Native Heavy Metal-Tolerant Plant Growth Promoting Rhizobacteria Improves Sulla spinosissima (L.) Growth in Post-Mining Contaminated Soils

Response of antioxidant activity, active constituent and rhizosphere microorganisms of Salvia miltiorrhiza to combined application of microbial inoculant, microalgae and biochar under Cu stress

Salvia miltiorrhiza, a widely used medicinal herb renowned for its properties in promoting blood circulation, removing blood stasis and alleviating pain, is currently facing quality degradation due to excessive heavy metal levels, posing a threat to medication safety. In order to investigate the effects of microbial inoculant, microalgae and biochar on the growth of Salvia miltiorrhiza under copper (Cu) stress, as well as its Cu absorption, antioxidant activity, active component contents and rhizosphere microbial community, a pot experiment was conducted. Salvia miltiorrhiza plants were cultivated in the soil containing 400 mg/kg of Cu for six months and treated with microbial inoculant, microalgae and biochar, either individually or in combination. Almost all soil amendment treatments led to an increase in root biomass. Notably, co-application of microbial inoculant and microalgae had the optimal effect with a 63.07 % increase compared to the group treated solely with Cu. Moreover, when microbial inoculant was applied alone or in combination with microalgae, the Cu content in plant roots was reduced by 19.29 % and 25.37 %, respectively, whereas other treatments failed to show a decreasing trend. Intriguingly, Cu stress increased the active component contents in plant roots, and they could also be enhanced beyond non-stress levels when microbial inoculant and microalgae were applied together or in combination with biochar. Analyses of plant antioxidant activity, soil properties and rhizosphere microorganisms indicated that these amendments may alleviate Cu stress by enhancing peroxidase activity, facilitating plant nutrient absorption, and enriching beneficial microorganisms capable of promoting plant growth and mitigating heavy metal-induced damage. This study suggests that the combined application of microbial inoculant and microalgae can reduce Cu levels in Salvia miltiorrhiza while enhancing its quality under Cu stress.

Bacterial indole-3-acetic acid: A key regulator for plant growth, plant-microbe interactions, and agricultural adaptive resilience

Indole-3-acetic acid (IAA), a fundamental phytohormone categorized under auxins, not only influences plant growth and development but also plays a critical role in plant-microbe interactions. This study reviews the role of IAA in bacteria-plant communication, with a focus on its biosynthesis, regulation, and the subsequent effects on host plants. Bacteria synthesize IAA through multiple pathways, which include the indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and several other routes, whose full mechanisms remain to be fully elucidated. The production of bacterial IAA affects root architecture, nutrient uptake, and resistance to various abiotic stresses such as drought, salinity, and heavy metal toxicity, enhancing plant resilience and thus offering promising routes to sustainable agriculture. Bacterial IAA synthesis is regulated through complex gene networks responsive to environmental cues, impacting plant hormonal balances and symbiotic relationships. Pathogenic bacteria have adapted mechanisms to manipulate the host's IAA dynamics, influencing disease outcomes. On the other hand, beneficial bacteria utilize IAA to promote plant growth and mitigate abiotic stresses, thereby enhancing nutrient use efficiency and reducing dependency on chemical fertilizers. Advancements in analytical methods, such as liquid chromatography-tandem mass spectrometry, have improved the quantification of bacterial IAA, enabling accurate measurement and analysis. Future research focusing on molecular interactions between IAA-producing bacteria and host plants could facilitate the development of biotechnological applications that integrate beneficial bacteria to improve crop performance, which is essential for addressing the challenges posed by climate change and ensuring global food security. This integration of bacterial IAA producers into agricultural practice promises to revolutionize crop management strategies by enhancing growth, fostering resilience, and reducing environmental impact.

Exploiting the role of plant growth promoting rhizobacteria in reducing heavy metal toxicity of pepper (Capsicum annuum L.)

