot disease control and sustainably enhancing maize yield.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03201-7.

Halotolerant biofilm-producing rhizobacteria mitigate seawater-induced salt stress and promote growth of tomato

Biofilm-producing rhizobacteria (BPR) enhance productivity and mitigate abiotic stresses in plants. This study showed that 21 out of 65 halotolerant rhizobacteria could build biofilms. The components of the biofilm matrices i.e., extracellular polymeric substances (EPS) are proteins, curli, nanocelloluse, nucleic acids, lipids, and peptidoglycans. Various functional groups including carbonyl, carboxyl, amino, hydroxyl, and phosphate were identified. Positions of these groups were shifted by application of 5% NaCl, suggesting Na⁺ biosorption. By sequencing, Glutamicibacter arilaitensis (ESK1, ESM4 and ESM7), G. nicotianae (ESK19, ESM8 and ESM16), Enterobacter ludwigii (ESK15, ESK17, ESM2 and ESM17), E. cloacae (ESM5 and ESM12), Exiguobacterium acetylicum (ESM24 and ESM25), Staphylococcus saprophyticus ESK6, Leclercia adecarboxylata ESK12, Pseudomonas poae ESK16, Bacillus subtilis ESM14, and P. putida ESM17 were identified. These rhizobacteria exhibited numerous plant growth-promoting (PGP) activities including producing IAA, ACC deaminase, and siderophores, and solubilizing phosphate. Under non-stress, bacterized plants increased biomass accumulation (8–23.2% roots and 23–49.4% shoots), while under seawater-induced salt stress only ESK12, ESM4, ESM12, and ESM14 enhanced biomass production (5.8–52.9% roots and 8.8–33.4% shoots). Bacterized plants induced antioxidant defense system (19.5–142% catalase and 12.3–24.2% DPPH radical scavenging activity), retained a greater relative water content (17–124%), showed lesser membrane injuries (19.9–26.5%), and a reduced Na⁺ (6–24% in roots) and increased K⁺/Na⁺ ratio (78.8 and 103% in roots by ESK12 and ESM24, respectively) than the non-bacterized plants in saline conditions. Thus, native halotolerant BPR can be utilized as ameliorators of salt stress.

Isolation of endophytic bacteria from the medicinal, forestal and ornamental tree Handroanthus impetiginosus

Plant interactions with endophytic bacteria produce mutual benefits and contribute to environmental sustainability. Handroanthus impetiginosus (Mart. ex DC.) Mattos 'pink lapacho' (syn. Tabebuia impetiginosa, Bignoniaceae) is a medicinal, ornamental and forestal native tree from South and Mesoamerica. Plant growth promoting bacteria (PGPB) isolated from pink lapacho are scarcely described. The aim of this work was to isolate and characterize native endophytic bacteria from pink lapacho. Ten bacterial strains were isolated from leaves and six from roots of naturally growing trees in Luján (Central-Eastern region of Argentina). Endophytes were identified as Bacillus, Paenibacillus, Pseudomonas, Rhizobium, Rummeliibacillus and Methylobacterium genera, according to 16S rRNA gene sequencing and phylogenetic analysis. In the present study, a strain of the Rummelibacillus genus (L14) has been first ever reported as endophyte. This strain was capable of growing in Nfb medium and exhibited zinc solubilization ability. A high percentage of strains showed PGPB traits; namely 88% fixed nitrogen, 63% solubilized zinc, 69% solubilized phosphate and 63% produced indole compounds such as IAA. Most strains were salt tolerant that confer them a potential competitive advantage to survive in saline conditions. To the best of our knowledge, this is the first study reporting an approach to assess the diversity of cultivable endophytic bacteria of H. impetiginosus tree and its plant growth promoting capacity. The knowledge about this kind of associations could contribute to environmental sustainability by developing effective biofertilizers that minimize the use of chemical fertilizers and pesticides.

Modulation in Biofertilization and Biofortification of Wheat Crop by Inoculation of Zinc-Solubilizing Rhizobacteria

Zinc is an important micronutrient needed for the optimum growth and development of plants. Contrary to chemical zinc fertilizers, the use of zinc-solubilizing bacteria is an environmentally friendly option for zinc enrichment in edible parts of crops. This study was conducted with the objective of selecting potential zinc-solubilizing rhizobacteria from the rhizosphere of chickpea grown in soils of eastern Uttar Pradesh and further assessing their impact on the magnitude of zinc assimilation in wheat crops. Among 15 isolates, CRS-9, CRS-17, CRS-30, and CRS-38 produced net soluble zinc in broth to the tune of 6.1, 5.9, 5.63, and 5.6 μg ml-1, respectively, in zinc phosphate with the corresponding pH of 4.48, 5.31, 5.2, and 4.76. However, the bacterial strains CRS-17, CRS-30, CRS-38, and CRS-9 showed maximum zinc phosphate solubilization efficiency of 427.79, 317.39, 253.57, and 237.04%, respectively. The four bacterial isolates were identified as Bacillus glycinifermentans CRS-9, Microbacterium oxydans CRS-17, Paenarthrobacter nicotinovorans CRS-30, and Bacillus tequilensis CRS-38 on the basis of morphological and biochemical studies and 16S rRNA gene sequencing. Bacterial inoculants significantly colonized the roots of wheat plants and formed a biofilm in the root matrix. These strains significantly increased seed germination (%) and vigor indices in wheat grown under glasshouse conditions. After 30 days of sowing of wheat under microcosm conditions, eight zinc transporter (TaZIP) genes were expressed maximally in roots, with concomitant accumulation of higher zinc content in the bacterially treated plant compared to the absolute control. Out of the four strains tested, two bacteria, B. tequilensis CRS-38 and P. nicotinovorans CRS-30, improved seed germination (%), vigor indices (2-2.5 folds), plant biomass, grain yield (2.39 g plant-1), and biofortificated grains (54.25 μg g-1Zn) of wheat. To the best of our knowledge, this may be the first report on the presence of zinc solubilization trait in B. glycinifermentans CRS-9, M. oxydans CRS-17, and P. nicotinovorans CRS-30.

