Endophyte-assisted promotion of biomass production and metal-uptake of energy crop sweet sorghum by plant-growth-promoting endophyte Bacillus sp. SLS18

Comparative analysis of the short-term germination and metal accumulation patterns of two Sorghum hybrids

Metal contamination poses a high risk for organisms, especially those with extensive food chain relevancy. Thus, elevated concentration of metals is considered a major cause for concern in crops. This study aimed to evaluate the short-term responses of sorghum and Sudan grass to different Cd/Zn doses in a complex germination test by assessing growth parameters, tissue metal concentrations, and metal interaction accountant for the ecophysiological and elemental alterations. To do so, radicle and hypocotyl length were measured, and Ca, K, Mg, Cd, Cu, Fe, and Zn concentrations were determined after 24, 72, and 120 h. Our results indicated significant (p < 0.05) differences in the radicle and hypocotyl length by species, contaminant dose, and exposure time. Further, the applied doses along the exposure time gradient significantly and variously affected tissue concentrations. Out of the comparisons involving single metal doses, two significant interactions were revealed: the concentrations of both Cu and Fe were significantly reduced by the increase in Cd concentration in Sudan grass tissues. It was concluded that both species have an excellent potential to indicate metal contamination and accumulate metals in the short term, however, with differences in their responses along the exposure gradient. Additionally, this study filled a literature gap by revealing major patterns and limitations in growth and metal accumulation for sorghum and Sudan grass, thereby supporting further research and practical implications.

Cyperus esculentus var. sativus Adapts to Multiple Heavy Metal Stresses Through the Assembly of Endophytic Microbial Communities

Interactions between plants and their endophytes alter their metabolic functions and ability to cope with abiotic stresses. In this study, high-throughput sequencing was used to analyze the species diversity and functions of endophytes in Cyperus esculentus var. sativus (CES) tubers under different heavy metal stress conditions. The results indicated that the number of observed endophytic species in the tubers increased under heavy metal stress (p < 0.05), leading to changes in species diversity and composition. The response of tuber endophytes to different metal concentrations varied, with certain endophytic bacteria and fungi, such as Pseudomonas, Novosphingobium, and Fusarium, showing increased abundance and becoming the dominant species in the tubers. Additionally, new endophytic genera, Actinophytocola and Monosporascus, emerged at specific metal concentrations (p < 0.05). Fatty acid salvage was enriched in the endophytes of CES, which may play an important role in assisting CES in responding to multiple heavy metal stresses. These findings showed that CES tuber endophytes undergo adaptive changes to support the ability of plants to cope with heavy metal stress.

Plant growth-promoting rhizobacterial secondary metabolites in augmenting heavy metal(loid) phytoremediation: An integrated green in situ ecorestorative technology

In recent times, increased geogenic and human-centric activities have caused significant heavy metal(loid) (HM) contamination of soil, adversely impacting environmental, plant, and human health. Phytoremediation is an evolving, cost-effective, environment-friendly, in situ technology that employs indigenous/exotic plant species as natural purifiers to remove toxic HM(s) from deteriorated ambient soil. Interestingly, the plant's rhizomicrobiome is pivotal in promoting overall plant nutrition, health, and phytoremediation. Certain secondary metabolites produced by plant growth-promoting rhizobacteria (PGPR) directly participate in HM bioremediation through chelation/mobilization/sequestration/bioadsorption/bioaccumulation, thus altering metal(loid) bioavailability for their uptake, accumulation, and translocation by plants. Moreover, the metallotolerance of the PGPR and the host plant is another critical factor for the successful phytoremediation of metal(loid)-polluted soil. Among the phytotechniques available for HM remediation, phytoextraction/phytoaccumulation (HM mobilization, uptake, and accumulation within the different plant tissues) and phytosequestration/phytostabilization (HM immobilization within the soil) have gained momentum in recent years. Natural metal(loid)-hyperaccumulating plants have the potential to assimilate increased levels of metal(loid)s, and several such species have already been identified as potential candidates for HM phytoremediation. Furthermore, the development of transgenic rhizobacterial and/or plant strains with enhanced environmental adaptability and metal(loid) uptake ability using genetic engineering might open new avenues in PGPR-assisted phytoremediation technologies. With the use of the Geographic Information System (GIS) for identifying metal(loid)-impacted lands and an appropriate combination of normal/transgenic (hyper)accumulator plant(s) and rhizobacterial inoculant(s), it is possible to develop efficient integrated phytobial remediation strategies in boosting the clean-up process over vast regions of HM-contaminated sites and eventually restore ecosystem health.

Unearthing the power of microbes as plant microbiome for sustainable agriculture

In recent years, research into the complex interactions and crosstalk between plants and their associated microbiota, collectively known as the plant microbiome has revealed the pivotal role of microbial communities for promoting plant growth and health. Plants have evolved intricate relationships with a diverse array of microorganisms inhabiting their roots, leaves, and other plant tissues. This microbiota mainly includes bacteria, archaea, fungi, protozoans, and viruses, forming a dynamic and interconnected network within and around the plant. Through mutualistic or cooperative interactions, these microbes contribute to various aspects of plant health and development. The direct mechanisms of the plant microbiome include the enhancement of plant growth and development through nutrient acquisition. Microbes have the ability to solubilize essential minerals, fix atmospheric nitrogen, and convert organic matter into accessible forms, thereby augmenting the nutrient pool available to the plant. Additionally, the microbiome helps plants to withstand biotic and abiotic stresses, such as pathogen attacks and adverse environmental conditions, by priming the plant's immune responses, antagonizing phytopathogens, and improving stress tolerance. Furthermore, the plant microbiome plays a vital role in phytohormone regulation, facilitating hormonal balance within the plant. This regulation influences various growth processes, including root development, flowering, and fruiting. Microbial communities can also produce secondary metabolites, which directly or indirectly promote plant growth, development, and health. Understanding the functional potential of the plant microbiome has led to innovative agricultural practices, such as microbiome-based biofertilizers and biopesticides, which harness the power of beneficial microorganisms to enhance crop yields while reducing the dependency on chemical inputs. In the present review, we discuss and highlight research gaps regarding the plant microbiome and how the plant microbiome can be used as a source of single and synthetic bioinoculants for plant growth and health.

Salinity-mediated enhancement of quaternary ammonium compounds resistance and removal in endophytic bacteria LSE01

The widespread usage of quaternary ammonium compounds (QACs) as disinfectants during the COVID-19 pandemic poses significant environmental risks, such as toxicity to organisms and the emergence of superbugs. In this study, different inorganic salts (NaCl, KCl, CaCl2, MgCl2) were used to induce endophytes LSE01 isolated from hyperaccumulating plants. After five generations of cultivation under 80 g/L NaCl, the minimum inhibitory concentration (MIC) of LSE01 to QACs increased by about 3-fold, while its degradation extent increased from 8% to 84% for C12BDMA-Cl and 5%-89% for C14BDMA-Cl. Transmission electron microscopy (TEM) and three-dimensional fluorescence spectra indicated that the cells induced by high concentration of salt caused plasmolysis and secreted more bound extracellular polymeric substances (B-EPS); these changes are likely to be an important reason for the observed increased resistance and enhanced degradation extent of LSE01 to QACs. Our findings suggest that salt-induction could be an effective way to enhance the resistance and removal of toxic organic pollutants by functional microorganisms.

