Enhanced Iron and Selenium Uptake in Plants by Volatile Emissions of Bacillus amyloliquefaciens (BF06)

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

History of a model plant growth-promoting rhizobacterium, Bacillus velezensis GB03: from isolation to commercialization

Bacillus velezensis strain GB03 is a Gram-positive rhizosphere bacterium known for its ability to promote plant growth and immunity. This review provides a comprehensive overview of the research on GB03 from its initial discovery in Australian wheat fields in 1971 to its current applications. Recognized as a model plant growth-promoting rhizobacterium (PGPR), GB03 has exhibited outstanding performance in enhancing the growth and protection of many crop plants including cucumber, pepper, wheat, barley, soybean, and cotton. Notably, GB03 has been reported to elicit plant immune response, referred to as induced systemic resistance (ISR), against above-ground pathogens and insect pests. Moreover, a pivotal finding in GB03 was the first-ever identification of its bacterial volatile compounds, which are known to boost plant growth and activate ISR. Research conducted over the past five decades has clearly demonstrated the potential of GB03 as an eco-friendly substitute for conventional pesticides and fertilizers. Validating its safety, the U.S. Environmental Protection Agency endorsed GB03 for commercial use as Kodiak® in 1998. Subsequently, other compounds, such as BioYield™, were released as a biological control agent against soil-borne pathogens and as a biofertilizer, utilizing a durable spore formulation. More recently, GB03 has been utilized as a keystone modulator for engineering the rhizosphere microbiome and for eliciting microbe-induced plant volatiles. These extensive studies on GB03 underscore its significant role in sustainable agriculture, positioning it as a safe and environmentally-friendly solution for crop protection.

Biofortification revisited: Addressing the role of beneficial soil microbes for enhancing trace elements concentration in staple crops

Trace element deficiency is a pervasive issue contributing to malnutrition on a global scale. The primary cause of this hidden hunger is related to low dietary intake of essential trace elements, which is highly prevalent in numerous regions across the world. To address deficiency diseases in humans, fortification of staple crops with vital trace elements has emerged as a viable solution. Current methods for fortifying crops encompass chemical amendments, genetic breeding, and transgenic approaches, yet these approaches possess certain limitations, constraining their agricultural application. In contrast, fortifying staple crops through the utilization of soil-beneficial microbes has emerged as a promising and economically feasible approach to enhance trace element content in crops. A specific subset of these beneficial soil microbes, referred to as plant growth-promoting microbes, have demonstrated their ability to influence the interactions between plants, soil, and minerals. These microbes facilitate the transport of essential soil minerals, such as zinc, iron, and selenium, into plants, offering the potential for the development of tailored bioinoculants that can enhance the nutritional quality of cereals, pulses, and vegetable crops. Nevertheless, further research efforts are necessary to comprehensively understand the molecular mechanisms underlying the uptake, transport, and augmentation of trace element concentrations in staple crops. By delving deeper into these mechanisms, customized bioinoculants of soil-beneficial microbes can be developed to serve as highly effective strategies in combating trace element deficiency and promoting global nutritional well-being.

Fungal volatile organic compounds: mechanisms involved in their sensing and dynamic communication with plants

Microbial volatile organic compounds (MVOCs) are mixtures of gas-phase hydrophobic carbon-based molecules produced by microorganisms such as bacteria and fungi. They can act as airborne signals sensed by plants being crucial players in triggering signaling cascades influencing their secondary metabolism, development, and growth. The role of fungal volatile organic compounds (FVOCs) from beneficial or detrimental species to influence the physiology and priming effect of plants has been well studied. However, the plants mechanisms to discern between FVOCs from friend or foe remains significantly understudied. Under this outlook, we present an overview of the VOCs produced by plant-associate fungal species, with a particular focus on the challenges faced in VOCs research: i) understanding how plants could perceive FVOCs, ii) investigating the differential responses of plants to VOCs from beneficial or detrimental fungal strains, and finally, iii) exploring practical aspects related to the collection of VOCs and their eco-friendly application in agriculture.

