Effect of endophytic Bacillus cereus ERBP inoculation into non-native host: Potentials and challenges for airborne formaldehyde removal

Removal of formaldehyde from indoor air by potted Sansevieria trifasciata plants: dynamic influence of physiological traits on the process

Plant-based removal of indoor formaldehyde is a widely studied method, yet little is known about the dynamic changes in this process. In this study, potted Sansevieria trifasciata Prain plants were exposed to 5-ppm formaldehyde gas concentration for 7 days. The results showed that formaldehyde exposure led to plant stress, affected photosynthesis, and damaged membrane lipids, as evidenced by a decrease in chlorophyll content, an increase in Chl a/b ratio and malondialdehyde content. However, the formaldehyde removal ability of the plants increased over the first 5 days, peaking at 18.02 mg h-1 kg-1 dry weight on the 5th day. This trend was correlated with changes in various indicators in the plant roots, including phytohormone and antioxidant enzymes. Notably, catalase activity in the roots behaved differently from other indicators. The indicators in the leaves showed turning points around the 3rd day due to the direct exposure of the leaves to formaldehyde. The relative abundance of endophytes indicated an increase in plant growth-promoting bacteria, which helped the plant cope with formaldehyde stress. The study suggests that under formaldehyde stress, plants manage active oxygen content by increasing phytohormones and regulating redox reactions. This enhances their tolerances to formaldehyde, thereby improving their ability to remove formaldehyde and aiding recovery after formaldehyde exposure.

Endophyte-assisted non-host plant Tillandsia brachycaulos enhance indoor formaldehyde removal

This study investigated the use of endophyte-assisted Tillandsia brachycaulos to enhance formaldehyde removal in indoor environments. A formaldehyde-degrading endophyte from the root of Epipremnum aureum, Pseudomonas plecoglossicida, was identified and used for inoculation. Among the inoculation methods, spraying proved to be the most effective, resulting in a significant 35 % increase in formaldehyde removal after 36 hours. The results of the light exposure experiment (3000 Lux) demonstrate that an increase in light intensity reduces the efficiency of the Tillandsia brachycaulos-microbial system in degrading formaldehyde. In a 15-day formaldehyde fumigation experiment at 2 ppm in a normal indoor environment, the inoculated Tillandsia brachycaulos exhibited removal efficiency ranging from 42.53 % to 66.13 %, while the uninoculated declined from 31.62 % to 3.17 %. The Pseudomonas plecoglossicida (referred to as PP-1) became the predominant bacteria within the Tillandsia brachycaulos after fumigation. Moreover, the endophytic inoculation effectively increased the resistance and tolerance of Tillandsia brachycaulos to formaldehyde, as evidenced by lower levels of hydroxyl radical, malondialdehyde (MDA), free protein, and peroxidase activity (POD), as well as higher chlorophyll content compared to uninoculated Tillandsia brachycaulos. These findings indicate that the combination of endophytic bacteria and Tillandsia brachycaulos has significant potential for improving indoor air quality.

Application of Acinetobacter indicus to promote cigarette smoke particulate matter phytoremediation: removal efficiency and plant-microbe interactions

Particulate matter (PM) is one of the most hazardous atmospheric pollutants. Several plant species show high potential to reduce air pollutants and are widely used as green belts to provide clean outdoor spaces for human well-being. However, high PM concentrations cause physiological changes and stress in plants. In this study, 11 species of Thai native perennial plants were exposed to PM generated from tobacco smoke. Wrightia religiosa (Teijsm. & Binn.) Benth. ex Kurz, Bauhinia purpurea DC. ex Walp. and Tectona grandis L.f. reduced PM effectively (which is in the typical range of 43.95 to 52.97%) compared to other plant species. In addition, the responses of perennial plants under PM stress at the proteomic level were also evaluated. Proteomic analysis of these three plant species showed that plants respond negatively to high PM concentrations, such as reducing several photosynthetic-related proteins and increasing plant stress response proteins. To improve PM phytoremediation efficiency and reduce plant stress from PM, perennial plant-microbe interactions were investigated. W. religiosa was inoculated with Acinetobacter indicus PS1, and high biosurfactant-producing strains clearly showed a higher PM removal efficiency than non-inoculated plants (9.48, 9.5 and 12.6% for PM1.0, PM2.5 and PM10, respectively). Inoculating W. religiosa with A. indicus PS1 maintained chlorophyll a and b concentrations. Moreover, the malondialdehyde (MDA) concentration of W. religiosa inoculated with A. indicus PS1 was lower than that of non-inoculated W. religiosa. The leaf wax content (µg/cm2) and biosurfactant (µg/cm2) of W. religiosa inoculated with A. indicus PS1 were also higher than those of non-inoculated W. religiosa. This study clearly showed that inoculating plants with A. indicus PS1 can help plants remediate PM and improve their PM stress response.

Fuelling phytoremediation: gasoline degradation by green wall systems-a case study

The capacity for indoor plants including green wall systems to remove specific volatile organic compounds (VOCs) is well documented in the literature; however under realistic settings, indoor occupants are exposed to a complex mixture of harmful compounds sourced from various emission sources. Gasoline vapour is one of the key sources of these emissions, with several studies demonstrating that indoor occupants in areas surrounding gasoline stations or with residentially attached garages are exposed to far higher concentrations of harmful VOCs. Here we assess the potential of a commercial small passive green wall system, commercially named the 'LivePicture Go' from Ambius P/L, Australia, to drawdown VOCs that comprise gasoline vapour, including total VOC (TVOC) removal and specific removal of individual speciated VOCs over time. An 8-h TVOC removal efficiency of 42.45% was achieved, along with the complete removal of eicosane, 1,2,3-trimethyl-benzene, and hexadecane. Further, the green wall also effectively reduced concentrations of a range of harmful benzene derivatives and other VOCs. These results demonstrate the potential of botanical systems to simultaneously remove a wide variety of VOCs, although future research is needed to improve upon and ensure efficiency of these systems over time and within practical applications.