Perspectives of pteridophytes microbiome for bioremediation in agricultural applications

The microbiome is the synchronised congregation of millions of microbial cells in a particular ecosystem. The rhizospheric, phyllospheric, and endospheric microbial diversity of lower groups of plants like pteridophytes, which includes the Ferns and Fern Allies, have also given numerous alternative opportunities to achieve greener and sustainable agriculture. The broad-spectrum bioactivities of these microorganisms, including bioremediation of heavy metals (HMs) in contaminated soil, have been drawing the attention of agricultural researchers for the preparation of bioformulations for applications in climate-resilient and versatile agricultural production systems. Pteridophytes have an enormous capacity to absorb HMs from the soil. However, their direct application in the agricultural field for HM absorption seems infeasible. At the same time, utilisation of Pteridophyte-associated microbes having the capacity for bioremediation have been evaluated and can revolutionise agriculture in mining and mineral-rich areas. In spite of the great potential, this group of microbiomes has been less studied. Under these facts, this prospective review was carried out to summarise the basic and applied research on the potential of Pteridophyte microbiomes for soil bioremediation and other agricultural applications globally. Gaps have also been indicated to present scopes for future research programmes.

Plant growth-promoting bacteria improve the Cd phytoremediation efficiency of soils contaminated with PE-Cd complex pollution by influencing the rhizosphere microbiome of sorghum

Composite pollution by microplastics and heavy metals poses a potential threat to the soilplant system and has received increasing attention. Plant growth-promoting bacteria (PGPB) have good application potential for the remediation of combined microplastic and heavy metal pollution, but few related studies exist. The present study employed a pot experiment to investigate the effects of inoculation with the PGPB Bacillus sp. SL-413 and Enterobacter sp. VY-1 on sorghum growth and Cd accumulation under conditions of combined cadmium (Cd) and polyethylene (PE) pollution. Cd+PE composite contamination led to a significant reduction in sorghum length and biomass due to increased toxicity. Inoculation with Bacillus sp. SL-413 and Enterobacter sp. VY-1 alleviated the stress caused by Cd+PE complex pollution, and the dry weight of sorghum increased by 25.7% to 46.1% aboveground and by 12.3% to 45.3% belowground. Bacillus sp. SL-413 and Enterobacter sp. VY-1 inoculation increased the Cd content and accumulation in sorghum and improved the phytoremediation efficiency of Cd. The inoculation treatment effectively alleviated the nutrient stress caused by the reduction in soil mineral nutrients due to Cd+PE composite pollution. The composition of the soil bacterial communities was also affected by the Cd, Cd+PE and bacterial inoculation treatments, which affected the diversity of the soil bacterial communities. Network analyses indicated that bacterial inoculation regulated the interaction of rhizospheric microorganisms and increased the stability of soil bacterial communities. The Mantel test showed that the changes in the soil bacterial community and function due to inoculation with Bacillus sp. SL-413 and Enterobacter sp. VY-1 were important factors influencing sorghum growth and Cd remediation efficiency. The results of this study will provide new evidence for the research on joint plantmicrobe remediation of heavy metal and microplastic composite pollution.

Screening of cadmium resistant bacteria and their growth promotion of Sorghum bicolor (L.) Moench under cadmium stress

Heavy metal pollution of agricultural soils, especially from cadmium (Cd) contaminationcaused serious problems in both food security and economy. Sorghum bicolor (L.) showed a great potential in phytoremediation of Cd contamination due to its fast growth, high yield and easy harvesting. However, the growth of S. bicolor plants tends to be inhibited under Cd exposure, which limited its application for Cd remediation. Plant growth-promoting rhizobacteria may enhance the Cd resistance of S. bicolor and thus improve its Cd removal efficiency. In this study, three Cd-resistant bacteria were screened based on Cd and acid tolerance and identified as Bacillus velezensis QZG6, Enterobacter cloacae QZS3 and Bacillus cereus QZS8, by 16S rRNA sequencing. Inoculation of hydroponic plants with strains QZG6, QZS3 or QZS8 significantly promoted the biomass of sorghum plants by 31.52%, 50.20% and 26.93%, respectively, compared with those of uninoculated plants under Cd exposure. The activity of SOD, POD and MDA content in Cd-stressed S. bicolor plants were reduced of 65.74%, 31.52%, and 80.91%, respectively, when inoculated with the strains QZS3. For pot experiment, strains QZG6, QZS3 and QZS8 significantly promoted the biomass of sorghum plants by 47.30%, 19.27% and 58.47%, compared with those of uninoculated plants under Cd exposure. The activity of SOD, POD and MDA content in Cd-stressed S. bicolor plants were reduced of 67.20%, 22.40%, and 40.65%, respectively, when inoculated with the strains QZS3. All these three strains significantly increased the Cd removal efficiency of the plants by 42.16% (QZG6), 18.76% (QZS3) and 21.06% (QZS8). To investigate the bacterial characteristics associated with growth promotion of S. bicolor plants, the ability on nitrogen fixation, phosphorus solubilization, siderophores production, and phytohormones production were determined. All the strains were able to fix nitrogen. Phosphorus release was observed for strains QZG6 (inorganic or organic phosphorus) and QZS3 (inorganic phosphorus). Both QZG6 and QZS8 were able to produce siderophores, while only QZG6 was positive for ACC deaminase. All the strains produced IAA, SA and GA. These results indicated that the three strains promoted the plant growth under Cd stress, probably through Cd detoxification by siderophores, as well as through growth regulation by N/P nutrient supply and phytohormone. The present study showed a great potential of the three Cd-resistant strains combined with S. bicolor plants in the remediation of Cd-polluted soils, which may provide a new insight into combining the advantages of microbes and plants to improve the remediation of Cd-contaminated soils.

Response of tomatoes to inactivated endophyte LSE01 under combined stress of high-temperature and drought

Endophytes can assist crops in adapting to high temperatures and drought conditions, thereby reducing agricultural losses. However, the mechanism through which endophytes regulate crop resistance to high temperatures and drought stress remains unclear, and concerns regarding safety and stability exist with active endophytes. Thus, heat-treated endophytic bacteria LSE01 (HTB) were employed as a novel microbial fertilizer to investigate their effects on plant adaptation to high temperatures and drought conditions. The results indicated that the diameter and weight of tomatoes treated with HTB under stress conditions increased by 23.04% and 71.15%, respectively, compared to the control. Tomato yield did not significantly decrease compared to non-stress conditions. Additionally, the contents of vitamin C, soluble sugars, and proteins treated with HTB increased by 18.81%, 11.54%, and 99.75%, respectively. Mechanistic research revealed that HTB treatment enhances tomato's stress resistance by elevating photosynthetic pigment and proline contents, enhancing antioxidant enzyme activities, and reducing the accumulation of MDA. Molecular biology research demonstrates that HTB treatment upregulates the expression of drought-resistant genes (GA2ox7, USP1, SlNAC3, SlNAC4), leading to modifications in stomatal conductance, plant morphology, photosynthetic intensity, and antioxidant enzyme synthesis to facilitate adaptation to dry conditions. Furthermore, the upregulation of the heat-resistant gene (SlCathB2-2) can increases the thickness of tomato cell walls, rendering them less vulnerable to heat stress. In summary, HTB endows tomatoes with the ability to adapt to high temperatures and drought conditions, providing new opportunities for sustainable agriculture.