Functional Investigation of Plant Growth Promoting Rhizobacterial Communities in Sugarcane

Plant microbiota are of great importance for host nutrition and health. As a C₄ plant species with a high carbon fixation capacity, sugarcane also associates with beneficial microbes, though mechanisms underlying sugarcane root-associated community development remain unclear. Here, we identify microbes that are specifically enriched around sugarcane roots and report results of functional testing of potentially beneficial microbes propagating with sugarcane plants. First, we analyzed recruitment of microbes through analysis of 16S rDNA enrichment in greenhouse cultured sugarcane seedlings growing in field soil. Then, plant-associated microbes were isolated and assayed for beneficial activity, first in greenhouse experiments, followed by field trials for selected microbial strains. The promising beneficial microbe SRB-109, which quickly colonized both roots and shoots of sugarcane plants, significantly promoted sugarcane growth in field trials, nitrogen and potassium acquisition increasing by 35.68 and 28.35%, respectively. Taken together, this report demonstrates successful identification and utilization of beneficial plant-associated microbes in sugarcane production. Further development might facilitate incorporation of such growth-promoting microbial applications in large-scale sugarcane production, which may not only increase yields but also reduce fertilizer costs and runoff.

Bioinoculants-Natural Biological Resources for Sustainable Plant Production

Agricultural sustainability is of foremost importance for maintaining high food production. Irresponsible resource use not only negatively affects agroecology, but also reduces the economic profitability of the production system. Among different resources, soil is one of the most vital resources of agriculture. Soil fertility is the key to achieve high crop productivity. Maintaining soil fertility and soil health requires conscious management effort to avoid excessive nutrient loss, sustain organic carbon content, and minimize soil contamination. Though the use of chemical fertilizers have successfully improved crop production, its integration with organic manures and other bioinoculants helps in improving nutrient use efficiency, improves soil health and to some extent ameliorates some of the constraints associated with excessive fertilizer application. In addition to nutrient supplementation, bioinoculants have other beneficial effects such as plant growth-promoting activity, nutrient mobilization and solubilization, soil decontamination and/or detoxification, etc. During the present time, high energy based chemical inputs also caused havoc to agriculture because of the ill effects of global warming and climate change. Under the consequences of climate change, the use of bioinputs may be considered as a suitable mitigation option. Bioinoculants, as a concept, is not something new to agricultural science, however; it is one of the areas where consistent innovations have been made. Understanding the role of bioinoculants, the scope of their use, and analysing their performance in various environments are key to the successful adaptation of this technology in agriculture.

Diversity of Bacterial Endophytes of Maize (Zea mays) and Their Functional Potential for Micronutrient Biofortification

Microorganisms due, to their immense metabolic diversity, have the potential to augment the uptake of iron and zinc in addition to other important nutrients in plants. In the present work, 129 different strains of endophytic bacteria were retrieved from stems and leaves of maize. Qualitative screening of these endophytes showed that 24.5% of these isolates were siderophore producers, while 14% could solubilize insoluble zinc compounds and 33% of them had phytase activity. Based on zinc solubilization efficiency and siderophore production ability, 10 isolates each from zinc solubilizers and siderophore producers were selected. Molecular identification indicated that the selected bacteria belonged to diverse genera Microbacterium, Pseudonocardia, Bacillus, Cellulosimicrobium, Staphylococcus, Luteimonas, Bordetella, Brevundimonas, Streptomyces, Cupriavidus, Sphingomonas, Ralstonia, Ochrobactrum, Conyzicola, Paenibacillus and Leifsonia. Quantitative analyses of Zn solubilization using Atomic absorption spectrophotometry (AAS) revealed that Microbacterium hydrothermale M10 and M. proteolyticum B2 were potential solubilizers of different forms of insoluble zinc compounds viz. ZnCO₃ (56.63-89.88 ppm), ZnO (106.38-120.08 ppm) and ZnS (3.62-5.56 ppm). Similarly, quantitative estimation of siderophore production activity revealed two endophytes viz. Bacillus altitudinis C7 (97.25% siderophore units) and Pseudonocardia alni M29 (92.05% siderophore units) as potential siderophore producers. These endophytes with potential to produce siderophores and phytases and ability to solubilize zinc can be an important starting material for trials on field to improve Fe and Zn content in edible portion of food crops.

Tyrosine Kinase Self-Phosphorylation Controls Exopolysaccharide Biosynthesis in Gluconacetobacter diazotrophicus Strain Pal5

The biosynthesis of exopolysaccharides (EPSs) is essential for endophytic bacterial colonisation in plants bacause this exopolymer both protects bacterial cells against the defence and oxidative systems of plants and acts on the plant colonisation mechanism in Gluconacetobacter diazotrophicus. The pathway involved in the biosynthesis of bacterial EPS has not been fully elucidated, and several areas related to its molecular regulation mechanisms are still lacking. G. diazotrophicus relies heavily on EPS for survival indirectly by protecting plants from pathogen attack as well as for endophytic maintenance and adhesion in plant tissues. Here, we report that EPS from G. diazotrophicus strain Pal5 is a signal polymer that controls its own biosynthesis. EPS production depends on a bacterial tyrosine (BY) kinase (Wzc) that consists of a component that is able to phosphorylate a glycosyltranferase or to self-phosphorylate. EPS interacts with the extracellular domain of Wzc, which regulates kinase activity. In G. diazotrophicus strains that are deficient in EPS production, the Wzc is rendered inoperative by self-phosphorylation. The presence of EPS promotes the phosphorylation of a glycosyltransferase in the pathway, thus producing EPS. Wzc-mediated self-regulation is an attribute for the control of exopolysaccharide biosynthesis in G. diazotrophicus.