Heavy metal-resistant rhizobacteria fosters to alleviate the cadmium toxicity in Arabidopsis by upregulating the plant physiological responses

This study was designed to investigate the role of Morganella morganii strains in alleviating Cd stress in Arabidopsis seedlings under controlled conditions. Both M. morganii strains ABT3 (ON316873) and ABT9 (ON316874) strains isolated from salt-affected areas showed higher resistance against Cd and possess plant growth-promoting traits such as nitrogen fixation, indole-acetic acid production, ammonia production, phosphate solubilization, and, catalase, gelatinase and protease enzyme production. Plant inoculation assay showed that varying concentration of Cd (1.5 mM and 2.5 mM) significantly reduced Arabidopsis growth, quantum yield (56.70%-66.49%), and chlorophyll content (31.90%-42.70%). Cd toxicity also triggered different associations between lipid peroxidation (43.61%-69.77%) and enzymatic antioxidant mechanisms. However, when both strains were applied to the Arabidopsis seedlings, the shoot and root length and fresh and dry weights were improved in the control and Cd-stressed plants. Moreover, both strains enhanced the resistance against Cd stress by increasing antioxidant enzyme activities [catalase (19.47%-27.39%) and peroxidase (37.50%-48.07%)]that ultimately cause a substantial reduction in lipid peroxidation (27.71%-41.90%). Both strains particularly ABT3 also showed positive results in improving quantum yield (73.84%-98.64%) and chlorophyll content (41.13%-48.63%), thus increasing the growth of Arabidopsis seedlings. The study suggests that PGPR can protect plants from Cd toxicity, and Cd-tolerant rhizobacterial strains can remediate heavy metal polluted sites and improve plant growth.

Dynamic mechanisms of cadmium accumulation and detoxification by Lolium perenne grown in soil inoculated with the cadmium-tolerant bacterium strain Cdq4-2

Cadmium (Cd) contamination is a serious threat to food security and human health. The cost-effective in situ method of remediating Cd-contaminated soil uses Cd-tolerant microorganisms and Cd-enriching plants. The present study investigated the dynamic effects of inoculating soil with a Cd-tolerant bacteria strain Cdq4-2 (Enterococcus sp.) on the physiological and biochemical properties of perennial ryegrass Lolium perenne. The combined effects of remediating Cd-contaminated soil with this plant and these bacteria were also studied. An experiment was used to compare three treatments of L. perenne crops: 1) CK (control soil without Cd), 2) C (20 mg/kg Cd-contaminated soil), and 3) CB (20 mg/kg Cd-contaminated soil inoculated with bacteria Cdq4-2). The results show that compared with treatment C, the aboveground biomass, underground biomass, and total biomass of CB were 46.83-69.31%, 131.76-462.79%, and 62.65-101.53% greater, respectively. The superoxide dismutase activity of CB was 17.62-54.63% lower, while its peroxidase activity was 67.49-146.51% higher. The malondialdehyde concentration in CB was 30.40-40.24% more significant, the ascorbic acid concentration was 6.20-188.22% higher, and its glutathione concentration was 16.25-63.63% lower. The Cd concentrations of aboveground parts of a plant in treatment CB were 18.55% and 30.53% higher than those of C at days 20 and 40, respectively, while that of underground parts was 24.25% higher on day 40. The bioconcentration factors of aboveground and underground parts were higher in treatment CB on day 40. The inoculation of Cd-contaminated soils with bacteria Cdq4-2 promoted growth in L. perenne, improved its antioxidant ability, and promoted the absorption, translocation, and accumulation of Cd. Hence, it improved the effectiveness of L. perenne in remediating Cd-contaminated soils.

Endophytes in Agriculture: Potential to Improve Yields and Tolerances of Agricultural Crops

Endophytic fungi and bacteria live asymptomatically within plant tissues. In recent decades, research on endophytes has revealed that their significant role in promoting plants as endophytes has been shown to enhance nutrient uptake, stress tolerance, and disease resistance in the host plants, resulting in improved crop yields. Evidence shows that endophytes can provide improved tolerances to salinity, moisture, and drought conditions, highlighting the capacity to farm them in marginal land with the use of endophyte-based strategies. Furthermore, endophytes offer a sustainable alternative to traditional agricultural practices, reducing the need for synthetic fertilizers and pesticides, and in turn reducing the risks associated with chemical treatments. In this review, we summarise the current knowledge on endophytes in agriculture, highlighting their potential as a sustainable solution for improving crop productivity and general plant health. This review outlines key nutrient, environmental, and biotic stressors, providing examples of endophytes mitigating the effects of stress. We also discuss the challenges associated with the use of endophytes in agriculture and the need for further research to fully realise their potential.

A systematic review on mitigation of common indoor air pollutants using plant-based methods: a phytoremediation approach

Environmental pollution, especially indoor air pollution, has become a global issue and affects nearly all domains of life. Being both natural and anthropogenic substances, indoor air pollutants lead to the deterioration of the ecosystem and have a negative impact on human health. Cost-effective plant-based approaches can help to improve indoor air quality (IAQ), regulate temperature, and protect humans from potential health risks. Thus, in this review, we have highlighted the common indoor air pollutants and their mitigation through plant-based approaches. Potted plants, green walls, and their combination with bio-filtration are such emerging approaches that can efficiently purify the indoor air. Moreover, we have discussed the pathways or mechanisms of phytoremediation, which involve the aerial parts of the plants (phyllosphere), growth media, and roots along with their associated microorganisms (rhizosphere). In conclusion, plants and their associated microbial communities can be key solutions for reducing indoor air pollution. However, there is a dire need to explore advanced omics technologies to get in-depth knowledge of the molecular mechanisms associated with plant-based reduction of indoor air pollutants.