Microbial volatile compounds (MVCs): an eco-friendly tool to manage abiotic stress in plants

Microbial volatile compounds (MVCs) are produced during the metabolism of microorganisms, are widely distributed in nature, and have significant applications in various fields. To date, several MVCs have been identified. Microbial groups such as bacteria and fungi release many organic and inorganic volatile compounds. They are typically small odorous compounds with low molecular masses, low boiling points, and lipophilic moieties with high vapor pressures. The physicochemical properties of MVCs help them to diffuse more readily in nature and allow dispersal to a more profound distance than other microbial non-volatile metabolites. In natural environments, plants communicate with several microorganisms and respond differently to MVCs. Here, we review the following points: (1) MVCs produced by various microbes including bacteria, fungi, viruses, yeasts, and algae; (2) How MVCs are effective, simple, efficient, and can modulate plant growth and developmental processes; and (3) how MVCs improve photosynthesis and increase plant resistance to various abiotic stressors.

A synthetic community of siderophore-producing bacteria increases soil selenium bioavailability and plant uptake through regulation of the soil microbiome

Dietary selenium (Se) is an effective strategy to meet Se requirement of human body, and Se biofortification in crops in seleniferous soils with selenobacteria represents an eco-friendly biotechnique. In this study, we tested the effectiveness of siderophore-producing bacterial (SPB) synthetic communities (SynComs) in promoting plant Se uptake in a subtropical seleniferous soil where the fixation of Se by ferric-oxides is severe. The results indicated that SPB SynComs drastically elevated soil bioavailable Se content by up to 68.7 %, and significantly increased plant Se concentration and uptake by up to 83.1 % and 92.2 %, respectively. Seven out of ten SPB isolates in the SynComs were enriched in soils after 120 days of inoculation. Additionally, variation partitioning analysis (VPA) revealed that the contribution of soil bacterial community (up to 42.8 %) to the increased plant Se uptake was much greater than that of soil bioavailable Se (up to 5.1 %), suggesting a direct pathway other than the pathway of mobilizing Se. The relative abundances of some operational taxonomic units (OTUs) showed significantly positive relationship with plant Se status but not with soil Se status, which supports the results of VPA. Network analysis indicates that some inoculated SPB isolates promoted plant Se uptake by regulating the native bacterial taxa. Taken together, this study demonstrates that SPB can be used in Se biofortification in crops, especially in subtropical soils.

Bacterial volatile organic compounds as biopesticides, growth promoters and plant-defense elicitors: Current understanding and future scope

Bacteria emit a large number of volatile organic compounds (VOCs) into the environment. VOCs are species-specific and their emission depends on environmental conditions, such as growth medium, pH, temperature, incubation time and interaction with other microorganisms. These VOCs can enhance plant growth, suppress pathogens and act as signaling molecules during plant-microorganism interactions. Some bacterial VOCs have been reported to show strong antimicrobial, nematicidal, pesticidal, plant defense, induced tolerance and plant-growth-promoting activities under controlled conditions. Commonly produced antifungal VOCs include dimethyl trisulfide, dimethyl disulfide, benzothiazole, nonane, decanone and 1-butanol. Species of Bacillus, Pseudomonas, Arthrobacter, Enterobacter and Burkholderia produce plant growth promoting VOCs, such as acetoin and 2,3-butenediol. These VOCs affect expression of genes involved in defense and development in plant species (i.e., Arabidopsis, tobacco, tomato, potato, millet and maize). VOCs are also implicated in altering pathogenesis-related genes, inducing systemic resistance, modulating plant metabolic pathways and acquiring nutrients. However, detailed mechanisms of action of VOCs need to be further explored. This review summarizes the bioactive VOCs produced by diverse bacterial species as an alternative to agrochemicals, their mechanism of action and challenges for employment of bacterial VOCs for sustainable agricultural practices. Future studies on technological improvements for bacterial VOCs application under greenhouse and open field conditions are warranted.

