Air-borne xylene degradation by Bougainvillea buttiana and the role of epiphytic bacteria in the degradation

Mining and expression analysis of color related genes in Bougainvillea glabra bracts based on transcriptome sequencing

Bract coloration is one of the key ornamental traits in Bougainvillea, yet research has predominantly focused on phenotypic color traits and pigment composition, with limited understanding of the molecular mechanisms underlying color formation. This gap hinders the improvement and innovation in bract coloration. To elucidate the regulatory mechanisms of bract coloration in Bougainvillea and to enhance the utilization of its germplasm resources, this study employed the Illumina Novaseq 6000 sequencing platform to conduct transcriptomic sequencing on 21 samples of bracts exhibiting seven distinct phenotypes. Comparative analysis against Nr, Pfam, EggNOG, GO, and KEGG databases annotated 90,279 unigenes. The highest annotation rates were achieved with the Nr (40.13%), GO (30.44%), and EggNOG (25.64%) databases. Among the species annotated, Beta vulgaris (20.08%) and Chenopodium quinoa (14.58%) shared the highest homology with Bougainvillea bract transcriptomes. WGCNA analysis identified 12 positively correlated tissue-specific modules, of which 2 are related to bract color formation. By comparing transcriptome data and genes within these specific modules against the KEGG database, a total of 321 unigenes associated with bract color formation in Bougainvillea were discovered. Among these, 220 unigenes are involved in anthocyanin synthesis, 43 unigenes are involved in betalain synthesis, 23 unigenes are annotated as Chlorophyll a-b binding protein genes, and 35 unigenes participate in carotenoid synthesis. Quantitative real-time PCR (qRT-PCR) validation of 16 differentially expressed genes (DEGs) including PAL2, CHS1, ANS, BZ1, 6GT, CDOPA5GT, ANR, CHS2, and DOPA, revealed significant expression differences among magenta, yellow, white, and cherry-colored bracts, suggesting their potential as candidate genes for bract color development. This study not only enriches the transcriptomic data of Bougainvillea but also identifies genes associated with bract coloration, providing a valuable theoretical basis for future gene cloning, genetic engineering, and breeding efforts in Bougainvillea.

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.

Assessing VOC emissions from different gas stations: impacts, variations, and modeling fluctuations of air pollutants

Gas stations distributed around densely populated areas are responsible for toxic pollutant emissions such as volatile organic compounds (VOCs). This study aims to measure VOCs emission from three different kinds of gas stations to determine the extent of pollution from the gas stations and the most frequent type of VOC compound emitted. The concentrations of ambient VOCs at three refueling stations with a different type of fuels in Mashhad were monitored. The result of this study showed that CNG fuel stations are less polluting than petrol stations. In all the studied sites, the highest concentrations were related to xylene isomers, irrespective of the fuel type. Total VOCs at the supply of both compressed natural gas (CNG) and gasoline stations was 482.36 ± 563.45 µg m-3. At a CNG station and a gasoline station, total VOC concentrations were 1363.4 ± 1975 µg m-3 and 410.29 ± 483.37 µg m-3, respectively. The differences in concentrations of toluene and m,p-xylene between the fuel stations can be related to the quality and type of fuel, vapor recovery technology, fuel reserves, dripless nozzles, traffic density in these stations, meteorological conditions and the location of sampling sites. The combination of a sine function and a quadratic function could model the fluctuation behavior of air pollutants like m,p-xylene. In all the sites, the highest concentrations were related to xylene isomers, irrespective of the type of fuel. The changing rate of m,p-xylene pollutant in each station was also modeled in this study.