Microorganisms are cost-effective and eco-friendly alternative methods for removing heavy metals (HM) from contaminated agricultural soils. Therefore, this study aims to identify and characterize HM-tolerant (HMT) plant growth-promoting rhizobacteria (PGPR) isolated from industry-contaminated soils to determine their impact as bioremediators on HM-stressed pepper plants. Four isolates [Pseudomonas azotoformans (Pa), Serratia rubidaea (Sr), Paenibacillus pabuli (Pp) and Bacillus velezensis (Bv)] were identified based on their remarkable levels of HM tolerance in vitro. Field studies were conducted to evaluate the growth promotion and tolerance to HM toxicity of pepper plants grown in HM-polluted soils. Plants exposed to HM stress showed improved growth, physio-biochemistry, and antioxidant defense system components when treated with any of the individual isolates, in contrast to the control group that did not receive PGPR. The combined treatment of the tested HMT PGPR was, however, relatively superior to other treatments. Compared to no or single PGPR treatment, the consortia (Pa+Sr+Pp+Bv) increased the photosynthetic pigment contents, relative water content, and membrane stability index but lowered the electrolyte leakage and contents of malondialdehyde and hydrogen peroxide by suppressing the (non) enzymatic antioxidants in plant tissues. In pepper, Cd, Cu, Pb, and Ni contents decreased by 88.0-88.5, 63.8-66.5, 66.2-67.0, and 90.2-90.9% in leaves, and 87.2-88.1, 69.4-70.0%, 80.0-81.3, and 92.3%% in fruits, respectively. Thus, these PGPR are highly effective at immobilizing HM and reducing translocation in planta. These findings indicate that the application of HMT PGPR could be a promising "bioremediation" strategy to enhance growth and productivity of crops cultivated in soils contaminated with HM for sustainable agricultural practices.

Plant species within Streptanthoid Complex associate with distinct microbial communities that shift to be more similar under drought

Prolonged water stress can shift rhizoplane microbial communities, yet whether plant phylogenetic relatedness or drought tolerance predicts microbial responses is poorly understood. To explore this question, eight members of the Streptanthus clade with varying affinity to serpentine soil were subjected to three watering regimes. Rhizoplane bacterial communities were characterized using 16S rRNA gene amplicon sequencing and we compared the impact of watering treatment, soil affinity, and plant species identity on bacterial alpha and diversity. We determined which taxa were enriched among drought treatments using DESeq2 and identified features of soil affinity using random forest analysis. We show that water stress has a greater impact on microbial community structure than soil affinity or plant identity, even within a genus. Drought reduced alpha diversity overall, but plant species did not strongly differentiate alpha diversity. Watering altered the relative abundance of bacterial genera within Proteobacteria, Firmicutes, Bacteroidetes, Planctomycetes, and Acidobacteria, which responded similarly in the rhizoplane of most plant species. In addition, bacterial communities were more similar when plants received less water. Pseudarthrobacter was identified as a feature of affinity to serpentine soil while Bradyrhizobium, Chitinophaga, Rhodanobacter, and Paenibacillus were features associated with affinity to nonserpentine soils among Streptanthus. The homogenizing effect of drought on microbial communities and the increasing prevalence of Gram-negative bacteria across all plant species suggest that effects of water stress on root-associated microbiome structure may be predictable among closely related plant species that inhabit very different soil environments. The functional implications of observed changes in microbiome composition remain to be studied.

Effect of a native bacterial consortium on growth, yield, and grain quality of durum wheat (Triticum turgidum L. subsp. durum) under different nitrogen rates in the Yaqui Valley, Mexico

A field experiment was carried out to quantify the effect of a native bacterial inoculant on the growth, yield, and quality of the wheat crop, under different nitrogen (N) fertilizer rates in two agricultural seasons. Wheat was sown under field conditions at the Experimental Technology Transfer Center (CETT-910), as a representative wheat crop area from the Yaqui Valley, Sonora México. The experiment was conducted using different doses of nitrogen (0, 130, and 250 kg N ha-1) and a bacterial consortium (BC) (Bacillus subtilis TSO9, B. cabrialesii subsp. tritici TSO2T, B. subtilis TSO22, B. paralicheniformis TRQ65, and Priestia megaterium TRQ8). Results showed that the agricultural season affected chlorophyll content, spike size, grains per spike, protein content, and whole meal yellowness. The highest chlorophyll and Normalized Difference Vegetation Index (NDVI) values, as well as lower canopy temperature values, were observed in treatments under the application of 130 and 250 kg N ha-1 (the conventional Nitrogen dose). Wheat quality parameters such as yellow berry, protein content, Sodium dodecyl sulfate (SDS)-Sedimentation, and whole meal yellowness were affected by the N dose. Moreover, the application of the native bacterial consortium, under 130 kg N ha-1, resulted in a higher spike length and grain number per spike, which led to a higher yield (+1.0 ton ha-1 vs. un-inoculated treatment), without compromising the quality of grains. In conclusion, the use of this bacterial consortium has the potential to significantly enhance wheat growth, yield, and quality while reducing the nitrogen fertilizer application, thereby offering a promising agro-biotechnological alternative for improving wheat production.