Deciphering the mechanisms, hormonal signaling, and potential applications of endophytic microbes to mediate stress tolerance in medicinal plants

The global healthcare market in the post-pandemic era emphasizes a constant pursuit of therapeutic, adaptogenic, and immune booster drugs. Medicinal plants are the only natural resource to meet this by supplying an array of bioactive secondary metabolites in an economic, greener and sustainable manner. Driven by the thrust in demand for natural immunity imparting nutraceutical and life-saving plant-derived drugs, the acreage for commercial cultivation of medicinal plants has dramatically increased in recent years. Limited resources of land and water, low productivity, poor soil fertility coupled with climate change, and biotic (bacteria, fungi, insects, viruses, nematodes) and abiotic (temperature, drought, salinity, waterlogging, and metal toxicity) stress necessitate medicinal plant productivity enhancement through sustainable strategies. Plants evolved intricate physiological (membrane integrity, organelle structural changes, osmotic adjustments, cell and tissue survival, reclamation, increased root-shoot ratio, antibiosis, hypersensitivity, etc.), biochemical (phytohormones synthesis, proline, protein levels, antioxidant enzymes accumulation, ion exclusion, generation of heat-shock proteins, synthesis of allelochemicals. etc.), and cellular (sensing of stress signals, signaling pathways, modulating expression of stress-responsive genes and proteins, etc.) mechanisms to combat stresses. Endophytes, colonizing in different plant tissues, synthesize novel bioactive compounds that medicinal plants can harness to mitigate environmental cues, thus making the agroecosystems self-sufficient toward green and sustainable approaches. Medicinal plants with a host set of metabolites and endophytes with another set of secondary metabolites interact in a highly complex manner involving adaptive mechanisms, including appropriate cellular responses triggered by stimuli received from the sensors situated on the cytoplasm and transmitting signals to the transcriptional machinery in the nucleus to withstand a stressful environment effectively. Signaling pathways serve as a crucial nexus for sensing stress and establishing plants' proper molecular and cellular responses. However, the underlying mechanisms and critical signaling pathways triggered by endophytic microbes are meager. This review comprehends the diversity of endophytes in medicinal plants and endophyte-mediated plant-microbe interactions for biotic and abiotic stress tolerance in medicinal plants by understanding complex adaptive physiological mechanisms and signaling cascades involving defined molecular and cellular responses. Leveraging this knowledge, researchers can design specific microbial formulations that optimize plant health, increase nutrient uptake, boost crop yields, and support a resilient, sustainable agricultural system.

Plant growth-promoting bacteria modulate gene expression and induce antioxidant tolerance to alleviate synergistic toxicity from combined microplastic and Cd pollution in sorghum

Microplastics (MPs) can act as carriers for environmental pollutants; therefore, MPs combined with heavy metal pollution are attracting increasing attention from researchers. In this study, the potential of the plant growth-promoting bacterium Bacillus sp. SL-413 to mitigate the stress caused by exposure to both MPs and cadmium (Cd) in sorghum plants was investigated. The effects of inoculation on sorghum biomass were investigated using hydroponic experiments, and evaluation of Cd accumulation and enzyme activity changes and transcriptomics approaches were used to analyze its effect on sorghum gene expression. The results showed that combined polyethylene (PE) and Cd pollution reduced the length and the fresh and dry weights of sorghum plants and thus exerted a synergistic toxic effect. However, inoculation with the strains alleviated the stress caused by the combined pollution and significantly increased the biomass. Inoculation increased the dry weights of the aboveground and belowground parts by 11.5-44.6% and 14.9-38.4%, respectively. Plant physiological measurements indicated that inoculation reduced the reactive oxygen species (ROS) content of sorghum by 10.5-27.2% and thereby alleviated oxidative stress. Transcriptome sequencing showed that exposure to combined Cd+MP contamination induced downregulation of gene expression, particularly that of genes related to amino sugar and nucleotide sugar metabolism, starch and sucrose metabolism, and plant hormone signal transduction, in sorghum. However, inoculation with Bacillus sp. SL-413 resulted in an increase in the proportion of upregulated genes involved in signal transduction, antioxidant defense, cell wall biology, and other metabolic pathways, which included the phenylpropanoid biosynthesis, photosynthesis, flavonoid biosynthesis, and MAPK signaling pathways. The upregulation of these genes promoted the tolerance of sorghum under combined Cd+MP pollution stress and alleviated the stress induced by these conditions. This study provides the first demonstration that plant growth-promoting bacteria can alleviate the stress caused by combined pollution with MPs and Cd by regulating plant gene expression. These findings provide a reference for the combined plant-microbial remediation of MPs and Cd.

Plant growth-promoting rhizobacteria: A good companion for heavy metal phytoremediation

Phytoremediation is an environment-friendly approach regarded as a potential candidate for remediating heavy metal (HM)-contaminated soils. However, the low efficacy of phytoremediation is a major limitation that hampers its large-scale application. Therefore, developing strategies to enhance phytoremediation efficacy for contaminated soils is crucial. Plant growth-promoting rhizobacteria (PGPR) considerably contribute to phytoremediation intensification. To improve the efficiency of plant-microbe symbiosis for remediation, the mechanisms underlying PGPR-stimulated HM accumulation and tolerance in plants should be comprehensively understood. This review focuses on hyperaccumulators, PGPR, and the mechanisms by which PGPR enhance phytoremediation from four aspects: providing nutrients to plants, secreting plant hormones and specific enzymes, inducing systemic resistance, and altering the bioavailability of HMs in soils. It also provides a theoretical and technical basis for future research on PGPR synergism in promoting the phytoextraction efficiency in HM-contaminated soils.

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.

A preliminary study to explain how Streptomyces pactum (Act12) works on phytoextraction: soil heavy metal extraction, seed germination, and plant growth

Streptomyces pactum (Act12) can both promote plant growth and strengthen heavy metal mobilization. Nevertheless, the mechanisms of how Act12 works during the phytoextraction process are still unknown. The present work investigated whether the metabolites produced by Act12 could influence the seed germination and the growth of potherb mustard and explored its mobilizing effect on soil cadmium (Cd) and zinc (Zn). The results showed that the germination potential and rate of potherb mustard seed treated with Act12 fermentation broth were 1.0- and 0.32-folds higher than those of control, probably due to the interruption of seed dormant stage. We also found that Act12 inoculation not only promoted the dry biomass (6.82%) of potherb mustard, but also increased the leaf chlorophyll (11.8%) and soluble protein (0.35%) production. The boosted seed germination rate under Act12 treatment (up to 63.3%) indicated that Act12 enhanced the resistance of potherb mustard seeds to Cd and Zn and alleviated their physiological toxicity. The generated metabolites during the Act12 fermentation posed positive impact on the availability of soil Cd and Zn. These findings bring new insight into the Act12-assisted phytoextraction of Cd and Zn from contaminated soils.

Trends in Harnessing Plant Endophytic Microbiome for Heavy Metal Mitigation in Plants: A Perspective

Plant microbiomes represent dynamic entities, influenced by the environmental stimuli and stresses in the surrounding conditions. Studies have suggested the benefits of commensal microbes in improving the overall fitness of plants, besides beneficial effects on plant adaptability and survival in challenging environmental conditions. The concept of 'Defense biome' has been proposed to include the plant-associated microbes that increase in response to plant stress and which need to be further explored for their role in plant fitness. Plant-associated endophytes are the emerging candidates, playing a pivotal role in plant growth, adaptability to challenging environmental conditions, and productivity, as well as showing tolerance to biotic and abiotic stresses. In this article, efforts have been made to discuss and understand the implications of stress-induced changes in plant endophytic microbiome, providing key insights into the effects of heavy metals on plant endophytic dynamics and how these beneficial microbes provide a prospective solution in the tolerance and mitigation of heavy metal in contaminated sites.

Regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi

Endophytic fungi exist widely in plants and play an important role in the growth and adaptation of plants. They could be used in phytoremediation techniques against heavy metal contaminated soil since beneficial microbial symbionts can endow plants with resistance to external heavy metal stresses. This review summarized the regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi. Potential endophytic fungi in enhancing plant's adaption to heavy metal stresses include arbuscular mycorrhizal fungi, dark septate endophytic fungi, plant growth promoting endophytic fungi. The mechanisms involve coevolution strategy, immune regulation and detoxification transport to improve the ability of plants to adapt to heavy metal stress. They can increase the synthesis of host hormones and maintaining the balance of endogenous hormones, strengthen osmotic regulation, regulate carbon and nitrogen metabolism, and increase immune activity, antioxidant enzyme and glutathione activity. They also help to improve the detoxification transport and heavy metal emission capacity of the host by significantly producing iron carrier, metallothionein and 1-aminocyclopropane-1-carboxylic acid deaminase. The combination of endophytic fungi and hyperaccumulation plants provides a promising technology for the ecological restoration of heavy metal contaminated soil. Endophytic fungi reserves further development on enhancing host plant's adaptability to heavy metal stresses.

Effects of the dual inoculation of dark septate endophytes and Trichoderma koningiopsis on blueberry growth and rhizosphere soil microorganisms

Blueberry is a shallow root plant in which the absorption of nutrients is inefficient, resulting in slow growth under artificial cultivation conditions. Endophytes play an important role in promoting plant growth; however, the effects of Trichoderma spp. and dark septate endophytes (DSEs) on host plant growth and soil microorganisms are still debatable. We isolated two endophytic fungal species, Trichoderma koningiopsis (TK) and a DSE (Amesia nigricolor; AN), from blueberry roots, which can solubilize insoluble phosphorus and produce amylase and cellulase to promote plant growth. We found that under dual inoculation, the colonization rate and colonization intensity of TK were higher than they were under single inoculation with TK, while the colonization rate and colonization intensity of AN were lower under dual inoculation than under single inoculation with AN. The plant nutrients, root activity, available potassium, and parts of soil phosphatase activities were highest under dual inoculation. TK inoculation resulted in the highest diversity and richness in the soil fungi and bacteria, followed by dual inoculation. The abundance of Ascomycota, Acidobacteriae, Firmicutes, and Actinobacteriota increased significantly, resulting in Trichoderma and Vicinamibacteria inoculated with TK, Chaetomium and Alicyclobacillales inoculated with AN, and Hypocreales and Burkholderiaceae with dual inoculation enriched in the soil.

Enhancing Mechanisms of the Plant Growth-Promoting Bacterial Strain Brevibacillus sp. SR-9 on Cadmium Enrichment in Sweet Sorghum by Metagenomic and Transcriptomic Analysis

To explore the mechanism by which the plant growth-promoting bacterium Brevibacillus sp. SR-9 improves sweet sorghum tolerance and enriches soil cadmium (Cd) under pot conditions, the effect of strain SR-9 inoculation on the microbial community of sorghum rhizosphere soil was analyzed by metagenomics. Gene expression in sweet sorghum roots was analyzed using transcriptomics. The results showed that strain SR-9 promoted the growth of sweet sorghum and improved the absorption and enrichment of Cd in the plants. Compared with the uninoculated treatment, the aboveground part and root dry weight in strain SR-9 inoculated with sorghum increased by 21.09% and 17.37%, respectively, and the accumulation of Cd increased by 135% and 53.41%, respectively. High-throughput sequencing showed that strain SR-9 inoculation altered the rhizosphere bacterial community, significantly increasing the relative abundance of Actinobacteria and Firmicutes. Metagenomic analysis showed that after inoculation with strain SR-9, the abundance of genes involved in amino acid transport metabolism, energy generation and conversion, and carbohydrate transport metabolism increased. KEGG functional classification showed that inoculation with strain SR-9 increased the abundance of genes involved in soil microbial metabolic pathways in the rhizosphere soil of sweet sorghum and the activity of soil bacteria. Transcriptome analysis identified 198 upregulated differentially expressed genes in sweet sorghum inoculated with strain SR-9, including those involved in genetic information processing, biological system, metabolism, environmental information processing, cellular process, and human disease. Most of the annotated differentially expressed genes were enriched in the metabolic category and were related to pathways such as signal transduction, carbohydrate metabolism, amino acid metabolism, and biosynthesis of other secondary metabolites. This study showed that plant growth-promoting bacteria can alter the rhizosphere bacterial community composition, increasing the activity of soil bacteria and upregulating gene expression in sweet sorghum roots. The findings enhance our understanding of the microbiological and botanical mechanisms by which plant growth-promoting bacterial inoculation improves the remediation of heavy metals by sorghum.

Perspectives and potential applications of endophytic microorganisms in cultivation of medicinal and aromatic plants

Ensuring food and nutritional security, it is crucial to use chemicals in agriculture to boost yields and protect the crops against biotic and abiotic perturbations. Conversely, excessive use of chemicals has led to many deleterious effects on the environment like pollution of soil, water, and air; loss of soil fertility; and development of pest resistance, and is now posing serious threats to biodiversity. Therefore, farming systems need to be upgraded towards the use of biological agents to retain agricultural and environmental sustainability. Plants exhibit a huge and varied niche for endophytic microorganisms inside the planta, resulting in a closer association between them. Endophytic microorganisms play pivotal roles in plant physiological and morphological characteristics, including growth promotion, survival, and fitness. Their mechanism of action includes both direct and indirect, such as mineral phosphate solubilization, fixating nitrogen, synthesis of auxins, production of siderophore, and various phytohormones. Medicinal and aromatic plants (MAPs) hold a crucial position worldwide for their valued essential oils and several phytopharmaceutically important bioactive compounds since ancient times; conversely, owing to the high demand for natural products, commercial cultivation of MAPs is on the upswing. Furthermore, the vulnerability to various pests and diseases enforces noteworthy production restraints that affect both crop yield and quality. Efforts have been made towards enhancing yields of plant crude drugs by improving crop varieties, cell cultures, transgenic plants, etc., but these are highly cost-demanding and time-consuming measures. Thus, it is essential to evolve efficient, eco-friendly, cost-effective simpler approaches for improvement in the yield and health of the plants. Harnessing endophytic microorganisms as biostimulants can be an effective and alternative step. This review summarizes the concept of endophytes, their multidimensional interaction inside the host plant, and the salient benefits associated with endophytic microorganisms in MAPs.

Progress and Applications of Plant Growth-Promoting Bacteria in Salt Tolerance of Crops

Saline soils are a major challenge in agriculture, and salinization is increasing worldwide due to climate change and destructive agricultural practices. Excessive amounts of salt in soils cause imbalances in ion distribution, physiological dehydration, and oxidative stress in plants. Breeding and genetic engineering methods to improve plant salt tolerance and the better use of saline soils are being explored; however, these approaches can take decades to accomplish. A shorter-term approach to improve plant salt tolerance is to be inoculated with bacteria with high salt tolerance or adjusting the balance of bacteria in the rhizosphere, including endosymbiotic bacteria (living in roots or forming a symbiont) and exosymbiotic bacteria (living on roots). Rhizosphere bacteria promote plant growth and alleviate salt stress by providing minerals (such as nitrogen, phosphate, and potassium) and hormones (including auxin, cytokinin, and abscisic acid) or by reducing ethylene production. Plant growth-promoting rhizosphere bacteria are a promising tool to restore agricultural lands and improve plant growth in saline soils. In this review, we summarize the mechanisms of plant growth-promoting bacteria under salt stress and their applications for improving plant salt tolerance to provide a theoretical basis for further use in agricultural systems.