A comprehensive review and perspective research in technology integration for the treatment of gaseous volatile organic compounds

Volatile organic compounds (VOCs) are harmful pollutants emitted from industrial processes. They pose a risk to human health and ecosystems, even at low concentrations. Controlling VOCs is crucial for good air quality. This review aims to provide a comprehensive understanding of the various methods used for controlling VOC abatement. The advancement of mono-functional treatment techniques, including recovery such as absorption, adsorption, condensation, and membrane separation, and destruction-based methods such as natural degradation methods, advanced oxidation processes, and reduction methods were discussed. Among these methods, advanced oxidation processes are considered the most effective for removing toxic VOCs, despite some drawbacks such as costly chemicals, rigorous reaction conditions, and the formation of secondary chemicals. Standalone technologies are generally not sufficient and do not perform satisfactorily for the removal of hazardous air pollutants due to the generation of innocuous end products. However, every integration technique complements superiority and overcomes the challenges of standalone technologies. For instance, by using catalytic oxidation, catalytic ozonation, non-thermal plasma, and photocatalysis pretreatments, the amount of bioaerosols released from the bioreactor can be significantly reduced, leading to effective conversion rates for non-polar compounds, and opening new perspectives towards promising techniques with countless benefits. Interestingly, the three-stage processes have shown efficient decomposition performance for polar VOCs, excellent recoverability for nonpolar VOCs, and promising potential applications in atmospheric purification. Furthermore, the review also reports on the evolution of mathematical and artificial neural network modeling for VOC removal performance. The article critically analyzes the synergistic effects and advantages of integration. The authors hope that this article will be helpful in deciding on the appropriate strategy for controlling interested VOCs.

Engineering green wall botanical biofiltration to abate indoor volatile organic compounds: A review on mechanisms, phyllosphere bioaugmentation, and modeling

Indoor air pollution affects the global population, especially in developed countries where people spend around 90% of their time indoors. The recent pandemic exacerbated the exposure by relying on indoor spaces and a teleworking lifestyle. VOCs are a group of indoor air pollutants with harmful effects on human health at low concentrations. It is widespread that plants can remove indoor VOCs. To this day, research has combined principles of phytoremediation, biofiltration, and bioremediation into a holistic and sustainable technology called botanical biofiltration. Overall, it is sustained that its main advantage is the capacity to break down and biodegrade pollutants using low energy input. This differs from traditional systems that transfer VOCs to another phase. Furthermore, it offers additional benefits like decreased indoor air health costs, enhanced work productivity, and well-being. However, many disparities exist within the field regarding the role of plants, substrate, and phyllosphere bacteria. Yet their role has been theorized; its stability is poorly known for an engineering approach. Previous research has not addressed the bioaugmentation of the phyllosphere to increase the performance, which could boost the system. Moreover, most experiments have studied passive potted plant systems at a lab scale using small chambers, making it difficult to extrapolate findings into tangible parameters to engineer the technology. Active systems are believed to be more efficient yet require more maintenance and knowledge expertize; besides, the impact of the active flow on the long term is not fully understood. Besides, modeling the system has been oversimplified, limiting the understanding and optimization. This review sheds light on the field's gains and gaps, like concepts, experiments, and modeling. We believe that embracing a multidisciplinary approach encompassing experiments, multiphysics modeling, microbial community analysis, and coworking with the indoor air sector will enable the optimization of the technology and facilitate its adoption.

Applied microbiology of the phyllosphere

The phyllosphere, or plant leaf surface, represents a microbial ecosystem of considerable size, holding extraordinary biodiversity and enormous potential for the discovery of new products, tools, and applications in biotechnology, agriculture, medicine, and elsewhere. This mini-review highlights the applied microbiology of the phyllosphere as an original field of study concerning itself with the genes, gene products, natural compounds, and traits that underlie phyllosphere-specific adaptations and services that have commercial and economic value for current or future innovation. Examples include plant-growth-promoting and disease-suppressive phyllobacteria, probiotics and fermented foods that support human health, as well as microbials that remedy foliar contamination with airborne pollutants, residual pesticides, or plastics. Phyllosphere microbes promote plant biomass conversion into compost, renewable energy, animal feed, or fiber. They produce foodstuffs such as thickening agents and sugar substitutes, industrial-grade biosurfactants, novel antibiotics and cancer drugs, as well as enzymes used as food additives or freezing agents. Furthermore, new developments in DNA sequence-based profiling of leaf-associated microbial communities allow for surveillance approaches in the context of food safety and security, for example, to detect enteric human pathogens on leafy greens, predict plant disease outbreaks, and intercept plant pathogens and pests on internationally traded goods. KEY POINTS: • Applied phyllosphere microbiology concerns leaf-specific adaptations for economic value • Phyllobioprospecting searches the phyllosphere microbiome for product development • Phyllobiomonitoring tracks phyllosphere microbial profiles for early risk detection.