Belowground plant-microbe communications via volatile compounds

Volatile compounds play important roles in rhizosphere biological communications and interactions. The emission of plant and microbial volatiles is a dynamic phenomenon that is affected by several endogenous and exogenous signals. Diffusion of volatiles can be limited by their adsorption, degradation, and dissolution under specific environmental conditions. Therefore, rhizosphere volatiles need to be investigated on a micro and spatiotemporal scale. Plant and microbial volatiles can expand and specialize the rhizobacterial niche not only by improving the root system architecture such that it serves as a nutrient-rich shelter, but also by inhibiting or promoting the growth, chemotaxis, survival, and robustness of neighboring organisms. Root volatiles play an important role in engineering the belowground microbiome by shaping the microbial community structure and recruiting beneficial microbes. Microbial volatiles are appropriate candidates for improving plant growth and health during environmental challenges and climate change. However, some technical and experimental challenges limit the non-destructive monitoring of volatile emissions in the rhizosphere in real-time. In this review, we attempt to clarify the volatile-mediated intra- and inter-kingdom communications in the rhizosphere, and propose improvements in experimental design for future research.

Endophytic bacteria promote biomass production and mercury-bioaccumulation of Bermuda grass and Indian goosegrass

Plant growth-promoting endophytic bacteria can potentially improve the biomass production of Hg-accumulating grasses, resulting in improved Hg extraction from contaminated soils. This study aimed to analyze the effect of inoculation of Hg-resistant endophytic bacteria (i) Jeotgalicoccus huakuii (B1) and (ii) Bacillus amyloliquefaciens (B2), as single and consortium inoculant, on biomass production and Hg bioaccumulation of Bermuda grass (R1) and Indian goosegrass (R2) planted as monoculture and mixed cropping. The grass seeds were surface-sterilized before the inoculation. Both inoculated seeds with B1 and B2 (treatments) and uninoculated seeds were sown separately in sterilized sand. Grass seedlings of both treatments and control were replanted in the washed and sterilized sand medium, spiked with HgCl₂ (100 mg kg^-1). A subset of grass was harvested at 0, 4, 6, and 10 weeks after planting to measure biomass production and Hg bioaccumulation. The results showed that bacterial inoculation enhanced the grass biomass by 52.68% and Hg bioaccumulation by 47.76%. Mercury residue of Hg-spiked sand treated with the bacterial consortium was reduced by 80%. This suggests that endophytic bacteria can improve grass biomass production and enhance Hg bioaccumulation in grass biomass.

Microbes: a potential tool for selenium biofortification

Selenium (Se) is a component of many enzymes and indispensable for human health due to its characteristics of reducing oxidative stress and enhancing immunity. Human beings take Se mainly from Se-containing crops. Taking measures to biofortify crops with Se may lead to improved public health. Se accumulation in plants mainly depends on the content and bioavailability of Se in soil. Beneficial microbes may change the chemical form and bioavailability of Se. This review highlights the potential role of microbes in promoting Se uptake and accumulation in crops and the related mechanisms. The potential approaches of microbial enhancement of Se biofortification can be summarized in the following four aspects: (1) microbes alter soil properties and impact the redox chemistry of Se to improve the bioavailability of Se in soil; (2) beneficial microbes regulate root morphology and stimulate the development of plants through the release of certain secretions, facilitating Se uptake in plants; (3) microbes upregulate the expression of certain genes and proteins that are related to Se metabolism in plants; and (4) the inoculation of microbes give rise to the generation of certain metabolites in plants contributing to Se absorption. Considering the ecological safety and economic feasibility, microbial enhancement is a potential tool for Se biofortification. For further study, the recombination and establishment of synthesis microbes is of potential benefit in Se-enrichment agriculture.

Bacterial volatile compound-based tools for crop management and quality

Bacteria produce a huge diversity of metabolites, many of which mediate ecological relations. Among these, volatile compounds cause broad-range effects at low doses and, therefore, may be exploited for plant defence strategies and agricultural production, but such applications are still in their early development. Here, we review the latest technologies involving the use of bacterial volatile compounds for phytosanitary inspection, biological control, plant growth promotion, and crop quality. We highlight a variety of effects with a potential applicative interest, based on either live biocontrol and/or biostimulant agents, or the isolated metabolites responsible for the interaction with hosts or competitors. Future agricultural technologies may benefit from the development of new analytical tools to understand bacterial interactions with the environment.