Biochar-based fertilizers and their applications in plant growth promotion and protection

Soil is an integral part of the ecosystem because it serves as a habitat for various microorganisms and lays the foundation for supporting plant growth and development. Therefore, factors such as increased anthropogenic activities hand by hand with other natural processes that harm the ecosystem may eventually lead to a decline in soil quality and fertility, hindering the growth of plants and soil microbial communities. Given the current global scenario of increasing human intervention, it is essential to find effective measures and reliable technologies to restore soil quality. Biochar is an emerging soil ameliorant employed for soil health restoration and is primarily generated through the anoxygenic pyrolysis of biomass. The biochar application in soil remediation may be beneficial due to biochar's unique physicochemical properties, including high carbon and metal fixation abilities. In addition, biochar possesses abilities to reduce the plant's environmental stress injuries. This review briefly overviewed the ingredients and mechanism of biochar productions. We then emphatically reviewed the advances in biochar applications in soil bioremediation, soil microflora growth stimulation, and the alleviation of various biotic and abiotic stresses in plants.

Potassium Deficiency in Rice Aggravates Sarocladium oryzae Infection and Ultimately Leads to Alterations in Endophyte Communities and Suppression of Nutrient Uptake

Sheath rot disease is an emerging fungal disease in rice, whose infection causes severe yield loss. Sarocladium oryzae (S. oryzae) is the major causal agent. Previous study has demonstrated that rice deficiency in potassium (K) aggravates S. oryzae infection. However, the effects of S. oryzae infection on the nutrient-uptake process, endophyte communities, and hormone level of host plant under K-deficiency condition remain unclear, the mechanism of K mediated S. oryzae infection needs to be further study. The present study analyzed alterations in the endophytic community and nutrient-uptake process of host plants through an exogenous inoculation of S. oryzae in pot and hydroponics experiments. S. oryzae infection sharply increased the relative abundance of Ascomycota and decreased the Shannon and Simpson index of the endophytic community. Compared with the K-sufficient rice infected with S. oryzae, K-starved rice infected with S. oryzae (-K + I) increased the relative abundance of Ascomycota in leaf sheaths by 52.3%. Likewise, the -K + I treatment significantly decreased the Shannon and Simpson indexes by 27.7 and 25.0%, respectively. Sufficient K supply increased the relative abundance of Pseudomonas spp. in the host plant. S. oryzae infection profoundly inhibited the nutrient uptake of the host plant. The accumulation of oleic acid and linoleic acid in diseased rice decreased the biosynthesis of jasmonic acid (JA), and the content of JA was lowest in the -K + I treatment, which suppressed K⁺ uptake. These results emphasize the importance of K in resistance to S. oryzae infection by modulating endophyte community diversity and enhancing the nutrient-uptake capacity of the host plant.

Heteroauxin-producing bacteria enhance the plant growth and lead uptake of Miscanthus floridulus (Lab.)

Soil lead (Pb) contamination has caused severe environmental threats and is in urgent need of remediation. This study was aimed to explore the feasibility of using the Miscanthus-microbe combination to reduce Pb pollution in the farmland surrounding a lead-zinc mining area. We have screened three heteroauxin (IAA)-producing microbes (Lelliottia jeotgali MR2, Klebsiella michiganensis TS8, and Klebsiella michiganensis ZR1) with high Pb tolerance. The IAA-producing ability of the mixed-species was stronger than that of the single bacterium. In pot experiments, the mixed-species of MR2-ZR1 and MR2-TS8 had better performance in enhancing the weight of Miscanthus grass (increased by 22.2-53.6% compared to the control group without inoculating microbes). The remediation efficiency of Pb was significantly higher in the MR2 (30.79%), MR2-TS8 (24.96%), and TS8-ZR1 (21.10%) groups than that in the control group (6.75%). We speculated that MR2 and mixed species of MR2-TS8 and TS8-ZR1 could promote the percentages of activated Pb fractions in soils and increase the Pb uptake of M. floridulus (Lab.). These results implied that the MR2-TS8 mixed-species might be selected as the effective microbial agent to simultaneously enhance the remediation efficiency of Pb-contaminated soils and the biomass of M. floridulus (Lab.).

Sweet sorghum for phytoremediation and bioethanol production

As an energy crop, sweet sorghum (Sorghum bicolor (L.) Moench) receives increasing attention for phytoremediation and biofuels production due to its good stress tolerance and high biomass with low input requirements. Sweet sorghum possesses wide adaptability, which also has high tolerances to poor soil conditions and drought. Its rapid growth with the large storage of fermentable saccharides in the stalks offers considerable scope for bioethanol production. Additionally, sweet sorghum has heavy metal tolerance and the ability to remove cadmium (Cd) in particular. Therefore, sweet sorghum has great potential to build a sustainable phytoremediation system for Cd-polluted soil remediation and simultaneous ethanol production. To implement this strategy, further efforts are in demand for sweet sorghum in terms of screening superior varieties, improving phytoremediation capacity, and efficient bioethanol production. In this review, current research advances of sweet sorghum including agronomic requirements, phytoremediation of Cd pollution, bioethanol production, and breeding are discussed. Furthermore, crucial problems for future utilization of sweet sorghum stalks after phytoremediation are combed.

Graphical Abstract

Chromium Stress Tolerance of a C4 (Zea mays L.) and C3 (Vigna radiata L.) Plants Primed with UV and Gamma-Treated Bacillus subtilis

Chromium stress is one of the deleterious abiotic factors that reduce crop production. Two anatomically different crops (C3 and C4) were compared for their chromium (0 and 50 ppm) tolerance and responses towards Bacillus subtilis (B. subtilis). Strains of B. subtilis were exposed to UV (30–210 min) and gamma irradiation (1–4 KGy), and the best mutants were selected on petri plates containing selective markers. Maize and mungbean were supplied with selected strains or the parent strain in rooting medium, along with a nutrient broth. A completely randomized design (five replicates) was adopted using nutrient broth as a control. Stress negatively affected plants grown without strains. Mungbean was more sensitive towards stress and treatments, maize had better root and shoot fresh weights, root and shoot lengths, proline levels, and MDA and GR activity. All strains of B. subtilis (parent, γ-irradiated and UV-irradiated) enhanced proline, total soluble protein, chlorophyll a, a + b and a/b levels, with negligible effects upon antioxidant enzymes. Irradiated strains proved their superiority to the parent strain, with reductions in H₂O₂ and MDA content. With comparable benefits, γ and UV irradiation may be adopted in future based upon technical availability.