Phytoremediation for the indoor environment: a state-of-the-art review

Poor indoor air quality has become of particular concern within the built environment due to the time people spend indoors, and the associated health burden. Volatile organic compounds (VOCs) off-gassing from synthetic materials, nitrogen dioxide and harmful outdoor VOCs such benzene, toluene, ethyl-benzene and xylene penetrate into the indoor environment through ventilation and are the main contributors to poor indoor air quality with health effects. A considerable body of literature over the last four decades has demonstrate the removal of gaseous contaminants through phytoremediation, a technology that relies on plant material and technologies to remediate contaminated air streams. In this review we present a state-of-the-art on indoor phytoremediation over the last decade. Here we present a review of 38 research articles on both active and passive phytoremediation, and describe the specific chemical removal efficiency of different systems. The literature clearly indicates the efficacy of these systems for the removal of gaseous contaminants in the indoor environment, however it is evident that the application of phytoremediation technologies for research purposes in-situ is currently significantly under studied. In addition, it is common for research studies to assess the removal of single chemical species under controlled conditions, with little relevancy to real-world settings easily concluded. The authors therefore recommend that future phytoremediation research be conducted both in-situ and on chemical sources of a mixed nature, such as those experienced in the urban environment like petroleum vapour, vehicle emissions, and mixed synthetic furnishings off-gassing. The assessment of these systems both in static chambers for their theoretical performance, and in-situ for these mixed chemical sources is essential for the progression of this research field and the widespread adoption of this technology.

Phytoremediation of indoor formaldehyde by plants and plant material

Formaldehyde evolves from various household items and is of environmental and public health concern. Removal of this contaminant from the indoor air is of utmost importance and currently, various practices are in the field. Among these practices, indoor plants are of particular importance because they help in controlling indoor temperature, moisture, and oxygen concentration. Plants and plant materials studied for the purpose have been reviewed hereunder. The main topics of the review are, mechanism of phytoremediation, plants and their benefits, plant material in formaldehyde remediation, and airtight environmental and health issues. Future research in the field is also highlighted which will help new researches to plan for the remediation of formaldehyde in indoor air. The remediation capacity of several plants has been tabulated and compared, which gives easy access to assess various plants for remediation of the target pollutant. Challenges and issues in the phytoremediation of formaldehyde are also discussed.Novelty statement: Phytoremediation is a well-known technique to mitigate various organic and inorganic pollutants. The technique has been used by various researchers for maintaining indoor air quality but its efficiency under real-world conditions and human activities is still a question and is vastly affected relative to laboratory conditions. Several modifications in the field are in progress, here in this review article we have summarized and highlighted new directions in the field which could be a better solution to the problem in the future.

Bacillus subtilis Impact on Plant Growth, Soil Health and Environment: Dr. Jekyll and Mr. Hyde

The increased dependence of farmers on chemical fertilizers poses a risk to soil fertility and ecosystem stability. Plant growth-promoting rhizobacteria (PGPR) are at the forefront of sustainable agriculture, providing multiple benefits for the enhancement of crop production and soil health. Bacillus subtilis is a common PGPR in soil that plays a key role in conferring biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation, and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Although it harbors several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota. In some instances, it affects the beneficial microbes of the rhizosphere. External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH, and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity. The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics, and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota. Bacillus subtilis has more positive attributes similar to the character of Dr. Jekyll and some negative attributes on plant growth, soil health, and the environment akin to the character of Mr. Hyde.

Particulate matter and volatile organic compound phytoremediation by perennial plants: Affecting factors and plant stress response

Air pollution by particulate matter (PM) and volatile organic compounds (VOCs) is a major global issue. Many technologies have been developed to address this problem. Phytoremediation is one possible technology to remediate these air pollutants, and a few studies have investigated the application of this technology to reduce PM and VOCs in a mixture of pollutants. This study aimed to screen plant species capable of PM and VOC phytoremediation and identify plant physiology factors to be used as criteria for plant selection for PM and VOC phytoremediation. Wrightia religiosa removed PM and VOCs. In addition, the relative water content in the plant and ethanol soluble wax showed positive relationships with PM and VOC phytoremediation, with a high correlation coefficient. For plant stress responses, several plant species maintained and/or increased the relative water content after short-term exposure to PM and VOCs. In addition, based on proteomic analysis, most of the proteins in W. religiosa leaves related to photosystems I and II were significantly reduced by PM_2.5. When a high water content was achieved in W. religiosa (80% soil humidity), W. religiosa can effectively remove PM. The results suggested that PM can reduce plant photosynthesis. In addition, plants might require a high water supply to maintain their health under PM and VOC stress.

Phyllosphere microbiome: Diversity and functions

Phyllosphere or aerial surface of plants represents the globally largest and peculiar microbial habitat that inhabits diverse and rich communities of bacteria, fungi, viruses, cyanobacteria, actinobacteria, nematodes, and protozoans. These hyperdiverse microbial communities are related to the host's specific functional traits and influence the host's physiology and the ecosystem's functioning. In the last few years, significant advances have been made in unravelling several aspects of phyllosphere microbiology, including diversity and microbial community composition, dynamics, and functional interactions. This review highlights the current knowledge about the assembly, structure, and composition of phyllosphere microbial communities across spatio-temporal scales, besides functional significance of different microbial communities to the plant host and the surrounding environment. The knowledge will help develop strategies for modelling and manipulating these highly beneficial microbial consortia for furthering scientific inquiry into their interactions with the host plants and also for their useful and economic utilization.

Identification and Characterization of Arbutus unedo L. Endophytic Bacteria Isolated from Wild and Cultivated Trees for the Biological Control of Phytophthora cinnamomi

Arbutus unedo L. is a resilient tree with a circum-Mediterranean distribution. Besides its ecological relevance, it is vital for local economies as a fruit tree. Several microorganisms are responsible for strawberry tree diseases, leading to production constrictions. Thus, the development of alternative plant protection strategies is necessary, such as bacterial endophytes, which may increase their host’s overall fitness and productivity. As agricultural practices are a driving factor of microbiota, this paper aimed to isolate, identify and characterize endophytic bacteria from strawberry tree leaves from plants growing spontaneously in a natural environment as well as from plants growing in orchards. A total of 62 endophytes were isolated from leaves and identified as Bacillus, Paenibacillus, Pseudomonas, Sphingomonas and Staphylococcus. Although a slightly higher number of species was found in wild plants, no differences in terms of diversity indexes were found. Sixteen isolates were tested in vitro for their antagonistic effect against A. unedo mycopathogens. B. cereus was the most effective antagonist causing a growth reduction of 20% in Glomerella cingulata and 40% in Phytophthora cinnamomi and Mycosphaerella aurantia. Several endophytic isolates also exhibited plant growth-promoting potential. This study provides insights into the diversity of endophytic bacteria in A. unedo leaves and their potential role as growth promoters and pathogen antagonists.