Screening of xylene degrading bacteria and optimization of their degradation characteristics in heavily polluted areas

Aiming at the problems of high xylene concentration and difficult removal in heavily polluted areas, high-efficient degrading bacteria of volatile organic compounds (VOCs) xylene in heavily polluted areas were selected and screened from sewage sludge, and their degradation characteristics were studied. The response surface methodology (RSM) optimized the optimal degradation conditions. The results showed that the screened degrading strain was identified as Klebsiella by the 16SrDNA technology and named H-16. During the start-up phase of the reactor, the removal rate of xylene by strain H-16 fluctuated, and it was stable above 71.3% for 150 min. At 40°C, the degradation rate is the highest, reaching 63.25%. With an increasing inoculum amount of strain H-16, the degradation rate of xylene gradually increased, and the degradation rate could reach 86.1% when the inoculation amount was 25%. A neutral environment was more conducive to the degradation and removal of xylene. Through the analysis of the model and RSM, the optimal conditions for the degradation of xylene by H-16 were obtained: 38.89°C, pH 6.94 and 18.07%. GC-MS results showed that the possible degradation pathway of xylene began with demethylation, formation of pentene diacid by benzene ring cleavage, and finally oxidation to generate CO2 and H2O.

Effect of airflow pattern and distance on removal of particulate matters and volatile organic compounds from cigarette smoke using Sansevieria trifasciata botanical biofilter

Botanical biofilters can effectively remove indoor air pollution. However, to apply botanical biofilters in situ, the distance of botanical biofilter to the pollutants and airflow pattern can be important factors impacting efficiency. This study examined the removal efficiency of particulate matters (PMs) and volatile organic compounds (VOCs) from cigarette smoke, such as formaldehyde and acetone, at various distances (100 cm, 175 cm, 240 cm, and 315 cm) using a Sansevieria trifasciata botanical biofilter. The botanical biofilter was placed inside a testing room (24 m³) and exposed to cigarette smoke. The pollutants removal efficiency was evaluated for six cycles (24 h/cycle) and one cycle as a recovery period where botanical biofilter was placed under normal conditions for 30 days. Results showed that the botanical biofilter could remove 140-250 μg m^-3, 147-257 μg m^-3, 212-455 μg m^-3 for PM₁, PM_2.5, and PM₁₀, respectively, at 8 h. Total VOCs, formaldehyde, and acetone removal were 40%-65%, 46%-69%, and 31%-61% at 24 h. PMs and VOCs removal efficiency can be affected by both distance and pattern of airflow in the testing room. The highest PM₁ and PM_2.5 elimination appeared at 240 cm and 315 cm, while VOCs removal was high at 100 cm. Botanical biofilter creates airflow vortices around 100 cm, indicating low removal of PMs. This is the first study that demonstrated the effect of airflow patterns on different pollutants removal efficiency.

Transcriptome analyses shed light on floral organ morphogenesis and bract color formation in Bougainvillea

BACKGROUND

Bougainvillea is a popular ornamental plant with brilliant color and long flowering periods. It is widely distributed in the tropics and subtropics. The primary ornamental part of the plant is its colorful and unusual bracts, rich in the stable pigment betalain. The developmental mechanism of the bracts is not clear, and the pathway of betalain biosynthesis is well characterized in Bougainvillea.