C4 Bacterial Volatiles Improve Plant Health

Plant growth-promoting rhizobacteria (PGPR) associated with plant roots can trigger plant growth promotion and induced systemic resistance. Several bacterial determinants including cell-wall components and secreted compounds have been identified to date. Here, we review a group of low-molecular-weight volatile compounds released by PGPR, which improve plant health, mostly by protecting plants against pathogen attack under greenhouse and field conditions. We particularly focus on C4 bacterial volatile compounds (BVCs), such as 2,3-butanediol and acetoin, which have been shown to activate the plant immune response and to promote plant growth at the molecular level as well as in large-scale field applications. We also disc/ uss the potential applications, metabolic engineering, and large-scale fermentation of C4 BVCs. The C4 bacterial volatiles act as airborne signals and therefore represent a new type of biocontrol agent. Further advances in the encapsulation procedure, together with the development of standards and guidelines, will promote the application of C4 volatiles in the field.

Selenium Biofortification: Roles, Mechanisms, Responses and Prospects

The trace element selenium (Se) is a crucial element for many living organisms, including soil microorganisms, plants and animals, including humans. Generally, in Nature Se is taken up in the living cells of microorganisms, plants, animals and humans in several inorganic forms such as selenate, selenite, elemental Se and selenide. These forms are converted to organic forms by biological process, mostly as the two selenoamino acids selenocysteine (SeCys) and selenomethionine (SeMet). The biological systems of plants, animals and humans can fix these amino acids into Se-containing proteins by a modest replacement of methionine with SeMet. While the form SeCys is usually present in the active site of enzymes, which is essential for catalytic activity. Within human cells, organic forms of Se are significant for the accurate functioning of the immune and reproductive systems, the thyroid and the brain, and to enzyme activity within cells. Humans ingest Se through plant and animal foods rich in the element. The concentration of Se in foodstuffs depends on the presence of available forms of Se in soils and its uptake and accumulation by plants and herbivorous animals. Therefore, improving the availability of Se to plants is, therefore, a potential pathway to overcoming human Se deficiencies. Among these prospective pathways, the Se-biofortification of plants has already been established as a pioneering approach for producing Se-enriched agricultural products. To achieve this desirable aim of Se-biofortification, molecular breeding and genetic engineering in combination with novel agronomic and edaphic management approaches should be combined. This current review summarizes the roles, responses, prospects and mechanisms of Se in human nutrition. It also elaborates how biofortification is a plausible approach to resolving Se-deficiency in humans and other animals.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

Applying rhizobacteria consortium for the enhancement of Scirpus grossus growth and phytoaccumulation of Fe and Al in pilot constructed wetlands

Pilot-scale constructed wetlands planted with Scirpus grossus, were used to investigate the effects of applying a three-rhizobacterial consortium (Bacillus cereus strain NII, Bacillus subtilis strain NII and Brevibacterium sp. strain NII) on the growth of S. grossus and also on the accumulation of iron (Fe) and aluminium (Al) in S. grossus. The experiment includes constructed wetlands with the addition of 2% of the consortium rhizobacteria and without the consortium rhizobacteria addition (acting as control). During each sampling day (0, 5, 10, 15, 20, 25, 30, 42, 72 and 102), plant height, concentration of Fe and Al and sand microbial community were investigated. The results for the constructed wetland with the addition of consortium rhizobacteria showed the growth of S. grossus increased significantly at 26% and 29% for plant height and dry weight, respectively. While the accumulation of Fe and Al in S. grossus were enhanced about 48% and 19% respectively. To conclude, the addition of the rhizobacteria consortium has enhanced both the growth of S. grossus and the metal accumulation. These results suggesting that rhizobacteria has good potential to restore Fe and Al contaminated water in general and particularly for mining wastewater.

