Evaluation of environmental bacterial communities as a factor affecting the growth of duckweed Lemna minor

Community interactions among microbes give rise to host-microbiome mutualisms in an aquatic plant

Microbiomes often benefit plants, conferring resistance to pathogens, improving stress tolerance, or promoting plant growth. As potential plant mutualists, however, microbiomes are not a single organism but a community of species with complex interactions among microbial taxa and between microbes and their shared host. The nature of ecological interactions among microbes in the microbiome can have important consequences for the net effects of microbiomes on hosts. Here, we compared the effects of individual microbial strains and 10-strain synthetic communities on microbial productivity and host growth using the common duckweed Lemna minor and a synthetic, simplified version of its native microbiome. Except for Pseudomonas protegens, which was a mutualist when tested alone, all of the single strains we tested were commensals on hosts, benefiting from plant presence but not increasing host growth relative to uninoculated controls. However, 10-strain synthetic microbial communities increased both microbial productivity and duckweed growth more than the average single-strain inoculation and uninoculated controls, meaning that host-microbiome mutualisms can emerge from community interactions among microbes on hosts. The effects of community inoculation were sub-additive, suggesting at least some competition among microbes in the duckweed microbiome. We also investigated the relationship between L. minor fitness and that of its microbes, providing some of the first empirical estimates of broad fitness alignment between plants and members of their microbiomes; hosts grew faster with more productive microbes or microbiomes.

IMPORTANCE

There is currently substantial interest in engineering synthetic microbiomes for health or agricultural applications. One key question is how multi-strain microbial communities differ from single microbial strains in their productivity and effects on hosts. We tested 20 single bacterial strains and 2 distinct 10-strain synthetic communities on plant hosts and found that 10-strain communities led to faster host growth and greater microbial productivity than the average, but not the best, single strain. Furthermore, the microbial strains or communities that achieved the greatest cell densities were also the most beneficial to their hosts, showing that both specific single strains and multi-strain synthetic communities can engage in high-quality mutualisms with their hosts. Our results suggest that ~5% of single strains, as well as multi-strain synthetic communities comprised largely of commensal microbes, can benefit hosts and result in effective host-microbe mutualisms.

Non-pathogenic microbiome associated to aquatic plants and anthropogenic impacts on this interaction

The microbiota associated with aquatic plants plays a crucial role in promoting plant growth and development. The structure of the plant microbiome is shaped by intricate interactions among hosts, microbes, and environmental factors. Consequently, anthropogenic pressures that disrupt these interactions can indirectly impact the ecosystem services provided by aquatic plants, such as CO2 fixation, provision of food resources, shelter to animals, nutrient cycling, and water purification. Presently, studies on plant-microbiota interactions primarily focus on terrestrial hosts and overlook aquatic environments with their unique microbiomes. Therefore, there is a pressing need for a comprehensive understanding of plant microbiomes in aquatic ecosystems. This review delves into the overall composition of the microbiota associated with aquatic plant, with a particular emphasis on bacterial communities, which have been more extensively studied. Subsequently, the functions provided by the microbiota to their aquatic plants hosts are explored, including the acquisition and mobilization of nutrients, production of auxin and related compounds, enhancement of photosynthesis, and protection against biotic and abiotic stresses. Additionally, the influence of anthropogenic stressors, such as climate change and aquatic contamination, on the interaction between microbiota and aquatic plants is discussed. Finally, knowledge gaps are highlighted and future directions in this field are suggested.