Diversity and community structure of endophytic Bacillus with antagonistic and antioxidant activity in the fruits of Xisha Wild Noni (Morinda citrifolia L.)

Noni (Morinda citrifolia L.) is a tropical crop with strong antibacterial, antioxidant and other abilities, and its fruit has a strong potential for adjuvant treatment of diseases. This study aimed to explore the dynamic change of endophytic bacteria in Noni fruit at different stages and the correlation between the antagonistic and antioxidant activity of the Bacillus which was screened and the change of the host's growth stage. In this study, though the high-throughput sequencing technology (HTS), 106 endophytic bacteria species were found in A, B, C, D, E and F stages of Noni fruit, among which the dominant group were Pantoea (0.3%-20.9%), and Candidatus_Uzinura (2.3%-35.2%) etc. The endophytic bacteria were isolated by culture-dependent method. Through their antagonistic experiments on Staphylococcus aureus and Escherichia coli, the results of 16S polyphasic taxonomic identification showed that the 34 antagonistic strains belonged to Bacillus. Five species of these Bacillus were identified by gyrA polyphase taxonomy, including Bacillus subtilis (76% of all Bacillus), Bacillus licheniformis (9%), Bacillus amyloliquefaciens (6%), Bacillus velezensis (6%) and Bacillus mojavensi (3%), and the RAPD showed these Bacillus are no signs of stable passage. In C, D, E and F stages, the average total antioxidant activity of Bacillus endophytic antagonists against Noni was 7.812 U/mL, 8.144 U/mL, 7.817 U/mL and 7.144 U/mL, which was much higher than that of Noni fruit, and antioxidant activity of Noni juice and Bacillus bacterial liquid vary with host's growth period showed the same trend, both rose slowly at first, and reached the highest in period E, then declined slightly in period F, it showed that the antagonistic Bacillus of Noni had synergistic function with Noni fruit. This study clarified the relationship of function between Noni fruit and endophytic bacteria, and laid a foundation for future study on the dynamic change of endophytic flora succession and efficacy.

A review on phytoremediation of contaminants in air, water and soil

There is a global need to use plants to restore the ecological environment. There is no systematic review of phytoremediation mechanisms and the parameters for environmental pollution. Here, we review this situation and describe the purification rate of different plants for different pollutants, as well as methods to improve the purification rate of plants. This is needed to promote the use of plants to restore the ecosystems and the environment. We found that plants mainly use their own metabolism including the interaction with microorganisms to repair their ecological environment. In the process of remediation, the purification factors of plants are affected by many conditions such as light intensity, stomatal conductance, temperature and microbial species. In addition the efficiency of phytoremediation is depending on the plants species-specific metabolism including air absorption and photosynthesis, diversity of soil microorganisms and heavy metal uptake. Although the use of nanomaterials and compost promote the restoration of plants to the environment, a high dose may have negative impacts on the plants. In order to improve the practicability of the phytoremediation on environmental restoration, further research is needed to study the effects of different kinds of catalysts on the efficiency of phytoremediation. Thus, the present review provides a recent update for development and applications of phytoremediation in different environments including air, water, and soil.

Mitigation of indoor air pollutants using Areca palm potted plants in real-life settings

Deterioration of indoor air quality (IAQ) has become a serious concern as people spend lots of time indoors and prolonged pollution exposure can result in adverse health outcomes. Indoor plants can phytoremediate a wide variety of indoor contaminants. Nonetheless, few experiments have demonstrated their efficacy in real-time environments. Therefore, the present study aims to experimentally assess the efficiency of Areca palm potted plants in phytoremediation of primary indoor air pollutant viz. total volatile organic compounds (TVOCs), carbon dioxide (CO₂), and carbon monoxide (CO) levels from real-world indoor spaces, for the first time. Four discrete naturally ventilated experimental sites (I-IV) situated at the Council of Scientific and Industrial Research- Institute of Himalayan Bioresource Technology (CSIR-IHBT) were used. For over a period of 4 months, the sites were monitored using zero plants as a control (1-4 week), three plants (5-8 week), six plants (9-12 week), and nine plants (13-16 week), respectively. Present results indicate that Areca palm potted plants can effectively reduce the TVOCs, CO₂, and CO levels by 88.16% in site IV, 52.33% and 95.70% in site III, respectively. The current study concluded that Areca palm potted plants offer an efficient, cost-effective, self-regulating, sustainable solution for improving indoor air quality and thereby human well-being and productivity in closed and confined spaces.

Expression of SidD gene and physiological characterization of the rhizosphere plant growth-promoting yeasts

There is increasing evidence that rhizosphere microbes contribute to the stress mitigation process, but the mechanisms of this plant-microbe interaction are not yet understood. Siderophores-producing microorganisms have been considered important for enhancing metal tolerance in plants. In this study, rhizosphere yeasts were isolated from wheat (Triticum aestivum L.) and examined for siderophores production and heavy metal resistance. Out of thirty-five isolates, only eight yeast strains showed heavy metal-resistance and plant-growth promotion properties. The highest inorganic phosphate-solubilization was shown by Trichosporon ovoides IFM 63839 (2.98 mg ml⁻¹) and Saccharomyces cerevisiae FI25-1F (2.54 mg ml⁻¹). Two strains, namely YEAST-6 and YEAST-16 showed high siderophore production and heavy metal-resistance, were investigated for sidD gene expression under different levels of Cd²⁺ and Pb²⁺ toxicity stress. The heavy metal-resistant yeast strains were characterized and identified based on the phenotypic characteristics and their 18S rRNA genes sequence. SidD gene expression was induced by yeasts growing under iron-limiting conditions and excess of other heavy metal, suggesting that expression of sidD gene increases in the presence of 600–800 μM heavy metal but under iron limitation. Extensive studies of the microbe-plant micronutrient interactions will enrich our understanding of the rhizosphere role in the terms of plant growth promotion.