In Vitro Culture Studies for the Mitigation of Heavy Metal Stress in Plants

Heavy metals are among the most common and dangerous contaminants; their action on plants, as well as the possibility for plants to effectively absorb and translocate them, have been studied for several years, mainly for exploitation in phytoremediation, an environmentally friendly and potentially effective technology proposed and studied for the recovery of contaminated soils and waters. In this work, the analysis has focused on the studies developed using in vitro techniques on the possibilities of mitigating, in plants, the stress due to the presence of heavy metals and/or improving their absorption. These objectives can be pursued with the use of different substances and organisms, which have been examined in detail. The following are therefore presented in this review: an analysis of the role of metals and metalloids; the use of several plant growth regulators, with their mechanisms of action in different physiological phases of the plant; the activity of bacteria and fungi; and the role of other effective compounds, such as ascorbic acid and glutathione.

Bacteria associated with Zn-hyperaccumulators Arabidopsis halleri and Arabidopsis arenosa from Zn-Pb-Cd waste heaps in Poland as promising tools for bioremediation

To identify metal adapted bacteria equipped with traits positively influencing the growth of two hyperaccumulator plant species Arabidopsis arenosa and Arabidopsis halleri, we isolated bacteria inhabiting rhizosphere and vegetative tissues (roots, basal and stem leaves) of plants growing on two old Zn-Pb-Cd waste heaps in Bolesław and Bukowno (S. Poland), and characterized their potential plant growth promoting (PGP) traits as well as determined metal concentrations in rhizosphere and plant tissues. To determine taxonomic position of 144 bacterial isolates, 16S rDNA Sanger sequencing was used. A metabolic characterization of isolated strains was performed in vitro using PGP tests. A. arenosa and A. halleri accumulate high amounts of Zn in their tissues, especially in stem leaves. Among in total 22 identified bacterial taxa, the highest level of the taxonomical diversity (H' = 2.01) was revealed in A. halleri basal leaf endophytes originating from Bukowno waste heap area. The 96, 98, 99, and 98% of investigated strains showed tolerant to Cd, Zn, Pb and Cu, respectively. Generally, higher percentages of bacteria could synthesize auxins, siderophores, and acetoin as well as could solubilize phosphate. Nine of waste heap origin bacterial strains were tolerant to toxic metals, showed in vitro PGP traits and are potential candidates for bioremediation.

Phyto-microbiome to mitigate abiotic stress in crop plants

Plant-associated microbes include taxonomically diverse communities of bacteria, archaebacteria, fungi, and viruses, which establish integral ecological relationships with the host plant and constitute the phyto-microbiome. The phyto-microbiome not only contributes in normal growth and development of plants but also plays a vital role in the maintenance of plant homeostasis during abiotic stress conditions. Owing to its immense metabolic potential, the phyto-microbiome provides the host plant with the capability to mitigate the abiotic stress through various mechanisms like production of antioxidants, plant growth hormones, bioactive compounds, detoxification of harmful chemicals and toxins, sequestration of reactive oxygen species and other free radicals. A deeper understanding of the structure and functions of the phyto-microbiome and the complex mechanisms of phyto-microbiome mediated abiotic stress mitigation would enable its utilization for abiotic stress alleviation of crop plants and development of stress-resistant crops. This review aims at exploring the potential of phyto-microbiome to alleviate drought, heat, salinity and heavy metal stress in crop plants and finding sustainable solutions to enhance the agricultural productivity. The mechanistic insights into the role of phytomicrobiome in imparting abiotic stress tolerance to plants have been summarized, that would be helpful in the development of novel bioinoculants. The high-throughput modern approaches involving candidate gene identification and target gene modification such as genomics, metagenomics, transcriptomics, metabolomics, and phyto-microbiome based genetic engineering have been discussed in wake of the ever-increasing demand of climate resilient crop plants.

Potential roles of the rhizospheric bacterial community in assisting Miscanthus floridulus in remediating multi-metal(loid)s contaminated soils