Influence of humic acid on the p-tert-Butylphenol removal efficiency by Spirodela polyrhiza-Tas13 association

Plant-microbe remediation technique is considered as a promising technology in removal of organic pollutants and its remediation efficiency is largely affected by a variety of surrounding environmental factors. Humic acid (HA) is the complex organic substance ubiquitous in environment, which characterized by its surfactant-like micelle microstructure and various reaction activity. In our study, a plant-microbe association with high p-tert-Butylphenol (PTBP) degradation potential constructed by Spirodela polyrhiza and Sphingobium phenoxybenzoativorans Tas13 has been used, and the influence of HA on the PTBP degradation efficiency of S. polyrhiza-Tas13 association was investigated. The result showed that the presence of HA greatly improved PTBP removal efficiency of S. polyrhiza-Tas13. The reason accounted for this may be due to the presence of HA promoted bacterial cell propagation, altered bacterial cell wall permeability, increased catechol 2,3-dioxygenase (C23O) enzyme activity of strain Tas13, rather than increasing the colonization ability of strain Tas13 on to the root surface. This study will greatly facilitate the application of aquatic plant-microbe association in environmental remediation.

Actinomycetospora lemnae sp. nov., A Novel Actinobacterium Isolated from Lemna aequinoctialis Able to Enhance Duckweed Growth

Duckweed-associated actinobacteria are co-existing microbes that affect duckweed growth and adaptation. In this study, we aimed to report a novel actinobacterium species and explore its ability to enhance duckweed growth. Strain DW7H6T was isolated from duckweed, Lemna aequinoctialis. Phylogenetic analysis based on its 16S rRNA gene sequence revealed that the strain was most closely related to Actinomycetospora straminea IY07-55T (99.0%), Actinomycetospora chibensis TT04-21T (98.9%), Actinomycetospora lutea TT00-04T (98.8%) and Actinomycetospora callitridis CAP 335T (98.4%). Chemotaxonomic and morphological characteristics of strain DW7H6T were consistent with members of the genus Actinomycetospora, while average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) between the draft genomes of this strain and its closely related type strains were below the proposed threshold values used for species discrimination. Based on chemotaxonomic, phylogenetic, phenotypic, and genomic evidence obtained, we describe a novel Actinomycetospora species, for which the name Actinomycetospora lemnae sp. nov. is proposed. The type strain is DW7H6T (TBRC 15165T, NBRC 115294T). Additionally, the duckweed-associated actinobacterium strain DW7H6T was able to enhance duckweed growth when compared to the control, in which the number of fronds and biomass dry weight were increased by up to 1.4 and 1.3 fold, respectively. Moreover, several plant-associated gene features in the genome of strain DW7H6T potentially involved in plant-microbe interactions were identified.

Effect of luminescent materials on the biochemistry, ultrastructure, and rhizobial microbiota of Spirodela polyrhiza

Fluorescent materials and technologies have become widely used in scientific research, and due to the ability to convert light wavelengths, their application to photosynthetic organisms can affect their development by altering light quality. However, the impacts of fluorescent materials on aquatic plants and their environmental risks remain unclear. To assess the effects of luminescent materials on floating aquatic macrophytes and their rhizosphere microorganisms, 4-(di-p-tolylamino)benzaldehyde-A (DTB-A) and 4-(di-p-tolylamino)benzaldehyde-M (DTB-M) (emitting blue-green and orange-red light, respectively) were added individually and jointly to Spirodela polyrhiza cultures and set at different concentrations (1, 10, and 100 μM). Both DTB-A and DTB-M exhibited phytotoxicity, which increased with concentration under separate treatment. Moreover, the combined group exhibited obvious stress relief at 10 μM compared to the individually treated group. Fluorescence imaging showed that DTB-A and DTB-M were able to enter the cell matrix and organelles of plant leaves and roots. Peroxidation induced cellular damage, contributing to a decrease in superoxide dismutase (SOD) and peroxidase (POD) activities and malondialdehyde (MDA) accumulation. Decomposition of organelle structures, starch accumulation in chloroplasts, and plasmolysis were observed under the ultrastructure, disrupting photosynthetic pigment content and photosynthesis. DTB-A and DTB-M exposure resulted in growth inhibition, dry weight loss, and leaf yellowing in S. polyrhiza. A total of 3519 Operational Taxonomic Units (OTUs) were identified in the rhizosphere microbiome. The microbial communities were dominated by Alphaproteobacteria, Oxyphotobacteria, and Gammaproteobacteria, with the abundance and diversity varied significantly among treatment groups according to Shannon, Simpson, and Chao1 indices. This study revealed the stress defense response of S. polyrhiza to DTB-A and DTB-M exposures, which provides a broader perspective for the bioremediation of pollutants using aquatic plants and supports the further development of fluorescent materials for applications.