Bioremediation of heavy metals from industrial effluents by endophytes and their metabolic activity: Recent advances

Worldwide, heavy metals pollution is mostly caused by rapid population growth and industrial development which is accumulated in food webs causing a serious public health risk. Endophytic microorganisms have a variety of mechanisms for metal sequestration having metal biosorption capacities.Endophytic organisms like bacteria and fungi provide beneficial qualities that help plants to improve their health, reduce stress, and detoxify metals. Endophytes have a higher proclivity for improving metal and mineral solubility by cells that secrete low-molecular-weight organic acids and metal-specific ligands like siderophores, which change the pH of the soil and improve binding activity. Protein-related approaches like chromatin immunoprecipitation sequencing (ChIP-Seq) and modified enzyme-linked immunosorbent assay (ELISA test) can represent endophytic bacterial community and DNA-protein interactions during metal reduction. This review explored the role of endophytes in bioremediation approaches that can help in analyzing the potential and prospects in response to industrial effluents' detoxification.

Assessment of the Streptomyces-plant system to mitigate the impact of Cr(VI) and lindane in experimental soils

Phytoremediation techniques have been proposed as ecological methods to clean up contaminated sites. This study is aimed to evaluate the effect of the Streptomyces sp. Waksman & Henrici and Zea mays L. plant system on the dissipation of Cr(VI) and/or lindane from a co-contaminated soil, being 2 mg kg^-1 of lindane and 150 mg kg^-1 of chromium used. Lindane dissipation was improved in the presence of plant-microorganism association; however, Cr(VI) removal was higher when plants or the microorganism were separately. In co-contaminated systems, chromium content in plant tissues was lower than metal content in plants grown only with Cr(VI), suggesting that lindane could interfere with metal accumulation in the plant. The high malondialdehyde (MDA) concentration detected in non-inoculated plants grown with chromium could be consequence of high metal concentration in plant tissues. Interestingly, plants inoculated with Streptomyces sp. Z38 growing with Cr(VI) showed decrease in MDA concentration, indicating that the bacterium could activate defense mechanisms in the plant. Also, inoculated plants showed the highest value of superoxide dismutase activity. Lettuce plants used as bioindicators grew better in biologically treated soils compared with lettuce grown on non-treated soil. The results presented in this work provide the basis that will allow the optimization of future trials on a larger scale.

Combination of osmotic stress and sugar stress response mechanisms is essential for Gluconacetobacter diazotrophicus tolerance to high-sucrose environments

Sugar-rich environments represent an important challenge for microorganisms. The osmotic and molecular imbalances resulting from this condition severely limit microbial metabolism and growth. Gluconacetobacter diazotrophicus is one of the most sugar-tolerant prokaryotes, able to grow in the presence of sucrose concentrations up to 30%. However, the mechanisms that control its tolerance to such conditions remain poorly exploited. The present work investigated the key mechanisms of tolerance to high sugar in G. diazotrophicus. Comparative proteomics was applied to investigate the main functional pathways regulated in G. diazotrophicus when cultivated in the presence of high sucrose. Among 191 proteins regulated by high sucrose, regulatory pathways related to sugar metabolism, nutrient uptake, compatible solute synthesis, amino acid metabolism, and proteolytic system were highlighted. The role of these pathways on high-sucrose tolerance was investigated by mutagenesis analysis, which revealed that the knockout mutants zwf::Tn5 (sugar metabolism), tbdr::Tn5 (nutrient uptake), mtlK::Tn5 (compatible solute synthesis), pepN::Tn5 (proteolytic system), metH::Tn5 (amino acid metabolism), and ilvD::Tn5 (amino acid metabolism) became more sensitive to high sucrose. Together, our results identified mechanisms involved in response to high sugar in G. diazotrophicus, shedding light on the combination of osmotolerance and sugar-tolerance mechanisms. KEY POINTS: • G. diazotrophicus intensifies glycolysis to metabolize the excess of sugar. • G. diazotrophicus turns down the uptake of nutrients in response to high sugar. • G. diazotrophicus requires amino acid availability to resist high sugar.

Isolation, Characterization of Zn Solubilizing Bacterium (Pseudomonas protegens RY2) and its Contribution in Growth of Chickpea (Cicer arietinum L) as Deciphered by Improved Growth Parameters and Zn Content

Background: Zinc is an essential micronutrient required for optimum plant growth. Zinc-solubilizing bacteria convert applied inorganic zinc to available forms that could be used by plants. Research design: In present study, experiments were conducted to isolate, characterize, and evaluate Zn solubilization potential of different bacteria. Results: Among 10 isolated strains, Pseudomonas protegens (RY2, MF351762) was found to be the most promising strain having zinc-solubilizing potential on 4 different insoluble zinc sources. In quantitative assay, Zn solubilization by RY2 was significantly higher than other strains at different incubation time. P. protegens RY2 was selected (based on zinc solubilizing and plant growth promoting activities like P solubilization and ACC deaminase) for plant experiments. Meanwhile, available Zn release rate in soil was determined at day 10 of incubation. Chickpea seeds were inoculated with RY2 strain and ZnO is used as zinc source. Growth parameters and quantifying zinc content of shoot and root using atomic absorption spectrophotometer were determined. Enhanced shoot and root dry weights and lengths were observed in chickpea plants compared to control. Maximum increase of 44%, 67%, and 75% in T2 (Soil + RY2), T5 (Soil + ZnO + RY2), and T7 (Soil + manure + ZnO + RY2), respectively, was found in shoot length compared to control (T1). Conclusion: The study indicated that zinc-solubilizing RY2 strain possesses potential for enhanced Zn in soil so it would allow reduced inorganic Zn application.