Biotechnology progress for removal of indoor gaseous formaldehyde

Formaldehyde is a ubiquitous carcinogenic indoor pollutant. The treatment of formaldehyde has attracted increasing social attention. Over the past few decades, an increasing number of publications have reported approaches for removing indoor formaldehyde. These potential strategies include physical adsorption, chemical catalysis, and biodegradation. Although physical adsorption is widely used, it does not really remove pollution. Chemical catalysis is very efficient but adds the risk of introducing secondary pollutants. Biological removal strategies have attracted more research attention than the first two methods, because it is more efficient, clean, and economical. Plants and bacteria are the common organisms used in formaldehyde removal. However, both have limitations and shortcomings when used alone. This review discusses the mechanisms, applications, and improvements of existing biological methods for the removal of indoor gaseous formaldehyde. A combination strategy relying on plants, bacteria, and physical adsorbents exhibits best ability to remove formaldehyde efficiently, economically, and safely. When this combination system is integrated with a heating, ventilation, air conditioning, and cooling (HVAC) system, a practical combined system can be established in formaldehyde removal. Multivariate interactions of biological and non-biological factors are needed for the future development of indoor formaldehyde removal. KEY POINTS: • Indoor gaseous formaldehyde removal is necessary especially for new residence. • Biological removal strategies have attracted increasing research attentions. • Combined system of plants, bacteria, and physical adsorbents exhibits best efficiency. • Integrated device of biological and non-biological factors will be potential practical.

Bisphenol A removal from a plastic industry wastewater by Dracaena sanderiana endophytic bacteria and Bacillus cereus NI

Understanding the significance of plant-endophytic bacteria for bisphenol A (BPA) removal is of importance for any application of organic pollutant phytoremediation. In this research, Dracaena sanderiana with endophytic Pantoea dispersa showed higher BPA removal than uninoculated plants at 89.54 ± 0.88% and 79.08 ± 1.20%, respectively. Quantitative Real-Time PCR (qPCR) showed that P. dispersa increased from 3.93 × 10⁷ to 8.80 × 10⁷ 16S rRNA gene copy number in root tissues from day 0 to day 5 which indicated that it could assist the plant in removing BPA during the treatment period. pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD), total dissolved solids (TDS), conductivity, and salinity were reduced after 5 days of the experimental period. Particularly, BOD significantly decreased due to activities of the plants and microorganisms. Furthermore, an indigenous bacterial strain, Bacillus cereus NI, from the wastewater could remove BPA in high TDS and alkalinity condition of the wastewater. This work suggests that D. sanderiana plants could be used as a tertiary process in a wastewater treatment system and should be combined with its endophytic bacteria. In addition, B. cereus NI could also be applied for BPA removal from wastewaters with high TDS and salinity.

Contribution of Bacillus cereus ERBP in ozone detoxification by Zamioculcas zamiifolia plants: Effect of ascorbate peroxidase, catalase and total flavonoid contents for ozone detoxification

Eighteen plant species were screened for ozone (O₃) removal in a continuous system. Zamioculcas zamiifolia had the highest O₃ removal efficiency. To enhance O₃ removal by Z. zamiifolia, adding a compatible endophytic bacteria, Bacillus cereus ERBP into Z. zamiifolia was studied. After operating under an O₃ continuous system (150-250 ppb) at a flow rate of 0.3 L min^-1 for 80 h, inoculated plants (74%) exhibited higher O₃ removal efficiency than non-inoculated ones (53%). In addition, after O₃ exposure (80 h), the population of B. cereus ERBP in inoculated plants was significantly increased in both shoots approximately 35 folds and leaves 13 folds compared to inoculated plants without O₃ exposure. The results also showed that B. cereus ERBP had the ability to protect Z. zamiifolia against O₃ stress conditions. The increase in B. cereus ERBP populations was attributed to the significant increase in ascorbate peroxidase (APX) and catalase (CAT) activity. In addition, increasing B. cereus ERBP populations led to raise total flavonoid contents which is one of antioxidant compounds. Increasing APX, CAT activities, and total flavonoid contents can enhance O₃ detoxification in plant tissues. The mechanism of B. cereus ERBP for enhancing O₃ phytoremediation was proposed in this study. The results suggested that B. cereus ERBP was a potential tool for alleviating O₃ stress on Z. zamiifolia and enhancing O₃ phytoremediation efficiency.

Metal resistant PGPR lowered Cd uptake and expression of metal transporter genes with improved growth and photosynthetic pigments in Lycopersicon esculentum under metal toxicity

Plant growth promoting rhizobacteria (PGPRs) are very effective in immobilization of heavy metals and reducing their translocation in plants via precipitation, complex formation and adsorption. The present study was therefore designed to understand the role of Pseudomonas aeruginosa and Burkholderia gladioli in mitigation of Cd stress (0.4 mM) in 10-days old L. esculentum seedlings. The present work investigated growth characteristics, photosynthetic pigments, metal tolerance index, metal uptake and the contents of metal chelating compounds (protein bound and non-protein bound thiols, total thiols) in microbes inoculated Cd treated L. esculentum seedlings. The gene expression profiling of different metal transporters was conducted in order to investigate the quantitative analysis. Our results revealed Cd generated toxicity in seedlings in terms of reduced growth (root length, shoot length and fresh weight) and photosynthetic pigments (chlorophyll, carotenoid and xanthophyll) which enhanced upon inoculations of P. aeruginosa and B. gladioli. Further, the metal uptake along with levels of protein and non-protein bound thiols was also enhanced in Cd-treated seedlings. Gene expression studies suggested enhanced expression in the metal transporter genes which were further declined in the microbe supplemented seedlings. Therefore, micro-organisms possess growth promoting traits that enable them to reduce metal toxicity in plants.