Phytoremediation technology is an important approach applied to heavy metal remediation, and how to improve its remediation efficiency is the key. In this study, we compared the rhizospheric bacterial communities and metals contents in Miscanthus floridulus (M. floridulus) of four towns, including Huayuan Town (HY), Longtan Town (LT), Maoer Village (ME), and Minle Town (ML) around the lead-zinc mining area in Huayuan County, China. The roles of rhizospheric bacterial communities in assisting the phytoremediation of M. floridulus were explored. It was found that the compositions of the rhizospheric bacterial community of M. floridulus differed in four regions, but majority of them were heavy metal-resistant bacteria that could promote plant growth. Results of bioconcentration factors showed the enrichment of Cu, Zn, and Pb by M. floridulus in these four regions were significantly different. The Zn enrichment capacity of ML was the strongest for Cu and stronger than LT and ME for Pb. The enrichment capacity of LT and ML was stronger than HY and ME. These bacteria may influence the different heavy metals uptake of M. floridulus by altering the soil physiochemical properties (e.g., soil peroxidase, pH and moisture content). In addition, co-occurrence network analysis also showed that LT and ML had higher network stability and complexity than HY and ME. Functional prediction analysis of the rhizospheric bacterial community showed that genes related to protein synthesis (e.g., zinc-binding alcohol dehydrogenase/oxidoreductase, Dtx R family transcriptional regulators and ACC deaminase) also contributed to phytoremediation in various ways. This study provides theoretical guidance for selecting suitable microorganisms to assist in the phytoremediation of heavy metals.

Rhizobacterial Colonization and Management of Bacterial Speck Pathogen in Tomato by Pseudomonas spp

Plants and soil microorganisms interact at every stage of growth. Pseudomonas spp. are highly regarded for their ability to increase crop production and protection from diseases. The aim of this study is to understand the mechanisms of the rhizobacterial colonization of tomato roots via chemotaxis assay and the activation of tomato resistance against the pathogenic bacterium, Pseudomonas syringae pv. tomato DC3000 (Pst). The capillary assay was used to evaluate the chemotaxis response of PGPRs (plant growth-promoting rhizobacteria). The activities of defense enzymes and the expressions of PR (pathogenesis-related) genes were measured using real-time qPCR. Chemotactic responses to malic and citric acids (the most important root exudates found in different plant species) at low concentrations varied substantially among the rhizobacterial isolates (63 species). Beneficial isolates including Pseudomonas resinovorans A5, P. vranovensis A30, P. resinovorans A28, P. umsongensis O26, P. stutzeri N42, and P. putida T15 reacted well to different concentrations of root exudates. P. putida T15 demonstrated the most potent anti-Pst activity. At three and six days after inoculation, the greatest levels of polyphenol oxidase and peroxidase activity were reported in the A5 and T15 groups. In tomato, transcript levels of four PR (pathogenesis-related) genes were elevated by rhizobacterial treatments. PGPR isolates alone or in combination with BABA (β-amino butyric acid) up-regulated the transcriptions of PR1, PR2, LOX, and PAL genes. Treatments with N42 and T15 resulted in the greatest improvements in tomato growth and yield traits. In conclusion, the results explain the mechanisms of rhizobacterial colonization for the improved management of Pst. Rhizobacterial isolates play a role in tomato's resistance to Pst via salicylic acid and jasmonic acid pathways.

Metal-tolerant and siderophore producing Pseudomonas fluorescence and Trichoderma spp. improved the growth, biochemical features and yield attributes of chickpea by lowering Cd uptake

Industrialization and human urbanization have led to an increase in heavy metal (HM) pollution which often cause negative/toxic effect on agricultural crops. The soil-HMs cannot be degraded biologically however, microbe-mediated detoxification of toxic HMs into lesser toxic forms are reported. Considering the potentiality of HMs-tolerant soil microbes in metal detoxification, Pseudomonas fluorescence PGPR-7 and Trichoderma sp. T-4 were recovered from HM-affected areas. Under both normal and cadmium stress, the ability of both microorganisms to produce different plant hormones and biologically active enzymes was examined. Strains PGPR-7 and T-4 tolerated cadmium (Cd) an up-to 1800 and 2000 µg mL^-1, respectively, and produced various plant growth regulating substances (IAA, siderophore, ACC deaminase ammonia and HCN) in Cd-stressed condition. The growth promoting and metal detoxifying ability of both strains were evaluated (either singly/combined) by applying them in chickpea (Cicer arietinum L.) plants endogenously contaminated with different Cd levels (0-400 µg kg^-1 soils). The higher Cd concentration (400 µg kg^-1 soils) negatively influenced the plant parameters which, however, improved following single/combined inoculation of P. fluorescence PGPR-7 and Trichoderma sp. T-4. Both microbial strains increased the growth of Cd-treated chickpeas however, their combined inoculation (PGPR-7 + T-4) caused the most positive effect. For instance, 25 µg Cd Kg^-1 + PGPR-7 + T4 treatment caused maximum increase in germination percentage (10%), root dry biomass (71.4%) and vigour index (33%), chl-a (38%), chl-b (41%) and carotenoid content (52%). Furthermore, combined inoculation of P. fluorescence PGPR-7 and Trichoderma sp. T-4 maximally decreased the proline, MDA content, POD and CAT activities by 50%, 43% and 62%, respectively following their application in 25 µg Cd kg^-1 soils-treated chickpea. Additionally, microbial strains lowered the plant uptake of Cd. For example, Cd-uptake in root tissues was decreased by 42 and 34% when 25 µg Cd Kg^-1- treated chickpea plants were inoculated with P. fluorescence PGPR-7, Trichoderma sp. T-4 and co-inoculation (PGPR-7 + T4) of both strains, respectively. Therefore, from the current observation, it is suggested that dual inoculation of metal tolerant P. fluorescence and Trichoderma sp. may potentially be used in detoxification and reclamation of metal-contaminated soils.