Bacteria Isolated from Wastewater Irrigated Agricultural Soils Adapt to Heavy Metal Toxicity While Maintaining Their Plant Growth Promoting Traits

The present study explored the plant growth promotion and bioremediation potential of bacteria inhabiting wastewater irrigated agricultural soils. Thirty out of 75 bacterial isolates (40%), 29/75 (39%) and 28/75 (37%) solubilized Zn, K and PO4 during plate essays respectively. Fifty-six percent of the isolates produced siderophores, while 30% released protease in vitro. Seventy-four percent of bacteria resisted Pb, Ni and Cd at various concentrations added to the culture media plates. Sixteen out of 75 (26%) isolates were able to fix N in Nbf medium. Among these 16 N fixers, N fixing nifH, nifD and nifK genes was detected through PCR in 8, 7 and 1 strain respectively using gene specific primers designed in the study with Enterobacter sp. having all three (nifHKD) genes. Isolated bacteria showed resemblance to diverse genera such as Bacillus, Pseudomonas, Enterobacter, Citrobacter, Acinetobacter, Serratia, Klebsiella and Enterococcus based on 16S rRNA gene sequence analysis. In addition to showing the best mineral solubilization and metal resistance potential, Citrobacter sp. and Enterobacter sp. also removed 87%, 79% and 43% and 86%, 78% and 51% of Ni, Cd and Pb, respectively, from aqueous solution. These potent bacteria may be exploited both for bioremediation and biofertilization of wastewater irrigated soils leading to sustainable agriculture.

Common Bean Yield and Zinc Use Efficiency in Association with Diazotrophic Bacteria Co-Inoculations

Enrichment of staple food with zinc (Zn) along with solubilizing bacteria is a sustainable and practical approach to overcome Zn malnutrition in human beings by improving plant nutrition, nutrient use efficiency, and productivity. Common bean (Phaseolus vulgaris L.) is one of a staple food of global population and has a prospective role in agronomic Zn biofortification. In this context, we evaluated the effect of diazotrophic bacterial co-inoculations (No inoculation, Rhizobium tropici, R. tropici + Azospirillum brasilense, R. tropici + Bacillus subtilis, R. tropici + Pseudomonas fluorescens, R. tropici + A. brasilense + B. subtilis, and R. tropici + A. brasilense + P. fluorescens) in association with soil Zn application (without and with 8 kg Zn ha−1) on Zn nutrition, growth, yield, and Zn use efficiencies in common bean in the 2019 and 2020 crop seasons. Soil Zn application in combination with R. tropici + B. subtilis improved Zn accumulation in shoot and grains with greater shoot dry matter, grain yield, and estimated Zn intake. Zinc use efficiency, recovery, and utilization were also increased with co-inoculation of R. tropici + B. subtilis, whereas agro-physiological efficiency was increased with triple co-inoculation of R. tropici + A. brasilense + P. fluorescens. Therefore, co-inoculation of R. tropici + B. subtilis in association with Zn application is recommended for biofortification and higher Zn use efficiencies in common bean in the tropical savannah of Brazil.

Contribution of zinc solubilizing bacterial isolates on enhanced zinc uptake and growth promotion of maize (Zea mays L.)

Zinc-solubilizing bacteria, namely Burkholderia cepacia and Acinetobacter baumannii (H1 and H3, respectively), able to grow in liquid basal medium supplemented with ZnO, Zn₃ (PO₄)₂, and ZnSO₄·7H₂O (0.1%), showed plant growth promoting properties. The treatment of Acinetobacter baumannii (H3) solubilizes the ZnO (1.42 ppm), Zn₃ (PO₄)₂ (1.15 ppm), and ZnSO₄·7H₂O (1.44 ppm).The maximum solubilization of ZnSO₄·7H₂O (1.42 ppm) was observed in Burkholderia cepacia (H1) after 15 days. Organic acids produced by the bacteria decreased the pH of the medium and helped in Zn solubilization. In pot experiment on maize, Burkholderia cepacia (H1) treatment significantly enhanced plant height and root length in the presence of ZnO (2%) added in 10 mL of inoculum in each pot. High-performance liquid chromatography (HPLC) analysis of maize root extract showed the presence of oxalic, maleic, tartaric, and fumaric acid after 60 days of the experiment. Bacterial treatments enhanced sugar and protein level in maize plants and were 55.2 and 42.55 µg/mL plant extract, respectively, under mixture of bacterial treatment. Hence, isolates H1 and H3 expressed highest potential throughout the experiments, as zinc solubilizers and plant growth-promoting strains. This study demonstrated that meticulous use of Zn-solubilizing bacterial strains could aid in enhanced plant growth and can be the potential bio-inoculants for biofortification of maize to overcome the problems of malnutrition.

Combined use of Bacillus strains and Miscanthus for accelerating biodegradation of poly(lactic acid) and poly(ethylene terephthalate)

Background

The aim of this study was to verify whether the presence of Bacillus strains and of miscanthus influence biodegradation and formed of biofilm of poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET).

Methods

The experiment conducted in compost soil showed that strains Bacillus subtilis and Bacillus cereus isolated from heavy metal contaminated environment have biochemical activity and accelerate biodegradation of both plastic materials.

Results

For PLA film it was found that the carbonyl index dropped by over 15% in the presence of B. subtilis, while the film tensile strength decreased by 35% and the oxygen to carbon O/C ratio was higher by 3% in the presence of B. cereus, and the presence of miscanthus resulted in a loss of weight. For PET film, a decrease in the carbonyl index by 16% was observed following inoculation with B. cereus. The metabolic activity of this strain contributed to the reduction of the film’s tensile strength by 17% and to the increase in the permeability to O₂ and CO₂. The most intense degradation of PET film was observed in the presence of bacteria and plants. B. subtilis strain combined with miscanthus plantings may be a promising method for accelerating PLA and PET degradation in compost soil.