Selenium Biofortification of Crop Food by Beneficial Microorganisms

Selenium (Se) is essential for human health, however, Se is deficient in soil in many places all around the world, resulting in human diseases, such as notorious Keshan disease and Keshin–Beck disease. Therefore, Se biofortification is a popular approach to improve Se uptake and maintain human health. Beneficial microorganisms, including mycorrhizal and root endophytic fungi, dark septate fungi, and plant growth-promoting rhizobacteria (PGPRs), show multiple functions, especially increased plant nutrition uptake, growth and yield, and resistance to abiotic stresses. Such functions can be used for Se biofortification and increased growth and yield under drought and salt stress. The present review summarizes the use of mycorrhizal fungi and PGPRs in Se biofortification, aiming to improving their practical use.

Understanding the molecular mechanisms for the enhanced phytoremediation of heavy metals through plant growth promoting rhizobacteria: A review

Rapid industrialization, modern agricultural practices and other anthropogenic activities add a significant quantity of toxic heavy metals into the environment, which induces severe toxic effects on all form of living organisms, alter the soil properties and its biological activity. Remediation of heavy metal contaminated sites has become an urgent necessity. Among the existing strategies, phytoremediation is an eco-friendly and much convincing tool for the remediation of heavy metals. However, the applicability of phytoremediation in contaminated sites is restricted by two prime factors such as i) slow growth rate at higher metal contaminated sites and ii) metal bioavailability. This circumstance could be minimized and accelerate the phytoremediation efficiency by incorporating the potential plant growth promoting rhizobacterial (PGPR) as a combined approach. PGPR inoculation might improve the plant growth through the production of plant growth promoting substances and improve the heavy metal remediation efficiency by the secretion of chelating agents, acidification and redox changes. Moreover, rhizobacterial inoculation consolidates the metal tolerance and uptake by regulating the expression of various metal transporters, tolerant and metal chelator genes. However, the exact underlying molecular mechanism of PGPR mediated plant growth promotion and phytoremediation of heavy metals is poorly understood. Thus, the present review provides clear information about the molecular mechanisms excreted by PGPR strains in plant growth promotion and phytoremediation of heavy metals.

A review on global metal accumulators-mechanism, enhancement, commercial application, and research trend

The biosphere is polluted with metals due to burning of fossil fuels, pesticides, fertilizers, and mining. The metals interfere with soil conservations such as contaminating aqueous waste streams and groundwater, and the evidence of this has been recorded since 1900. Heavy metals also impact human health; therefore, the emancipation of the environment from these environmental pollutants is critical. Traditionally, techniques to remove these metals include soil washing, removal, and excavation. Metal-accumulating plants could be utilized to remove these metal pollutants which would be an alternative option that would simultaneously benefit commercially and at the same time clean the environment from these pollutants. Commercial application of pollutant metals includes biofortification, phytomining, phytoremediation, and intercropping. This review discusses about the metal-accumulating plants, mechanism of metal accumulation, enhancement of metal accumulation, potential commercial applications, research trends, and research progress to enhance the metal accumulation, benefits, and limitations of metal accumulators. The review identified that the metal accumulator plants only survive in low or medium polluted environments with heavy metals. Also, more research is required about metal accumulators in terms of genetics, breeding potential, agronomics, and the disease spectrum. Moreover, metal accumulators' ability to uptake metals need to be optimized by enhancing metal transportation, transformation, tolerance to toxicity, and volatilization in the plant. This review would benefit the industries and environment management authorities as it provides up-to-date research information about the metal accumulators, limitation of the technology, and what could be done to improve the metal enhancement in the future.

Irrigation Water Quality—A Contemporary Perspective

In the race to enhance agricultural productivity, irrigation will become more dependent on poorly characterized and virtually unmonitored sources of water. Increased use of irrigation water has led to impaired water and soil quality in many areas. Historically, soil salinization and reduced crop productivity have been the primary focus of irrigation water quality. Recently, there is increasing evidence for the occurrence of geogenic contaminants in water. The appearance of trace elements and an increase in the use of wastewater has highlighted the vulnerability and complexities of the composition of irrigation water and its role in ensuring proper crop growth, and long-term food quality. Analytical capabilities of measuring vanishingly small concentrations of biologically-active organic contaminants, including steroid hormones, plasticizers, pharmaceuticals, and personal care products, in a variety of irrigation water sources provide the means to evaluate uptake and occurrence in crops but do not resolve questions related to food safety or human health effects. Natural and synthetic nanoparticles are now known to occur in many water sources, potentially altering plant growth and food standard. The rapidly changing quality of irrigation water urgently needs closer attention to understand and predict long-term effects on soils and food crops in an increasingly fresh-water stressed world.