Aspergillus sp. A31 and Curvularia geniculata P1 mitigate mercury toxicity to Oryza sativa L

Aspergillus sp. A31 and Curvularia geniculata P1 are endophytes that colonize the roots of Aeschynomene fluminensis Vell. and Polygonum acuminatum Kunth. in humid environments contaminated with mercury. The two strains mitigated mercury toxicity and promoted Oryza sativa L growth. C. geniculata P1 stood out for increasing the host biomass by fourfold and reducing the negative effects of the metal on photosynthesis. Assembling and annotation of Aspergillus sp. A31 and C. geniculata P1 genomes resulted in 28.60 Mb (CG% 53.1; 10,312 coding DNA sequences) and 32.92 Mb (CG% 50.72; 8,692 coding DNA sequences), respectively. Twelve and 27 genomes of Curvularia/Bipolaris and Aspergillus were selected for phylogenomic analyzes, respectively. Phylogenetic analysis inferred the separation of species from the genus Curvularia and Bipolaris into different clades, and the separation of species from the genus Aspergillus into three clades; the species were distinguished by occupied niche. The genomes had essential gene clusters for the adaptation of microorganisms to high metal concentrations, such as proteins of the phytoquelatin-metal complex (GO: 0090423), metal ion binders (GO: 0046872), ABC transporters (GO: 0042626), ATPase transporters (GO: 0016887), and genes related to response to reactive oxygen species (GO: 0000302) and oxidative stress (GO: 0006979). The results reported here help to understand the unique regulatory mechanisms of mercury tolerance and plant development.

Influence of electrical fields enhanced phytoremediation of multi-metal contaminated soil on soil parameters and plants uptake in different soil sections

The influence of electrical fields on phytoremediation of multi-metal (Cd, Cu, and Zn) naturally contaminated soils has been investigated based on different soil sections. After ryegrass and hybrid penisetum were sowed for 30 d, electrical fields were applied during 30 days with the switching polarity every 30 min and continuing for 16 h d^-1. After electrokinetic (EK) assisted phytoremediation process, soil electrical conductivity (EC) in anode section and available soil potassium (K) in cathode section were obviously elevated. Plants biomass in middle and cathode sections were increased in both plants, especially in middle section the overall biomass of hybrid penisetum increased by 68.8%. The influence of electrical field on the contents of heavy metals in plants was different depending on the species of plants, kind of heavy metals and soil section. For Cd, Cu, and Zn co-contaminated soils, shoot metals accumulation in middle section in both plants were improved at least about 20% (with the exception of Zn in ryegrass). Electrical fields had the most significant effect on copper absorption by ryegrass and shoot Cu accumulation were elevated 32.5% in all the section. The soil EC maybe an important factor that affected electrical fields enhanced plants growth, plant metals concentrations and remediation efficiency.

The Endophytic Microbiome as a Hotspot of Synergistic Interactions, with Prospects of Plant Growth Promotion

The plant root is the primary site of interaction between plants and associated microorganisms and constitutes the main components of plant microbiomes that impact crop production. The endophytic bacteria in the root zone have an important role in plant growth promotion. Diverse microbial communities inhabit plant root tissues, and they directly or indirectly promote plant growth by inhibiting the growth of plant pathogens, producing various secondary metabolites. Mechanisms of plant growth promotion and response of root endophytic microorganisms for their survival and colonization in the host plants are the result of complex plant-microbe interactions. Endophytic microorganisms also assist the host to sustain different biotic and abiotic stresses. Better insights are emerging for the endophyte, such as host plant interactions due to advancements in 'omic' technologies, which facilitate the exploration of genes that are responsible for plant tissue colonization. Consequently, this is informative to envisage putative functions and metabolic processes crucial for endophytic adaptations. Detection of cell signaling molecules between host plants and identification of compounds synthesized by root endophytes are effective means for their utilization in the agriculture sector as biofertilizers. In addition, it is interesting that the endophytic microorganism colonization impacts the relative abundance of indigenous microbial communities and suppresses the deleterious microorganisms in plant tissues. Natural products released by endophytes act as biocontrol agents and inhibit pathogen growth. The symbiosis of endophytic bacteria and arbuscular mycorrhizal fungi (AMF) affects plant symbiotic signaling pathways and root colonization patterns and phytohormone synthesis. In this review, the potential of the root endophytic community, colonization, and role in the improvement of plant growth has been explained in the light of intricate plant-microbe interactions.

Evaluation of phytoremediation potential of five Cd (hyper)accumulators in two Cd contaminated soils

A phytoextraction experiment with five Cd hyperaccumulators (Amaranthus hypochondriacus, Celosia argentea, Solanum nigrum, Phytolacca acinosa and Sedum plumbizincicola) was conducted in two soils with different soil pH (5.93 and 7.43, respectively). Most accumulator plants grew better in the acidic soil, with 19.59-39.63% higher biomass than in the alkaline soil, except for S. plumbizincicola. The potential for a metal-contaminated soil to be cleaned up using phytoremediation is determined by the metal uptake capacity of hyperaccumulator, soil properties, and mutual fitness of plant-soil relationships. In the acidic soil, C. argentea and A. hypochondriacus extracted the highest amount of Cd (1.03 mg pot^-1 and 0.92 mg pot^-1, respectively). In the alkaline soil, S. plumbizincicola performed best, mainly as a result of high Cd accumulation in plant tissue (541.36 mg kg^-1). Most plants achieved leaf Cd bioconcentration factor (BCF) of >10 in the acidic soil, compared to <4 in the alkaline soil. Soil Cd availability was chiefly responsible for such contrasting metal extraction capacity, with 5.02% fraction and 48.50% fraction of total Cd being available in the alkaline and acidic soil, respectively. In the alkaline soil, plants tended to increase rhizosphere soil available Cd mainly through excreting more low molecular weight organic acids, not through changing the soil pH. In the acidic soil, plants slightly decreased soil available Cd. Those species which have high Ca, Zn, Fe uptake capacity extract more Cd from soil, and a positive correlation was found between the concentrations of Cd and Ca, Zn, Fe in leaves. Soil available Ca²⁺, Mg²⁺, SO₄^2-, Cl^- did not play a key role in Cd uptake by plants. In summary, acidic soil was of higher potential to recover from Cd contamination by phytoextraction, while in the alkaline soil, S. plumbizincicola showed potential for Cd phytoextraction.

Synergistic removal of cadmium and organic matter by a microalgae-endophyte symbiotic system (MESS): An approach to improve the application potential of plant-derived biosorbents

Plant-derived materials as environmentally friendly biosorbents to remove heavy metals from wastewater have been extensively studied. However, the chemical oxygen demand (COD) increase caused by the plant-derived biosorbent has not been considered previously. In this study, water hyacinth was used as biosorbent to remove Cd(II) from wastewater. About 66% of Cd(II) was removed by the biosorbent with a maximum biosorption capacity (q_max) of 21.6 mg g^-1. However, the COD of the filtrate increased from 0 to 292 mg L^-1 during this process. Subsequently, endophytes, microalgae and the microalgae-endophyte symbiotic system (MESS) were assessed for the simultaneous Cd(II) and COD removal. Among these three systems, the MESS achieved the best performance. After 3 d of inoculation, the extent of total Cd(II) removal increased to 99.2% while COD decreased to 77 mg L^-1. This study provides a new insight into the application of a plant-derived biosorbent in combination with microalgae and endophytes for the effective treatment of heavy metal-bearing wastewater.