Impacts and tolerance responses of Coprinus comatus and Pleurotus cornucopiae on cadmium contaminated soil

Large amounts of cadmium (Cd) have been discharged into soil with the rapid development of industry. In this study, we revealed the impacts of Coprinus comatus (C. comatus) and Pleurotus cornucopiae (P. cornucopiae) on soil and the tolerance responses of macrofungi in the presence of Cd by the analysis of soil biochemical properties and macrofungi growth indexes. Results showed that with the cultivation of C. comatus and P. cornucopiae, the HOAc-extractable Cd in soil individually reduced by 9.53% and 11.35%, the activities of soil urease, acid phosphatase, dehydrogenase, and Fluorescein diacetate (FDA) hydrolysis increased by 18.11-101.45%, 8.39-18.24%, 9.37-55.50% and 28.94-41.92%, respectively. Meanwhile, different soil bacterial communities were observed with various macrofungi cultivations. Also, Cd accumulation significantly enhanced the macrofungi antioxidant enzyme activities, which increased by 24.10-45.43%, 30.11-61.53% and 7.03-26.81% for catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) activities in the macrofungi, respectively. Moreover, the enhanced macrofungi endophytic bacterial diversities with Cd existence was firstly observed in the present experiment. These findings revealed the possible Cd resistance mechanisms in macrofungi, suggesting C. comatus and P. cornucopiae were promising ameliorators for Cd contaminated soil.

Enterobacter hormaechei (MF957335) enhanced yield, disease and salinity tolerance in tomato

Soil salinity is one of the major limiting factors for poor crop yield in the world. Increasing salinity in the soil is a challenge for agriculture. In the recent past, plant growth-promoting rhizobacteria (PGPR) are being used to enhance plant growth in various conditions. However, the saline-tolerant PGPR are of great use for plant growth under saline condition. In the present study, saline-tolerant E. hormaechei (MF957335) was isolated from saline water. E. hormaechei (MF957335) was tested for its potassium and calcium solubilizing efficiency using Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM-EDX). E. hormaechei (MF957335) and K-Feldspar treatments significantly increased plant growth as compared to untreated plants (negative control). E. hormaechei (MF957335) significantly increased fresh biomass, shoot and root length of tomato plants. Among all the NaCl treatments, maximum fruits (9.66) were achieved in 250 mM NaCl + E. hormaechei treatment. Similar results with increased fruit numbers were obtained in K-Feldspar-treated plants. Apart from the plant growth, fresh biomass and fruit numbers, tomatoes from K-Feldspar-treated plants were large, fleshy and deep red colored. The study could demonstrate bioavailability of potassium from K-feldspar for tomato cultivation. Control plants tomato were small, non-fleshy, yellowish red, and infected with calcium deficiency disease blossom-end rot. The present study demonstrates the role of E. hormaechei (MF957335) in plant growth, yield promotion and disease tolerance by potassium and calcium solubilization, respectively. The study showed that E. hormaechei (MF957335) could be applied to saline and non-saline soils to enhance tomato yield.

Plant-Growth Endophytic Bacteria Improve Nutrient Use Efficiency and Modulate Foliar N-Metabolites in Sugarcane Seedling

Beneficial plant–microbe interactions lead to physiological and biochemical changes that may result in plant-growth promotion. This study evaluated the effect of the interaction between sugarcane and endophytic bacterial strains on plant physiological and biochemical responses under two levels of nitrogen (N) fertilization. Six strains of endophytic bacteria, previously selected as plant growth-promoting bacteria (PGPB), were used to inoculate sugarcane mini stalks, with and without N fertilization. After 45 days, biomass production; shoot nutrient concentrations; foliar polyamine and free amino acid profiles; activities of nitrate reductase and glutamine synthase; and the relative transcript levels of the GS1, GS2, and SHR5 genes in sugarcane leaves were determined. All six endophytic strains promoted sugarcane growth, increasing shoot and root biomass, plant nutritional status, and the use efficiency of most nutrients. The inoculation-induced changes at the biochemical level altered the foliar free amino acid and polyamine profiles, mainly regarding the relative concentrations of citrulline, putrescine, glycine, alanine, glutamate, glutamine, proline, and aspartate. The transcription of GS1, GS2, and SHR5 was higher in the N fertilized seedlings, and almost not altered by endophytic bacterial strains. The endophytic strains promoted sugarcane seedlings growth mainly by improving nutrient efficiency. This improvement could not be explained by their ability to induce the production of amino acid and polyamine composts, or GS1, GS2, and SHR5, showing that complex interactions may be associated with enhancement of the sugarcane seedlings’ performance by endophytic bacteria. The strains demonstrated biotechnological potential for sugarcane seedling production.

The Response of the Associations of Grass and Epichloë Endophytes to the Increased Content of Heavy Metals in the Soil

The rapid development of civilization increases the area of land exposed to the accumulation of toxic compounds, including heavy metals, both in water and soil. Endophytic fungi associated with many species of grasses are related to the resistance of plants to biotic and abiotic stresses, which include heavy metals. This paper reviews different aspects of symbiotic interactions between grass species and fungal endophytes from the genera Epichloë with special attention paid to the elevated concentration of heavy metals in growing substrates. The evidence shows the high resistance variation of plant endophyte symbiosis on the heavy metals in soil outcome. The fungal endophytes confer high heavy metal tolerance, which is the key feature in its practical application with their host plants, i.e., grasses in phytoremediation.

Characterization of zinc solubilization potential of arsenic tolerant Burkholderia spp. isolated from rice rhizospheric soil