RESULTS

At the whole-genome level, we found 23,469 protein-coding genes by assembling the RNA-Seq and Iso-Seq data of floral and leaf tissues. Genome evolution analysis revealed that Bougainvillea is related to spinach; the two diverged approximately 52.7 million years ago (MYA). Transcriptome analysis of floral organs revealed that flower development of Bougainvillea was regulated by the ABCE flower development genes; A-class, B-class, and E-class genes exhibited high expression levels in bracts. Eight key genes of the betalain biosynthetic pathway were identified by homologous alignment, all of which were upregulated concurrently with bract development and betalain accumulation during the bract initiation stage of development. We found 47 genes specifically expressed in stamens, including seven highly expressed genes belonging to the pentose and glucuronate interconversion pathways. BgSEP2b, BgSWEET11, and BgRD22 are hub genes and interacted with many transcription factors and genes in the carpel co-expression network.

CONCLUSIONS

We assembled protein-coding genes of Bougainvilea, identified the floral development genes, and constructed the gene co-expression network of petal, stamens, and carpel. Our results provide fundamental information about the mechanism of flower development and pigment accumulation in Bougainvillea, and will facilitate breeding of cultivars with high ornamental value.

Phylloplane Biodiversity and Activity in the City at Different Distances from the Traffic Pollution Source

The phylloplane is an integrated part of green infrastructure which interacts with plant health. Taxonomic characterization of the phylloplane with the aim to link it to ecosystem functioning under anthropogenic pressure is not sufficient because only active microorganisms drive biochemical processes. Activity of the phylloplane remains largely overlooked. We aimed to study the interactions among the biological characteristics of the phylloplane: taxonomic diversity, functional diversity and activity, and the pollution grade. Leaves of Betula pendula were sampled in Moscow at increasing distances from the road. For determination of phylloplane activity and functional diversity, a MicroResp tool was utilized. Taxonomic diversity of the phylloplane was assessed with a combination of microorganism cultivation and molecular techniques. Increase of anthropogenic load resulted in higher microbial respiration and lower DNA amount, which could be viewed as relative inefficiency of phylloplane functioning in comparison to less contaminated areas. Taxonomic diversity declined with road vicinity, similar to the functional diversity pattern. The content of Zn in leaf dust better explained the variation in phylloplane activity and the amount of DNA. Functional diversity was linked to variation in nutrient content. The fraction of pathogenic fungi of the phylloplane was not correlated with any of the studied elements, while it was significantly high at the roadsides. The bacterial classes Gammaproteobacteria and Cytophagia, as well as the Dothideomycetes class of fungi, are exposed to the maximal effect of distance from the highway. This study demonstrated the sensitivity of the phylloplane to road vicinity, which combines the effects of contaminants (mainly Zn according to this study) and potential stressful air microclimatic conditions (e.g., low relative air humidity, high temperature, and UV level). Microbial activity and taxonomic diversity of the phylloplane could be considered as an additional tool for bioindication.

Construction of integrative transcriptome to boost systematic exploration of Bougainvillea

Members of the genus Bougainvillea are rich sources of natural dyes, pigments, and traditional medicines. They are also commonly used as ornamentals in roadside landscape construction. However, the horticultural development of Bougainvillea flowers with extended growth periods and coloration is not always feasible. One reason is limited molecular knowledge and no genomic information for Bougainvillea. Here, we compiled an integrative transcriptome of all expressed transcripts for Bougainvillea × buttiana Miss Manila by integrating 20 Illumina-sequencing RNA transcriptomes. The integrative transcriptome consisted of 97,623 distinct transcripts. Of these, 47,006 were protein-coding, 31,109 were non-coding, and 19,508 were unannotated. In addition, we affirmed that the integrative transcriptome could serve as a surrogate reference to the genome in aiding accurate transcriptome assembly. For convenience, we curated the integrative transcriptome database for Bougainvillea, namely InTransBo, which can be freely accessed at http://www.bio-add.org/InTransBo/index.jsp . To the best of our knowledge, the present study is the most comprehensive genomic resource for Bougainvillea up-to-date. The integrative transcriptome helps fill the genomic gap and elucidate the transcriptional nature of Bougainvillea. It may also advance progress in the precise regulation of flowering in horticulture. The same strategy can be readily applied toward the systematic exploration of other plant species lacking complete genomic information.