Metal resistant PGPR lowered Cd uptake and expression of metal transporter genes with improved growth and photosynthetic pigments in Lycopersicon esculentum under metal toxicity

Plant growth promoting rhizobacteria (PGPRs) are very effective in immobilization of heavy metals and reducing their translocation in plants via precipitation, complex formation and adsorption. The present study was therefore designed to understand the role of Pseudomonas aeruginosa and Burkholderia gladioli in mitigation of Cd stress (0.4 mM) in 10-days old L. esculentum seedlings. The present work investigated growth characteristics, photosynthetic pigments, metal tolerance index, metal uptake and the contents of metal chelating compounds (protein bound and non-protein bound thiols, total thiols) in microbes inoculated Cd treated L. esculentum seedlings. The gene expression profiling of different metal transporters was conducted in order to investigate the quantitative analysis. Our results revealed Cd generated toxicity in seedlings in terms of reduced growth (root length, shoot length and fresh weight) and photosynthetic pigments (chlorophyll, carotenoid and xanthophyll) which enhanced upon inoculations of P. aeruginosa and B. gladioli. Further, the metal uptake along with levels of protein and non-protein bound thiols was also enhanced in Cd-treated seedlings. Gene expression studies suggested enhanced expression in the metal transporter genes which were further declined in the microbe supplemented seedlings. Therefore, micro-organisms possess growth promoting traits that enable them to reduce metal toxicity in plants.

Inoculated Clitoria ternatea with Bacillus cereus ERBP for enhancing gaseous ethylbenzene phytoremediation: Plant metabolites and expression of ethylbenzene degradation genes

Air pollutants especially polyaromatic hydrocarbons pose countless threats to the environment. This issue demands for an effective phytoremediation technology. In this study we report the beneficial interactions of Clitoria ternatea and its plant growth promoting endophytic bacteria Bacillus cereus ERBP by inoculating it for the remediation of 5 ppm airborne ethylbenzene (EB). The percentage efficiency for ethylbenzene removal among B. cereus ERBP inoculated and non-inoculated sterile and natural C. ternatea has also been determined. The inoculation of B. cereus ERBP has significantly increased EB removal efficiency of both sterile and natural C. ternatea. The inoculated natural C. ternatea seedlings showed 100% removal efficiency within 84 h for the aforementioned pollutant compared with the sterile inoculated C. ternatea seedlings (108 h). The degradation of EB by C. ternatea seedlings with and without B. cereus ERBP was assessed by measuring the intermediates of EB including 1-phenylethanol, acetophenon, benzaldehyde and benzoic acid. In addition, cytochrome P450s monooxygenase (CYP83D1) and dehydrogenases (LOC100783159) involved in the oxidation of hydrocarbons are well reported for their bio catalytic activities under xenobiotic stress conditions. Hence, the co-effect of the native endophyte B. cereus ERBP inoculation and EB exposure on the expression level of CYP83D1 and dehydrogenase were also determined. The targeted genes CYP83D1and dehydrogenases have shown an increased expression level under the 5 ppm of EB exposure enabling C. ternatea to withstand and remediate the pollutant.

Towards practical indoor air phytoremediation: A review

Indoor air quality has become a growing concern due to the increasing proportion of time people spend indoors, combined with reduced building ventilation rates resulting from an increasing awareness of building energy use. It has been well established that potted-plants can help to phytoremediate a diverse range of indoor air pollutants. In particular, a substantial body of literature has demonstrated the ability of the potted-plant system to remove volatile organic compounds (VOCs) from indoor air. These findings have largely originated from laboratory scale chamber experiments, with several studies drawing different conclusions regarding the primary VOC removal mechanism, and removal efficiencies. Advancements in indoor air phytoremediation technology, notably active botanical biofilters, can more effectively reduce the concentrations of multiple indoor air pollutants through the action of active airflow through a plant growing medium, along with vertically aligned plants which achieve a high leaf area density per unit of floor space. Despite variable system designs, systems available have clear potential to assist or replace existing mechanical ventilation systems for indoor air pollutant removal. Further research is needed to develop, test and confirm their effectiveness and safety before they can be functionally integrated in the broader built environment. The current article reviews the current state of active air phytoremediation technology, discusses the available botanical biofiltration systems, and identifies areas in need of development.

Effect of microorganisms on reducing cadmium uptake and toxicity in rice (Oryza sativa L.)

This study analyzed the application of three microorganism inoculums, including Bacillus subtilis, Bacillus cereus, and commercial effective microorganism (EM) solution in order to determine cadmium (Cd) reduction in rice (Oryza sativa L.) and rice growth promotion. Rice was grown in Cd-contaminated soil (120 mg/kg) and selected microorganisms were inoculated. Cd concentration and rice weight were measured at 45 and 120 days of the experiment. The result showed that B. subtilis inoculation into rice can highly reduce Cd accumulation in every part of rice roots and shoots (45 days), and grains (120 days). This species can effectively absorb Cd compared to other inoculums, which might be the main mechanism to reduce Cd transportation in rice plants. Interestingly, plants that were inoculated with bacterial species individually harbored higher calcium (Ca) and magnesium (Mg) accumulation; B. subtilis-inoculated plants had the highest levels of Ca and Mg compared to other inoculated ones. Moreover, inoculating rice plants with these microorganisms could increase the dry weight of the plant and protect them from Cd stress because all the inoculums presented the ability to solubilize phosphate, produce indole-3-acetic acid (IAA), and control ethylene levels by 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. After 120 days, quantification of each inoculum by quantitative polymerase chain reaction (qPCR) confirmed the root colonization of bacterial species, with B. subtilis showing higher 16S rRNA gene copy numbers than the other species. B. subtilis was classified as a non-human pathogenic strain, reassuring the safe application of this plant growth-promoting bacterium as a crop inoculum.