In this study, experiments were conducted to isolate, characterize, and evaluate rice rhizosphere bacteria for their arsenic (As) tolerance ability and zinc (Zn) solubilization potential in culture media and soil. Among 20 bacterial isolates recovered, six were found to solubilize inorganic Zn salt(s) efficiently under in vitro culture conditions. 16S rRNA gene sequence-based phylogenetic analysis indicated the affiliation of efficient Zn solubilizing bacteria (ZSB) to Burkholderia vietnamiensis and Burkholderia seminalis. Zinc solubilizing efficiency (ZSE) of the bacteria varied with the concentrations and types of Zn salts used in the experiments. Increasing trend in ZSE of the bacteria was noticed when the percentage of ZnO increased from 0.1 to 0.5 but the same decreased at 1.0%. Increased Zn solubilization was noticed when bacteria were incubated with lower concentration of Zn₃(PO₄)₂ and ZnCO₃. In general, Zn solubilization increased with increasing incubation time in lower volume medium, while some isolates failed to solubilize one or more tested Zn salts. However, enriched concentrated cells of the ZSB in glucose amended medium with 0.5% ZnO showed an increasing trend of Zn solubilization with time and were able to solubilize more than 300 mg/L Zn. This increased rate of Zn release by the ZSB was attributed to marked decline in pH that might be due to the enhanced gluconic acid production from glucose. As evident from the decreased ZSE of the bacteria in the presence of As(V) in particular, it seems arsenic imparts a negative effect on Zn solubilization. The ZSB were also able to increase the rate of Zn release in soil. A microcosm-based soil incubation study amending the enriched bacteria and 0.5% ZnO in soil showed an elevated level of both water-soluble and available Zn compared to un-inoculated control. During Zn solubilization in microcosms, viable cells in terms of colony-forming unit (CFU) declined by the same order of magnitude both in the presence and absence of ZnO that might be due to the nutrients limiting condition aroused during the incubation period rather than Zn toxicity. The bacteria in this study also exhibited plant growth promoting traits, such as growth in nitrogen-free medium, production of indole acetic acid (IAA), and solubilization of potassium and phosphate. Our findings suggested that Burkholderia spp. could be the potential candidates for enhancing Zn dissolution in the soil that might reduce the rate of inorganic Zn fertilization in agricultural soil.

Biofilm Producing Rhizobacteria With Multiple Plant Growth-Promoting Traits Promote Growth of Tomato Under Water-Deficit Stress

Plant growth-promoting rhizobacteria (PGPR) not only enhance plant growth but also control phytopathogens and mitigate abiotic stresses, including water-deficit stress. In this study, 21 (26.9%) rhizobacterial strains isolated from drought-prone ecosystems of Bangladesh were able to form air–liquid (AL) biofilms in the glass test tubes containing salt-optimized broth plus glycerol (SOBG) medium. Based on 16S rRNA gene sequencing, Pseudomonas chlororaphis (ESR3 and ESR15), P. azotoformans ESR4, P. poae ESR6, P. fluorescens (ESR7 and ESR25), P. gessardii ESR9, P. cedrina (ESR12, ESR16, and ESR23), P. veronii (ESR13 and ESR21), P. parafulva ESB18, Stenotrophomonas maltophilia ESR20, Bacillus cereus (ESD3, ESD21, and ESB22), B. horikoshii ESD16, B. aryabhattai ESB6, B. megaterium ESB9, and Staphylococcus saprophyticus ESD8 were identified. Fourier transform infrared spectroscopy studies showed that the biofilm matrices contain proteins, polysaccharides, nucleic acids, and lipids. Congo red binding results indicated that these bacteria produced curli fimbriae and nanocellulose-rich polysaccharides. Expression of nanocellulose was also confirmed by Calcofluor binding assays and scanning electron microscopy. In vitro studies revealed that all these rhizobacterial strains expressed multiple plant growth-promoting traits including N₂ fixation, production of indole-3-acetic acid, solubilization of nutrients (P, K, and Zn), and production of ammonia, siderophores, ACC deaminase, catalases, lipases, cellulases, and proteases. Several bacteria were also tolerant to multifarious stresses such as drought, high temperature, extreme pH, and salinity. Among these rhizobacteria, P. cedrina ESR12, P. chlororaphis ESR15, and B. cereus ESD3 impeded the growth of Xanthomonas campestris pv. campestris ATCC 33913, while P. chlororaphis ESR15 and B. cereus ESD21 prevented the progression of Ralstonia solanacearum ATCC^® 11696^TM. In a pot experiment, tomato plants inoculated with P. azotoformans ESR4, P. poae ESR6, P. gessardii ESR9, P. cedrina ESR12, P. chlororaphis ESR15, S. maltophilia ESR20, P. veronii ESR21, and B. aryabhattai ESB6 exhibited an increased plant growth compared to the non-inoculated plants under water deficit-stressed conditions. Accordingly, the bacterial-treated plants showed a higher antioxidant defense system and a fewer tissue damages than non-inoculated plants under water-limiting conditions. Therefore, biofilm-producing PGPR can be utilized as plant growth promoters, suppressors of plant pathogens, and alleviators of water-deficit stress.

DegP protease is essential for tolerance to salt stress in the plant growth-promoting bacterium Gluconacetobacter diazotrophicus PAL5

The use of plant growth-promoting bacteria represents an alternative to the massive use of mineral fertilizers in agriculture. However, some abiotic stresses commonly found in the environment, like salinity, can affect the efficiency of this approach. Here, we investigated the key mechanisms involved in the response of the plant growth-promoting bacterium Gluconacetobacter diazotrophicus to salt stress by using morphological and cell viability analyses, comparative proteomics, and reverse genetics. Our results revealed that the bacteria produce filamentous cells in response to salt at 100 mM and 150 mM NaCl. However, such a response was not observed at higher concentrations, where cell viability was severely affected. Proteomic analysis showed that salt stress modulates proteins involved in several pathways, including iron uptake, outer membrane efflux, osmotic adjustment, cell division and elongation, and protein transport and quality control. Proteomic data also revealed the repression of several extracytoplasmic proteins, especially those located at periplasm and outer membrane. The role of such pathways in the tolerance to salt stress was analyzed by the use of mutant defectives for Δtbdr (iron uptake), ΔmtlK and ΔotsA (compatible solutes synthesis), and ΔdegP (quality control of nascent extracytoplasmic proteins). ΔdegP presented the highest sensitivity to salt stress, Δtbdr, andΔmtlK also showed increased sensitivity, but ΔotsA was not affected. This is the first demonstration that DegP protein, a protease with minor chaperone activity, is essential for tolerance to salt stress in G. diazotrophicus. Our data contribute to a better understanding of the molecular bases that control the bacterial response/tolerance to salt stress, shedding light on quality control of nascent extracytoplasmic proteins.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Comparative proteomics reveals essential mechanisms for osmotolerance in Gluconacetobacter diazotrophicus