Role of PGPR and silver nanoparticles on the physiology of Momordica charantia L. irrigated with polluted water comprising high Fe and Mn

The current investigation designed to estimate the bioremediation potential of plant growth-promoting rhizobacteria (PGPR) and Ag-nanoparticles. Tube well and HIT water comprising Mn and Fe above recommended values were used as treatments while tap water irrigation was treated as control. The HIT water showed 24, 200, and 64.11% higher content of Na, K Ca over control. Seeds were sterilized in 95% ethanol and soaked for 3 h before sowing in 73 h old culture of Pseudomonas stutzeri (Kx574858) @ 108 cells/ml. Phytotoxic effect of Fe and Mn reduce plant biomass and suppress photosynthetic activity indicates. The carotenoids, proline, and proline activity were 366, 450, and 678% higher in tube well water with combined PGPR and Ag-nanoparticles treatments. Pseudomonas stutzeri was more effective than Ag-nanoparticles to reduce oxidative stress with higher production of carotenoids, flavonoids, proline content, and enzyme SOD and CAT activities in HIT water. It is contingent that the high Mn and Fe bearing waste water enhance PGPR bioremediation potential to reduce metal stress in plants with synergistic action of PGPR and organic matter to alleviate oxidative stresses under metal stress. The residual effect of P. stutzeri on organic matter content of the rhizosphere soil and germination rate was higher for Momordica charantia L.

Bioremediation of Heavy Metals by Rhizobacteria

Heavy elements accumulate rapidly in the soil due to industrial activities and the industrial revolution, which significantly impact the morphology, physiology, and yield of crops. Heavy metal contamination will eventually affect the plant tolerance threshold and cause changes in the plant genome and genetic structure. Changes in the plant genome lead to changes in encoded proteins and protein sequences. Consuming these mutated products can seriously affect human and animal health. Bioremediation is a process that can be applied to reduce the adverse effects of heavy metals in the soil. In this regard, bioremediation using plant growth–promoting rhizobacteria (PGPRs) as beneficial living agents can help to neutralize the negative interaction between the plant and the heavy metals. PGPRs suppress the adverse effects of heavy metals and the negative interaction of plant-heavy elements by different mechanisms such as biological adsorption and entrapment of heavy elements in extracellular capsules, reduction of metal ion concentration, and formation of complexes with metal ions inside the cell.

Overview of biofertilizers in crop production and stress management for sustainable agriculture

With the increase in world population, the demography of humans is estimated to be exceeded and it has become a major challenge to provide an adequate amount of food, feed, and agricultural products majorly in developing countries. The use of chemical fertilizers causes the plant to grow efficiently and rapidly to meet the food demand. The drawbacks of using a higher quantity of chemical or synthetic fertilizers are environmental pollution, persistent changes in the soil ecology, physiochemical composition, decreasing agricultural productivity and cause several health hazards. Climatic factors are responsible for enhancing abiotic stress on crops, resulting in reduced agricultural productivity. There are various types of abiotic and biotic stress factors like soil salinity, drought, wind, improper temperature, heavy metals, waterlogging, and different weeds and phytopathogens like bacteria, viruses, fungi, and nematodes which attack plants, reducing crop productivity and quality. There is a shift toward the use of biofertilizers due to all these facts, which provide nutrition through natural processes like zinc, potassium and phosphorus solubilization, nitrogen fixation, production of hormones, siderophore, various hydrolytic enzymes and protect the plant from different plant pathogens and stress conditions. They provide the nutrition in adequate amount that is sufficient for healthy crop development to fulfill the demand of the increasing population worldwide, eco-friendly and economically convenient. This review will focus on biofertilizers and their mechanisms of action, role in crop productivity and in biotic/abiotic stress tolerance.