Biological approaches of fluoride remediation: potential for environmental clean-up

Fluoride (F), anion of fluorine which is naturally present in soil and water, behaves as toxic inorganic pollutant even at lower concentration and needs immediate attention. Its interaction with flora, fauna and other forms of life, such as microbes, adversely affect various physiochemical parameters by interfering with several metabolic pathways. Conventional methods of F remediation are time-consuming, laborious and cost intensive, which renders them uneconomical for sustainable agriculture. The solution lies in cracking down this environmental contaminant by adopting economic, eco-friendly, cost-effective and modern technologies. Biological processes, viz. bioremediation involving the use of bacteria, fungi, algae and higher plants that holds promising alternative to manage F pollution, recover contaminated soil and improve vegetation. The efficiency of indigenous natural agents may be enhanced, improved and selected over the hazardous chemicals in sustainable agriculture. This review article emphasizes on various biological approaches for the remediation of F-contaminated environment, and exploring their potential applications in environmental clean-up. It further focuses on thorough systemic study of modern biotechnological approaches such as gene editing and gene manipulation techniques for enhancing the plant-microbe interactions for F degradation, drawing attention towards latest progresses in the field of microbial assisted treatment of F-contaminated ecosystems. Future research and understanding of the molecular mechanisms of F bioremediation would add on to the possibilities of the application of more competent strains showing striking results under diverse ecological conditions.

Phyllospheric Microbiomes: Diversity, Ecological Significance, and Biotechnological Applications

The phyllosphere referred to the total aerial plant surfaces (above-ground portions), as habitat for microorganisms. Microorganisms establish compositionally complex communities on the leaf surface. The microbiome of phyllosphere is rich in diversity of bacteria, fungi, actinomycetes, cyanobacteria, and viruses. The diversity, dispersal, and community development on the leaf surface are based on the physiochemistry, environment, and also the immunity of the host plant. A colonization process is an important event where both the microbe and the host plant have been benefited. Microbes commonly established either epiphytic or endophytic mode of life cycle on phyllosphere environment, which helps the host plant and functional communication with the surrounding environment. To the scientific advancement, several molecular techniques like metagenomics and metaproteomics have been used to study and understand the physiology and functional relationship of microbes to the host and its environment. Based on the available information, this chapter describes the basic understanding of microbiome in leaf structure and physiology, microbial interactions, especially bacteria, fungi, and actinomycetes, and their adaptation in the phyllosphere environment. Further, the detailed information related to the importance of the microbiome in phyllosphere to the host plant and their environment has been analyzed. Besides, biopotentials of the phyllosphere microbiome have been reviewed.

Characterization of a Versatile Plant Growth-Promoting Rhizobacterium Pseudomonas mediterranea Strain S58

Plant growth-promoting rhizobacterial strain S58 was isolated from the tobacco rhizosphere. It showed strong antagonism against a battery of plant pathogenic fungi and bacteria, and controlled wheat sharp eyespot and tobacco wildfire diseases efficiently. Further tests showed that strain S58 solubilized organic phosphate and produced siderophore, protease, ammonia, and indole-3-acetic acid. In Arabidopsis thaliana, it promoted plant growth and changed root system architecture by restricting the growth of primary roots and increasing lateral root numbers. We relied on morphological, biochemical, physiological characteristics, and molecular phylogenic analysis to identify strain S58 as Pseudomonas mediterranea. The complete genome of strain S58 has a single circular chromosome of 6,150,838 bp with a 61.06% G+C content. The bacterial genome contained 5,312 predicted genes with an average length of 992.90 bp. A genome analysis suggested that P. mediterranea S58 was a rich cyclic lipopeptide (CLP)-producing strain that possessed seven non-ribosomal peptide gene clusters for CLP synthesis. Leaf inoculation of the bacterial culture and supernatants triggered cell death-like immunity in tobacco. Quantitative real-time PCR assays showed that the strain S58 induced the expression of pattern-triggered immunity and cell death marker genes, but not jasmonic acid marker genes. The results suggested that P. mediterranea S58 is a novel, versatile plant growth-promoting agent with multiple beneficial traits for plants.

Phytoremediation of cadmium-contaminated soil by Sorghum bicolor and the variation of microbial community

In this paper, the growth of S. bicolor in Cd-polluted sandy clay loam soil in north China, Cd accumulation in plant and the corresponding soil microbial community were characterized when the plant matured (140 d of growth). Cadmium promoted the growth of mature S. bicolor with higher height and heavier dry mass, especially at the spiked level of 1 mg kg^-1 soil (P < 0.05). The higher microbial diversity was found under Cd stress at the spiked level of 15 mg kg^-1, which basically corresponded with its influence on the plant growth. High-throughput sequencing data demonstrated that the predominant bacterial phyla include Proteobacteria (35.99% for Cd-polluted soil and 35.22% for the control soil), Chloroflexi (21.33% and 20.58%), Actinobacteria (12.00% and 12.89%), Acidobacteria (7.47% and 11.14%), Bacteroidetes (7.37% and 6.96%), Gemmatimonadetes (5.60% and 6.65%), Firmicutes (2.82% and 1.86%), Planctomycetes (2.47% and 0.95%), Saccharibacteria (1.26% and 1.11%). The predominant fungal phyla was Ascomycota, with the relative abundance of 89.96% for the control soil and 86.2% for the Cd-polluted soil. S. bicolor could grow well in sandy clay loam soil in northern China at low Cd lvel, but it could not accumulate cadmium at higher cadmium level. S. bicolor could be used for phytoextraction of cadmium from the lightly Cd-polluted soil.

Inoculation of plant growth promoting bacteria from hyperaccumulator facilitated non-host root development and provided promising agents for elevated phytoremediation efficiency

Plant growth promoting bacteria (PGPB) have been reported to have the ability to promote plant growth, development and increase heavy metals (HMs) uptake. Therefore, PGPB inoculation as soil remediation agents into plants with larger biomass and potential of phytoextraction is of great importance to increase bioremediation efficiency. In this study, 12 PGPB strains isolated from a cadmium (Cd)/zinc hyperaccumulator Sedum alfredii Hance were inoculated into non-host plant Brassica juncea and their effects on plant growth and Cd uptake were determined. The results showed that inoculation of most PGPB strains promoted plant growth, boosted root development and improved chlorophyll content in the absence of Cd. Inoculation of PGPB strains promoted plant growth up to 111% in shoot and 358% in root when treated with 2 μM Cd. In addition, PGPB inoculation not only ameliorated plant root morphology including the total root length (RL), total surface area (SA), total root volume (RV) and number of root tips (RT), but also facilitated Cd uptake up to 126%. Furthermore, inoculation of PGPB strains promoted plant Cd accumulation up to 261% in shoot and up to 8.93-fold increase in root. Among all the 12 PGPB strains, Burkholdria SaMR10 and Sphingomonas SaMR12 were identified as the promising microbes for improving phytoremediation efficiency of Cd contaminated soils. These results not only provided useful findings for further investigation of interacting mechanisms between different bacterial strains and plants, but also facilitated the development of microbe-assisted phytoremediation application for HM contaminated soil.

Promotion of the root development and Zn uptake of Sedum alfredii was achieved by an endophytic bacterium Sasm05

Endophyte-assisted phytoremediation has gained increasing attention. However, the interacting mechanisms of endophytes and metal hyperaccumulators are still not clear. An endophytic bacterium Pseudomonas fluorescens Sasm05 inoculation promoted Sedum alfredii Hance rooting and root development, in which the specific root length (SRL) and average number of root tips (ART) increased to 2.09- and 3.35-fold, respectively. Sasm05 inoculation promoted plant growth, increased the chlorophyll content, and elevated Zn uptake of plant at excess Zn supply. At 200 μM Zn treatment level, Sasm05 inoculation increased plant biomass and the chlorophyll content by more than 40%, and root Zn content by 40%. Furthermore, Sasm05 inoculation upregulated the expression of the Zn transporter SaIRT1 to 3.43-fold in the roots, while another transporter SaNramp1 expression was increased to 38.66-fold in the roots and 7.53-fold in the shoots. Time course study showed the best effects of Sasm05 on plant biomass and the chlorophyll content were detected at 30 d, while for Zn content at 3 d. These results firstly provided molecular evidences of endophytic bacteria in facilitating host plant Zn uptake, which will absolutely benefit the understanding of interacting mechanisms between hyperaccumulators and their endophytes.