Plant growth-promoting bacteria are a promising alternative to improve agricultural sustainability. Gluconacetobacter diazotrophicus is an osmotolerant bacterium able to colonize several plant species, including sugarcane, coffee, and rice. Despite its biotechnological potential, the mechanisms controlling such osmotolerance remain unclear. The present study investigated the key mechanisms of resistance to osmotic stress in G. diazotrophicus. The molecular pathways regulated by the stress were investigated by comparative proteomics, and proteins essential for resistance were identified by knock-out mutagenesis. Proteomics analysis led to identify regulatory pathways for osmotic adjustment, de novo saturated fatty acids biosynthesis, and uptake of nutrients. The mutagenesis analysis showed that the lack of AccC protein, an essential component of de novo fatty acid biosynthesis, severely affected G. diazotrophicus resistance to osmotic stress. Additionally, knock-out mutants for nutrients uptake (Δtbdr and ΔoprB) and compatible solutes synthesis (ΔmtlK and ΔotsA) became more sensitive to osmotic stress. Together, our results identified specific genes and mechanisms regulated by osmotic stress in an osmotolerant bacterium, shedding light on the essential role of cell envelope and extracytoplasmic proteins for osmotolerance.

Biodegradation of carbendazim by a potent novel Chryseobacterium sp. JAS14 and plant growth promoting Aeromonas caviae JAS15 with subsequent toxicity analysis

In the present study, carbendazim (MBC) degrading bacterial strains were isolated and identified as Chryseobacterium sp. JAS14 and Aeromonas caviae JAS15. Both the strains completely degraded 200 mg l^-1 of MBC in the aqueous medium and soil within 4-9 days of incubation. In an aqueous medium, the degradation process was characterized by a rate constant of 53.16 day^-1 and 42.60 day^-1, following zero order model and DT₅₀ was 1.8 days and 2.34 days for Chryseobacterium sp. JAS14 and A. caviae JAS15, respectively. A Chryseobacterium sp. JAS14 and A. caviae JAS15 inoculated into the soil without the addition of nutrients showed the degradation rate constant of 27.30 day^-1 and 23.87 day^-1, and DT₅₀ was 3.66 days and 4.18 days, respectively. The metabolites during MBC biodegradation by Chryseobacterium sp. JAS14 and A. caviae JAS15 were identified as 2-aminobenzimidazole, 2-hydroxybenzimidazole, 1, 2 diaminobenzene and catechol. To our knowledge, this is the first study of the detailed biodegradation pathway of MBC by Chryseobacterium sp. JAS14 was proposed. Phytotoxicity and cytogenotoxicity assays showed that the toxicity of the MBC reduced after biodegradation by Chryseobacterium sp. JAS14 and A. caviae JAS15. In addition, A. caviae JAS15 possess important plant growth promoting traits under normal and MBC stress condition. These results suggest the Chryseobacterium sp. JAS14 and A. caviae JAS15 could be used as a bioresource for the reclamation of MBC contaminated soil.

Isolation, characterization and inoculation of Cd tolerant rice endophytes and their impacts on rice under Cd contaminated environment

Cadmium (Cd) contamination in paddy soil becomes increasingly prominent in recent years, which endangers the safe production of food crops. Cd-tolerant endophytes are ideal mediators for decreasing Cd content in rice plants, but their effects on the rice endophytic microbial community and gene expression profile have not yet been well elucidated. In this study, 58 endophytic bacteria from rice seeds were isolated and characterized. Five strains of them were selected based on their potential growth-promoting traits and strong Cd tolerance that could grow well under 4 mM Cd²⁺. By 16S ribosomal RNA (rRNA) identification, these five strains were designated as Enterobacter tabaci R2-7, Pantoea agglomerans R3-3, Stenotrophomonas maltophilia R5-5, Sphingomonas sanguinis R7-3 and Enterobacter tabaci R3-2. Pot experiments in relieving Cd stress in rice plants showed that the S. maltophilia R5-5 performed the strongest potential for reducing the Cd content in root and blade by 81.33% and 77.78%, respectively. The endophytic microbial community diversity, richness and composition were significantly altered in S. maltophilia R5-5 inoculated rice plants. Reverse-transcription qPCR (RT-qPCR) showed that the expression of Cd transporters, OsNramp5 and OsHMA2, were down-regulated in S. maltophilia R5-5-innoculated rice roots. The results indicate that the inoculation of endophytic bacteria S. maltophilia R5-5 provides a reference for alleviating the heavy metal contamination in paddy fields and can be a better alternative for guaranteeing the safe production of crops. Changes in the relative abundance of Cd-resistant microorganisms and the expression of Cd transporters might be the intrinsic factors affecting cadmium content in rice.

Zinc biosorption, biochemical and molecular characterization of plant growth-promoting zinc-tolerant bacteria