Induced Systemic Resistance (ISR) and Fe Deficiency Responses in Dicot Plants

Plants develop responses to abiotic stresses, like Fe deficiency. Similarly, plants also develop responses to cope with biotic stresses provoked by biological agents, like pathogens and insects. Some of these responses are limited to the infested damaged organ, but other responses systemically spread far from the infested organ and affect the whole plant. These latter responses include the Systemic Acquired Resistance (SAR) and the Induced Systemic Resistance (ISR). SAR is induced by pathogens and insects while ISR is mediated by beneficial microbes living in the rhizosphere, like bacteria and fungi. These root-associated mutualistic microbes, besides impacting on plant nutrition and growth, can further boost plant defenses, rendering the entire plant more resistant to pathogens and pests. In the last years, it has been found that ISR-eliciting microbes can induce both physiological and morphological responses to Fe deficiency in dicot plants. These results suggest that the regulation of both ISR and Fe deficiency responses overlap, at least partially. Indeed, several hormones and signaling molecules, like ethylene (ET), auxin, and nitric oxide (NO), and the transcription factor MYB72, emerged as key regulators of both processes. This convergence between ISR and Fe deficiency responses opens the way to the use of ISR-eliciting microbes as Fe biofertilizers as well as biopesticides. This review summarizes the progress in the understanding of the molecular overlap in the regulation of ISR and Fe deficiency responses in dicot plants. Root-associated mutualistic microbes, rhizobacteria and rhizofungi species, known for their ability to induce morphological and/or physiological responses to Fe deficiency in dicot plant species are also reviewed herein.

Revisiting bacterial volatile-mediated plant growth promotion: lessons from the past and objectives for the future

BACKGROUND

Bacterial volatile compounds (BVCs) are important mediators of beneficial plant-bacteria interactions. BVCs promote above-ground plant growth by stimulating photosynthesis and sugar accumulation and by modulating phytohormone signalling. These compounds also improve below-ground mineral uptake and modify root system architecture.

SCOPE

We review advances in our understanding of the mode of action and practical applications of BVCs since the discovery of BVC-mediated plant growth promotion in 2003. We also discuss unanswered questions about the identity of plant receptors, the effectiveness of combination of two or more BVCs on plant growth, and the potential side effects of these compounds for human and animal health.

CONCLUSION

BVCs have good potential for use as biostimulants and protectants to improve plant health. Further advances in the development of suitable technologies and preparing standards and guidelines will help in the application of BVCs in crop protection and health.

Sniffing bacterial volatile compounds for healthier plants

Bacterial volatile compounds (BVCs) are not waste or by-products of primary metabolism but rather have critical roles in the biology and ecological competence of bacteria. BVCs are exploited as a source of nutrients and information in plant-bacteria interactions. They target key points in plant physiology, activating downstream metabolic pathways by a domino effect. BVCs are an ancient signal and are involved in plant-bacteria communication, which was shaped during evolutionary history and established before the development of higher plants. This type of communication is not exclusive to mutualistic interactions, because pathogens also use volatiles to alter plant physiology. Here, fragmented information is drawn together to provide a clearer view of how BVCs affect such interactions.