Characterization of cadmium-resistant rhizobacteria and their promotion effects on Brassica napus growth and cadmium uptake

Excessive cadmium (Cd) accumulation in soil can adversely affect plants, animals, microbes, and humans; therefore, novel and uncharacterized Cd-resistant plant-growth-promoting rhizobacteria (PGPR) are required to address this issue. In the paper, 13 bacteria were screened, their partial 16S rRNA sequences determined, and the isolates, respectively, clustered into Curtobacterium (7), Chryseobacterium (4), Cupriavidus (1), and Sphingomonas (1). Evaluation of PGP traits, including indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, siderophore secretion, and cyanhydric acid production, identified Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 as promising candidates for PGPR based on high IAA or ACC deaminase production. Additionally, root-elongation assays indicated that inoculating GX_5, _15, or _31 increased Brassica napus root length both in the presence and absence of Cd by 19.75-29.96% and 19.15-31.69%, respectively. Pot experiments indicated that inoculating B. napus with GX_5, _15, and _31 significantly increased the dry weight of above-ground tissues and root biomass by 40.97-85.55% and 18.99-103.13%, respectively. Moreover, these isolates significantly increased Cd uptake in the aerial parts and root tissue of B. napus by 7.38-11.98% and 48.09-79.73%, respectively. These results identified GX_5, _15, or _31 as excellent promoters of metal remediation by using microorganism-associated phytoremediation.

Bioactive Products From Plant-Endophytic Gram-Positive Bacteria

Endophytes constitute plant-colonizing microorganisms in a mutualistic symbiosis relationship. They are found in most ecosystems reducing plant crops' biotic and abiotic stressors by stimulating immune responses, excluding plant pathogens by niche competition, and participating in antioxidant activities and phenylpropanoid metabolism, whose activation produces plant defense, structural support, and survival molecules. In fact, metabolomic studies have demonstrated that endophyte genes associated to specific metabolites are involved in plant growth promotion (PGP) by stimulating plant hormones production such as auxins and gibberellins or as plant protective agents against microbial pathogens, cancer, and insect pests, but eco-friendly and eco-safe. A number of metabolites of Gram-positive endophytes isolated from agriculture, forest, mangrove, and medicinal plants, mainly related to the Firmicutes phyla, possess distinctive biocontrol and plant growth-promoting activities. In general, Actinobacteria and Bacillus endophytes produce aromatic compounds, lipopeptides, plant hormones, polysaccharides, and several enzymes linked to phenylpropanoid metabolism, thus representing high potential for PGP and crop management strategies. Furthermore, Actinobacteria have been shown to produce metabolites with antimicrobial and antitumor activities, useful in agriculture, medicine, and veterinary areas. The great endophytes diversity, their metabolites production, and their adaptation to stress conditions make them a suitable and unlimited source of novel metabolites, whose application could reduce agrochemicals usage in food and drugs production.

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.

Isolation and identification of Ammodendron bifolium endophytic bacteria and the action mechanism of selected isolates-induced seed germination and their effects on host osmotic-stress tolerance

This study aimed to identify Ammodendron bifolium endophytic bacteria, and to evaluate promoting mechanism of selected isolates on seed germination and their effects on host osmotic-stress tolerance. Forty-five strains were isolated from A. bifolium and were classified into 13 different genera by 16S rDNA gene sequence analysis. AY3, AY9 and AG18, which were identified as Staphylococcus, Kocuria, Bacillus sp., promoted host seed ethylene release during germination. Ethrel and 1-aminocyclopropane-1-carboxylic acid (ACC) imitated the effect of AY3, AY9 and AG18 on seed germination. The data suggest that ethylene mediates AY3-, AY9-, AG18-induced A. bifolium seed germination. In addition, osmotic stress prevented seed germination and radicle elongation. However, the inhibitory effect of osmotic stress on seed germination and radicle elongation were rescued by AY3, AY9 and AG18. The results show that AY3, AY9 and AG18 increased osmotic-stress tolerance in A. bifolium. AY3, AY9, AG18 induced A. bifolium seed germination through promoting ethylene production during endophytic bacteria-plant interaction, and increase osmotic-stress tolerance in A. bifolium. AY3, AY9 and AG18 are potential candidates for the protection of A. bifolium.

Effect of Phosphate-Solubilizing Bacteria on the Mobility of Insoluble Cadmium and Metabolic Analysis

Phosphate-solubilizing bacteria (PSB) can promote plant growth by dissolving insoluble phosphate. Therefore, PSB may have the potential to improve the mobility of heavy metals in soils and enhance phytoextraction. This study isolated a few PSB strains that could dissolve CdCO₃ and solid Cd in soil. Two typical PSB, namely, high- and low-Cd-mobilizing PSB (Pseudomonas fluorescens gim-3 and Bacillus cereus qh-35, respectively), were selected to analyze the metabolic profiles, metabolic pathways, and mechanisms of mobilization of insoluble Cd. A total of 34 metabolites secreted by the two PSB strains were identified. Gluconic acid was the main contributor to Cd dissolution (42.4%) in high-Cd-mobilizing PSB. By contrast, gluconic acid was not secreted in low-Cd-mobilizing PSB. Metabolic pathway analysis showed that gluconic acid was produced by the peripheral direct oxidation pathway. Hence, PSB with peripheral direct oxidation pathway were likely to have high-Cd-mobilizing capacity.

Evaluation of the plant-growth-promoting abilities of endophytic bacteria from the psammophyteAmmodendron bifolium

The objective of this study was to assess the plant-growth-promoting abilities of 45 endophytic bacterial isolates from Ammodendron bifolium through physiological characteristics detection and endophytic bacteria–plant interaction. Each of these isolates exhibited 1 or more plant-growth-promoting traits, but only 11 isolates belonging to the genera Bacillus, Staphylococcus, and Kocuria were capable of promoting seed germination and radicle growth. These results together with the results of the correlation analysis revealed that the completion of seed germination may not be due to IAA production, phosphate solubilization, pellicle formation, and ACC deaminase, protease and lipase production by endophytic bacteria, but may be closely related to amylase and cellulase production. Further, endophytic bacterial isolates with plant-growth-promoting traits may also provide beneficial effects to host plants at different growth stages. Thus, these results are of value for understanding the ecological roles of endophytic bacteria in host plant habitats and can serve as a foundation for further studies of their potential in plant regeneration.

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro-organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress-tolerant plants.

What Is There in Seeds? Vertically Transmitted Endophytic Resources for Sustainable Improvement in Plant Growth

Phytobeneficial microbes, particularly endophytes, such as fungi and bacteria, are concomitant partners of plants throughout its developmental stages, including seed germination, root and stem growth, and fruiting. Endophytic microbes have been identified in plants that grow in a wide array of habitats; however, seed-borne endophytic microbes have not been fully explored yet. Seed-borne endophytes are of great interest because of their vertical transmission; their potential to produce various phytohormones, enzymes, antimicrobial compounds, and other secondary metabolites; and improve plant biomass and yield under biotic and abiotic stresses. This review addresses the current knowledge on endophytes, their ability to produce metabolites, and their influence on plant growth and stress mitigation.