Chlorophytum comosum-bacteria interactions for airborne benzene remediation: Effect of native endophytic Enterobacter sp. EN2 inoculation and blue-red LED light

This study was performed to determine the effect of plant-endophytic Enterobacter sp. EN2 interactions and blue-red LED light conditions on gaseous benzene removal by plants. It was found that under consecutive benzene fumigation for three cycles (18 days), inoculation of the strain EN2 into sterilized and non-sterilized native C. comosum resulted in significantly increased gaseous benzene removal compared to that in non-inoculated groups under the same light conditions (P < 0.05). Remarkably, EN2 colonization in inoculated plants under LED conditions was higher than under fluorescence conditions as the EN2 could grow better under LED conditions. Strain EN2 possesses NADPH that is used to facilitate benzene degradation and modulate plant growth under benzene stress by bacterial IAA production and ACC deaminase activity; higher IAA and lower ethylene levels were found in inoculated plants compared to non-inoculated ones. These contributed to better benzene removal efficiency. Interestingly, under fumigation for 16 cycles (67 days), there was no difference in gaseous benzene removal between inoculated plants and non-inoculated plants under the same light conditions at initial benzene concentrations of 5 ppm. This is probably due to EN2 reaching maximum growth under all treatments. However, C. comosum exhibited better benzene removal under LED conditions than under fluorescence conditions during 16 cycles, possibly due to better photosynthetic performance and plant growth, leading to more NADPH, and eventually enhanced benzene removal efficiency. Hence, the most efficient acceleration of benzene removal was provided by inoculation of strain EN2 onto C. comosum under blue-red LED light conditions.

Dracaena sanderiana endophytic bacteria interactions: Effect of endophyte inoculation on bisphenol A removal

Bisphenol A (BPA) is one of the most abundant endocrine-disrupting compounds which is found in the aquatic environment. However, actual knowledge regarding the effect of plant-bacteria interactions on enhancing BPA removal is still lacking. In the present study, Dracaena sanderiana endophytic bacteria interactions were investigated to evaluate the effect of bacterial inoculation on BPA removal under hydroponic conditions. Two plant growth-promoting (PGP) bacterial strains, Bacillus thuringiensis and Pantoea dispersa, which have high BPA tolerance and can utilize BPA for growth, were used as plant inocula. P. dispersa-inoculated plants showed the highest BPA removal efficiency at 92.32 ± 1.23% compared to other inoculated and non-inoculated plants. This was due to a higher population of the endophytic inoculum within the plant tissues which resulted in maintained levels of indole-3-acetic acid (IAA) for the plant's physiological needs and lower levels of reactive oxygen species (ROS). In contrast, B. thuringiensis-inoculated plants had a lower BPA removal efficiency. However, individual B. thuringiensis possessed a significantly higher BPA removal efficiency compared to P. dispersa. This study provides convincing evidence that not all PGP endophytic bacteria-plant interactions could improve the BPA removal efficiency. Different inocula and inoculation times should be investigated before using plant inoculation to enhance phytoremediation.

Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes

Air pollution is air contaminated by anthropogenic or naturally occurring substances in high concentrations for a prolonged time, resulting in adverse effects on human comfort and health as well as on ecosystems. Major air pollutants include particulate matters (PMs), ground-level ozone (O3), sulfur dioxide (SO2), nitrogen dioxides (NO2), and volatile organic compounds (VOCs). During the last three decades, air has become increasingly polluted in countries like China and India due to rapid economic growth accompanied by increased energy consumption. Various policies, regulations, and technologies have been brought together for remediation of air pollution, but the air still remains polluted. In this review, we direct attention to bioremediation of air pollutants by exploiting the potentials of plant leaves and leaf-associated microbes. The aerial surfaces of plants, particularly leaves, are estimated to sum up to 4 × 108 km2 on the earth and are also home for up to 1026 bacterial cells. Plant leaves are able to adsorb or absorb air pollutants, and habituated microbes on leaf surface and in leaves (endophytes) are reported to be able to biodegrade or transform pollutants into less or nontoxic molecules, but their potentials for air remediation has been largely unexplored. With advances in omics technologies, molecular mechanisms underlying plant leaves and leaf associated microbes in reduction of air pollutants will be deeply examined, which will provide theoretical bases for developing leaf-based remediation technologies or phylloremediation for mitigating pollutants in the air.

Impact of endophytic colonization patterns on Zamioculcas zamiifolia stress response and in regulating ROS, tryptophan and IAA levels under airborne formaldehyde and formaldehyde-contaminated soil conditions

Deeper understanding of plant-endophyte interactions under abiotic stress would provide new insights into phytoprotection and phytoremediation enhancement. Many studies have investigated the positive role of plant-endophyte interactions in providing protection to the plant against pollutant stress through auxin (indole-3-acetic acid (IAA)) production. However, little is known about the impact of endophytic colonization patterns on plant stress response in relation to reactive oxygen species (ROS) and IAA levels. Moreover, the possible effect of pollutant phase on plant stress response is poorly understood. Here, we elucidated the impact of endophytic colonization patterns on plant stress response under airborne formaldehyde compared to formaldehyde-contaminated soil. ROS, tryptophan and IAA levels in the roots and shoots of endophyte-inoculated and non-inoculated plants in the presence and absence of formaldehyde were measured. Strain-specific quantitative polymerase chain reaction (qPCR) was used to investigate dynamics of endophyte colonization. Under the initial exposure to airborne formaldehyde, non-inoculated plants accumulated more tryptophan in the shoots (compared to the roots) to synthesize IAA. However, endophyte-inoculated plants behaved differently as they synthesized and accumulated more tryptophan in the roots and, hence, higher levels of IAA accumulation and exudation within roots which might act as a signaling molecule to selectively recruit B. cereus ERBP. Under continuous airborne formaldehyde stress, higher levels of ROS accumulation in the shoots pushed the plant to synthesize more tryptophan and IAA in the shoots (compared to the roots). Higher levels of IAA in the shoots might act as the potent driving force to relocalize B. cereus ERBP from roots to the shoots. In contrast, under formaldehyde-contaminated soil, B. cereus ERBP colonized root tissues without moving to the shoots since there was a sharp increase in ROS, tryptophan and IAA levels of the roots without any significant increase in the shoots. Pollutant phase affected endophytic colonization patterns and plant stress responses differently.