Zinc plays a key role in plant nutrition at low levels; however, at higher concentrations Zn ions can be highly phytotoxic and plant growth-promoting rhizobacteria can be used to reduce such metal toxicity. In the present investigation we had reported the zinc biosorption and molecular characterization of plant growth-promoting zinc-tolerant bacteria. Initially, thirty bacteria having zinc solubilizing ability were screened for MIC against zinc ion and displayed high value of MIC ranging from 2.5 to 62.5 mM. Biochemically, all the 30 isolates showed significant difference in the 6 biochemical tests performed. The molecular diversity studies based on the repetitive DNA PCR viz, REP, ERIC and BOX elements showed significant genetic diversity among these 30 zinc-tolerant bacteria. These ZTB strains also showed multiple PGP activities and all ZTB strains were found positive for production of IAA, GA₃ and ammonia, whereas 24 were found positive for ACC deaminase activity, 8 showed siderophore production and 9 ZTB isolates were positive for HCN production. Out of 30 isolates, 24 showed phosphorus solubilization activity, 30 showed potash solubilization, 15 showed silica solubilization and 27 showed phytase production activities. All the 30 ZTB stains showed zinc solubilization up to 0.25% insoluble ZnO in the medium, whereas at 2% ZnO in MSM only 12 isolates showed solubilization which were further selected for zinc biosorption and pot studies. The heavy metal removal studies revealed that ZTB stains were able to remove zinc ions effectively from the medium efficiently and the highest zinc biosorption (< 90%) was recorded with the bacterial strain Z-15. Further, the inoculation of ZTB strains under zinc stress conditions (pot containing 1000 mg/kg Zn) resulted in significant increase of shoot length, root length and total chlorophyll content in maize seedlings compared with the uninoculated control. The partial 16S rDNA sequence of the potential ZTB isolates viz. Z-15, Z-24, Z-28 and Z-29 revealed their identity as Serratia sp. The ability of these zinc-tolerant bacteria to tolerate the toxic level of zinc may serve as suitable candidates for developing microbial formulations for the growth of crop plants in Zn-contaminated areas.

Functionally dissimilar soil organisms improve growth and Pb/Zn uptake by Stachys inflata grown in a calcareous soil highly polluted with mining activities

Micro- and macro-organisms are key components of sustainable soil-plant systems; and are involved in plant growth stimulation and accumulation of heavy metals in the plant, with great contribution to phytoremediation of heavy metal-contaminated soils. However, the combined effect of arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR) and earthworms on plant growth and metal uptake is not yet clear. The main objective of this study was to examine the combined influences of these soil organisms on the growth and metal uptake by a native plant species (Stachys inflata), and subsequently on potential phytoremediation in a soil highly polluted with Pb/Zn mining activities. Metal tolerant AMF, PGPR and earthworms were used either alone or in combination in a factorial pot experiment under greenhouse conditions for 4 months. Inoculation of AMF, PGPR and earthworm led to an increase of soil Pb/Zn availability with the greatest Zn availability (about 2 times) with triple inoculation of all the organisms and the highest Pb availability (about 3 times) with co-inoculation of PGPR and earthworms. Triple inoculation of soil organisms increased the total plant biomass 3 times, total Pb uptake 6 times and total Zn uptake 5 times as compared with the uninoculated plants. The growth-stimulating effect of combined soil organisms was much greater than that of individual or dually-inoculated organisms. These results showed that it is possible to use the combination of metal-tolerant soil organisms as a potential bioaugmentation tool to accelerate metal phytoremediation rate in calcareous soils polluted by Pb/Zn mining activity under arid conditions. This could occur as a consequence of the synergistic effects of AMF, PGPR and earthworms on metal availability in the soil, S. inflata growth, metal tolerance and uptake. However, it would practically require more than hundreds of cropping cycles (360 for Pb and 250 for Zn) to achieve metal maximum permissible limits under the conditions of this pot experiment.

Characterization of the Cd-resistant fungus Aspergillus aculeatus and its potential for increasing the antioxidant activity and photosynthetic efficiency of rice

Considerable evidence exists that microorganisms play a significant role in the remediation of soil contaminated with heavy metals. Aspergillus aculeatus (A. aculeatus) isolated from Cd-polluted soil has been shown to increase the tolerance of turfgrasses to Cd stress. In this study, we assessed the tolerance, biosorption capacity for Cd and surface characteristics of this fungus and investigated the effect of plant inoculation with A. aculeatus on the lipid peroxidation, antioxidant activities and photosynthetic rates in rice cultivated in Cd-contaminated soil. The results indicated that the removal efficiency of A. aculeatus was 46.8% at a Cd concentration of 10 mg L^-1. The A. aculeatus strains had the capacity to produce indole acetic acid, siderophore, and 1-aminocyclopropane-1-carboxylate deaminase and to solubilize phosphate. The O₂^- accumulation and the amount of MDA in rice roots inoculated with A. aculeatus were significantly lower than those in uninoculated plants. Nevertheless, no decrease in leaf ROS accumulation and photosynthetic activity was observed between the inoculated and uninoculated plants. Inoculation with A. aculeatus contained more of the ROS-scavenging metabolite GSH, a higher GSH/GSSG ratio, and higher antioxidative enzyme (SOD, POD, and CAT) activities, possibly explaining the lower ROS concentrations observed in inoculated roots in the presence of Cd. These results suggest that application of A. aculeatus has the potential to protect crops against Cd stress.

Integrated phytobial heavy metal remediation strategies for a sustainable clean environment - A review

Heavy metal contamination in the environment is a global threat which accelerated after the industrial revolution. Remediation of these noxious elements has been widely investigated and multifarious technologies have been practiced for many decades. Phytoremediation has attracted much attention from researchers. Under this technology, heavy metal hyperaccumulator plants have been extensively employed to extract extraordinary concentrations of heavy metals but slow growth, limited biomass and stresses caused by heavy metals imperil the efficiency of hyperaccumulators. Plant growth promoting rhizobacteria (PGPR) can help overcome/lessen heavy metal-induced adversities. PGPR produce several metabolites, including growth hormones, siderophores and organic acids, which aid in solubilization and provision of essential nutrients (e.g. Fe and Mg) to the plant. Hyperaccumulator plants may be employed to remediate metal contaminated sites. Use of PGPR to enhance growth of hyperaccumulator plant species may enhance their metal accumulating capacity by increasing metal availability and also by alleviating plant stress induced by the heavy metals. Combined use of hyperaccumulator plants and PGPR may prove to be a cost effective and environmentally friendly technology to clean heavy metal contaminated sites on a sustainable basis. This review discusses the current status of PGPR in improving the growth and development of hyperaccumulator plants growing in metal contaminated environments. The mechanisms used by these rhizosphere bacteria in increasing the availability of heavy metals to plants and coping with heavy metal stresses are also described.