Involvement of abscisic acid in microbe-induced saline-alkaline resistance in plants

Soil salinity-alkalinity is one of abiotic stresses that lead to plant growth inhibition and yield loss. It has recently been indicated that plant growth promoting rhizobacteria (PGPR) can enhance the capacity of plants to counteract negative effects caused by adverse environments. However, whether PGPR confers increased saline-alkaline resistance of plants and the underlying mechanisms remain unclear. We thus investigated the effects of Bacillus licheniformis (strain SA03) on Chrysanthemum plants grown under saline-alkaline conditions. Soil inoculation with SA03 significantly mitigated saline-alkaline stress in plants with augmented photosynthesis, biomass and survival rates. Moreover, the inoculated plants accumulated more Fe and less Na⁺ content than the non-inoculated plants under the stress. However, the inoculation with SA03 failed to trigger a series of saline-alkaline stress responses in abscisic acid (ABA)- and nitric oxide (NO)-deficient plants. Furthermore, NO acted as a secondary messenger of ABA to regulate the stress responses and tolerance in Chrysanthemum plants. Therefore, these findings indicated that B. licheniformis SA03 could be employed to improve saline-alkaline tolerance of plants by mediating cellular ABA levels.

Bacillus licheniformis SA03 Confers Increased Saline-Alkaline Tolerance in Chrysanthemum Plants by Induction of Abscisic Acid Accumulation

Soil saline-alkalization is a major abiotic stress that leads to low iron (Fe) availability and high toxicity of sodium ions (Na+) for plants. It has recently been shown that plant growth promoting rhizobacteria (PGPR) can enhance the ability of plants to tolerate multiple abiotic stresses such as drought, salinity, and nutrient deficiency. However, the possible involvement of PGPR in improving saline-alkaline tolerance of plants and the underlying mechanisms remain largely unknown. In this study, we investigated the effects of Bacillus licheniformis (strain SA03) on the growth of Chrysanthemum plants under saline-alkaline conditions. Our results revealed that inoculation with SA03 alleviated saline-alkaline stress in plants with increased survival rates, photosynthesis and biomass. The inoculated plants accumulated more Fe and lower Na+ concentrations under saline-alkaline stress compared with the non-inoculated plants. RNA-Sequencing analyses further revealed that SA03 significantly activated abiotic stress- and Fe acquisition-related pathways in the stress-treated plants. However, SA03 failed to increase saline-alkaline tolerance in plants when cellular abscisic acid (ABA) and nitric oxide (NO) synthesis were inhibited by treatment with fluridone (FLU) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), respectively. Importantly, we also found that NO acted downstream of SA03-induced ABA to activate a series of adaptive responses in host plants under saline-alkaline stress. These findings demonstrated the potential roles of B. licheniformis SA03 in enhancing saline-alkaline tolerance of plants and highlighted the intricate integration of microbial signaling in regulating cellular Fe and Na+ accumulation.

Bacillus amyloliquefaciens SAY09 Increases Cadmium Resistance in Plants by Activation of Auxin-Mediated Signaling Pathways

Without physical contact with plants, certain plant growth-promoting rhizobacteria (PGPR) can release volatile organic compounds (VOCs) to regulate nutrient acquisition and induce systemic immunity in plants. However, whether the PGPR-emitted VOCs can induce cadmium (Cd) tolerance of plants and the underlying mechanisms remain elusive. In this study, we probed the effects of Bacillus amyloliquefaciens (strain SAY09)-emitted VOCs on the growth of Arabidopsis plants under Cd stress. SAY09 exposure alleviates Cd toxicity in plants with increased auxin biosynthesis. RNA-Seq analyses revealed that SAY09 exposure provoked iron (Fe) uptake- and cell wall-associated pathways in the Cd-treated plants. However, SAY09 exposure failed to increase Cd resistance of plants after treatment with 1-naphthylphthalamic acid (NPA) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO). Under Cd stress, SAY09 exposure markedly promoted Fe absorption in plants with the increased hemicellulose 1 (HC1) content and Cd deposition in root cell wall, whereas these effects were almost abrogated by treatment with NPA or c-PTIO. Moreover, exogenous NPA remarkably repressed the accumulation of nitric oxide (NO) in the SAY09-exposed roots under Cd stress. Taken together, the findings indicated that NO acted as downstream signals of SAY09-induced auxin to regulate Fe acquisition and augment Cd fixation in roots, thereby ameliorating Cd toxicity.