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.

Microbial degradation of pyrene in holm oak (Quercus ilex) phyllosphere: Role of particulate matter in regulating bioaccessibility

In this study we first measured the mineralization of pyrene on leaves of urban holm oak (Quercus ilex) by autochthonous microorganisms and an inoculated PAH degrading bacterium (i.e., Mycobacterium gilvum), selected as a model phyllosphere species, as well as the leaf-water (K_LW) and leaf-air (K_LA) partition coefficients for this chemical. Mineralization was investigated in two different experimental systems in terms of leaf and microorganism environment. Additionally, the influence on pyrene partitioning and mineralization when particulate matter (PM) was present on the leaf surface or removed was studied. Mineralization of ¹⁴C-labeled pyrene by autochthonous microorganisms was lower than 1% after approximately two weeks, while M. gilvum mineralized 5% to 17% of pyrene. These extents corresponded to mineralization half-lives that ranged between ~30 to ~200 days. We proposed that PM present at the leaf surface reduced the accumulation of pyrene by inner compartments (cuticle) distantly located from microbial cells and enhanced the bioaccessibility of pyrene, speeding up microbial activity and therefore mineralization. These results highlight that plant-phyllosphere microorganism interaction is more complex than currently established and deserves additional studies to further comprehend the air purification ecosystem service of phyllosphere microorganisms.

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.

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.

Bougainvillea Genus: A Review on Phytochemistry, Pharmacology, and Toxicology

This review discusses the current knowledge of the phytochemistry and in vitro and in vivo evaluations carried out using the extracts and, where appropriate, the main active components isolated from the genus Bougainvillea. Out of 18 species, most phytochemical, pharmacological, and toxicological studies focused on four species with different cultivars and one hybrid. Some plants are used for the treatment of various health disorders. Numerous phytochemical investigations of plants in this genus confirm the presence of aliphatic hydrocarbons, fatty acids, fatty alcohols, volatile compounds, phenolic compounds, peltogynoids, flavonoids, phytosterols, terpenes, carbohydrates, and betalains. Various studies have confirmed that these extracts or active substances that were isolated from the genus Bougainvillea have multiple pharmacological activities. Some species of Bougainvillea have emerged as sources of traditional medicine in human health. More studies of the phytochemical, pharmacological, and toxicological properties and their mechanisms of action, safety, and efficacy in all Bougainvillea species, cultivars, and hybrids are advisable for future research.

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.

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.

Diversity and hydrocarbon-degrading potential of epiphytic microbial communities on Platanus x acerifolia leaves in an urban area

Plants and their associated bacteria have been suggested to play a role in air pollution mitigation, especially in urban areas. Particularly, epiphytic bacteria might be able to degrade atmospheric hydrocarbons. However, phyllospheric bacterial communities are highly variable depending on several factors, e.g. tree species, leaf age and physiology, environmental conditions. In this work, bacterial communities hosted by urban Platanus x acerifolia leaves were taxonomically characterized using high throughput sequencing of 16S rRNA gene, and their temporal and spatial variability was assessed by comparing samples collected from different locations in the city of Milan (Italy) and in different months. The diversity of alkane hydroxylase (alkB) phylotypes harboured by phyllospheric bacteria associated to urban Platanus trees was also evaluated. Results revealed that temporal changes, which are related to seasonality, acted as a stronger driver both on Platanus phyllospheric community structure and on alkB phylotype diversity than sampling location. Biodiversity of bacterial communities decreased along the growing season, leading to a strong dominance by the genus Stenotrophomonas. On the contrary, diversity of hydrocarbon-degrading populations increased over the months, although it resulted lower than that reported for other habitats. It was therefore hypothesized that atmospheric hydrocarbons might play a key role in the selection of phyllospheric populations in urban areas.