Trimethylamine removal by plant capsule of Sansevieria kirkii in combination with Bacillus cereus EN1

Trimethylamine (TMA) contamination produces a strong "fishy" odor and can cause pathological changes in humans. By screening native microorganisms from Sansevieria kirkii exposed to 100 ppm TMA, it was shown that endophytic bacteria number 1 (EN1) and number 2 (EN2) have a higher TMA tolerance and removal capacity than other bacteria species in a closed system. In addition, EN1 and EN2 demonstrated the ability to produce high quantities of indole-3-acetic acid (IAA) and use 1-aminocyclopropane-1-carboxylic acid (ACC), which is found normally in plant growth-promoting bacteria (PGPB). Moreover, 16S ribosomal DNA (rDNA) sequences of EN1 and EN2 identification showed that EN1 and EN2 was the same bacteria species, Bacillus cereus. B. cereus EN1 was chosen to apply with S. kirkii to remove TMA in a plant capsule, which was compared to control conditions. It was found that 500 g of soil with S. kirkii inoculated with B. cereus EN1 had a higher TMA removal efficiency than other conditions. Moreover, the flow rate of TMA-contaminated gas was varied (0.03-1 L min^-1) to calculate the loading rate and elimination capacity. The maximum loading rate of 500 g soil with B. cereus EN1-inoculated S. kirkii was 2500 mg m^-3 h^-1, while other conditions showed only around 250-750 mg m^-3 h^-1. Therefore, a plant capsule with B. cereus EN1-inoculated S. kirkii had the potential to be applied in TMA-contaminated air.

Efficient phytoremediation of organic contaminants in soils using plant-endophyte partnerships

Soil pollution with organic contaminants is one of the most intractable environmental problems today, posing serious threats to humans and the environment. Innovative strategies for remediating organic-contaminated soils are critically needed. Phytoremediation, based on the synergistic actions of plants and their associated microorganisms, has been recognized as a powerful in situ approach to soil remediation. Suitable combinations of plants and their associated endophytes can improve plant growth and enhance the biodegradation of organic contaminants in the rhizosphere and/or endosphere, dramatically expediting the removal of organic pollutants from soils. However, for phytoremediation to become a more widely accepted and predictable alternative, a thorough understanding of plant-endophyte interactions is needed. Many studies have recently been conducted on the mechanisms of endophyte-assisted phytoremediation of organic contaminants in soils. In this review, we highlight the superiority of organic pollutant-degrading endophytes for practical applications in phytoremediation, summarize alternative strategies for improving phytoremediation, discuss the fundamental mechanisms of endophyte-assisted phytoremediation, and present updated information regarding the advances, challenges, and new directions in the field of endophyte-assisted phytoremediation technology.

Euphorbia milii-native bacteria interactions under airborne formaldehyde stress: Effect of epiphyte and endophyte inoculation in relation to IAA, ethylene and ROS levels

Better understanding of plant-bacteria interactions under stress is of the prime importance for enhancing airborne pollutant phytoremediation. No studies have investigated plant-epiphyte interactions compared to plant-endophyte interactions under airborne formaldehyde stress in terms of plant Indole-3-acetic acid (IAA), ethylene, reactive oxygen species (ROS) levels and pollutant removal efficiency. Euphorbia milii was inoculated with native plant growth-promoting (PGP) endophytic and epiphytic isolates individually to investigate plant-endophyte compared to plant-epiphyte interactions under continuous formaldehyde fumigation. Under airborne formaldehyde stress, endophyte interacts with its host plant closely and provides higher levels of IAA which protected the plant against formaldehyde phytotoxicity by lowering intracellular ROS, ethylene levels and maintaining shoot epiphytic community; hence, higher pollutant removal. However, plant-epiphyte interactions could not provide enough IAA to confer protection against formaldehyde stress; thus, increased ROS and ethylene levels, large decrease in shoot epiphytic population and lower pollutant removal although epiphyte contacts with airborne pollutant directly (has greater access to gaseous formaldehyde). Endophyte-inoculated plant synthesized more tryptophan as a signaling molecule for its associated bacteria to produce IAA compared to the epiphyte-inoculated one. Under stress, PGP endophyte interacts with its host closely; thus, better protection against stress and higher pollutant removal compared to epiphyte which has limited interactions with the host plant; hence, lower pollutant removal.

Euphorbia milii-Endophytic Bacteria Interactions Affect Hormonal Levels of the Native Host Differently Under Various Airborne Pollutants

This study was conducted to assess the effect of plant-native endophytic bacteria interactions on indole-3-acetic acid (IAA), ethylene levels, and hormonal balance of Euphorbia milii under different airborne pollutants. IAA levels and airborne formaldehyde removal by E. milii enhanced when inoculated with endophytic isolates. However, one isolate, designated as root endophyte 4, with the highest levels of IAA production individually, declined gaseous formaldehyde removal of plant, since it disturbed hormonal balance of E. milii, leading to IAA levels higher than physiological concentrations, which stimulated ethylene biosynthesis and stomatal closure under light conditions. However, plant-root endophyte 4 interactions favored airborne benzene removal, since benzene was more phytotoxic and the plant needed more IAA to protect against benzene phytotoxicity. As trimethylamine (TMA) was not toxic, it did not affect plant-endophyte interactions. Therefore, IAA levels of root endophyte 4-inoculated E. milii was not significantly different from a noninoculated one. Under mixed-pollutant stress (formaldehyde, benzene, TMA), root endophyte 4-inoculated E. milii removed benzene at the lowest rate, since benzene was the most phytotoxic pollutant with the greatest molecular mass. However, TMA (with greater molecular mass) was removed faster than formaldehyde due to higher phytotoxicity of formaldehyde. Plant-endophyte interactions were affected differently under various airborne pollutants.