Duckweed-associated bacteria as plant growth-promotor to enhance growth of Spirodela polyrhiza in wastewater effluent from a poultry farm

Duckweed has been highlighted as an invaluable resource because of its abilities to remove nitrogen and phosphorus from wastewater coupling with the production of high starch/protein-containing plant biomass. Duckweed recruits microbes and particularly forms a stable "core" bacterial microbiota, which greatly reduces the colonization efficiency of plant growth-promoting bacteria (PGPB). In this study, natural duckweeds were enriched in a sterilized-partially treated wastewater effluent from a poultry farm. After 24 days of cultivation, the duckweed-associated bacteria (DAB) were isolated and evaluated for their plant growth-promoting (PGP) potentials by co-cultivation with axenic Spirodela polyrhiza. Ten species were found in more than one location and could be considered candidates for the stable "core" DAB. Among them, all isolates of Acinetobacter soli, Acidovorax kalamii, Brevundimonas vesicularis, Pseudomonas toyotomiensis, and Shinella curvata increased duckweed growth in Hoagland medium. The highest PGP ability was observed in Sh. curvata W12-8 (with EPG value of 208.72%), followed by Paracoccus marcusii W7-16 (171.31%), Novosphingobium subterraneum W5-13 (156.96%), and Ac. kalamii W7-18 (156.96%). However, the highest growth promotion in the wastewater was observed when co-cultured with W7-16, which was able to increase biomass dry weight and root length of duckweed by 3.17 and 2.26 folds, respectively.

Presence of microbiome decreases fitness and modifies phenotype in the aquatic plant Lemna minor

Plants live in close association with microbial organisms that inhabit the environment in which they grow. Much recent work has aimed to characterize these plant-microbiome interactions, identifying those associations that increase growth. Although most work has focused on terrestrial plants, Lemna minor, a floating aquatic angiosperm, is increasingly used as a model in host-microbe interactions and many bacterial associations have been shown to play an important role in supporting plant fitness. However, the ubiquity and stability of these interactions as well as their dependence on specific abiotic environmental conditions remain unclear. Here, we assess the impact of a full L. minor microbiome on plant fitness and phenotype by assaying plants from eight natural sites, with and without their microbiomes, over a range of abiotic environmental conditions. We find that the microbiome systematically suppressed plant fitness, although the magnitude of this effect varied among plant genotypes and depended on the abiotic environment. Presence of the microbiome also resulted in phenotypic changes, with plants forming smaller colonies and producing smaller fronds and shorter roots. Differences in phenotype among plant genotypes were reduced when the microbiome was removed, as were genotype by environment interactions, suggesting that the microbiome plays a role in mediating the plant phenotypic response to the environment.

Characterization of the physico-chemical properties of the natural habitat and in vitro culture effects on the biochemistry, proliferation and morphology of Lemna minuta

In this study, the ecological conditions of the natural habitat of Lemna minuta Kunth in Morocco were investigated, and the impact of five synthetic growth media (Murashige-Skoog (MS), Schenk-Hildebrand (SH), Hoagland medium (HM), 10X Algal Assay Procedure (AAP), and Swedish Standard Institute medium (SIS)) on the morphophysiological and biochemical parameters was analysed. The morphophysiological parameters included root length, frond surface area, and fresh weight, while the biochemical parameters included photosynthetic pigments, carbohydrates, and protein content. The study was conducted in vitro in two phases: an uncontrolled aeration system (Phase I) and a controlled aeration system (Phase II).The results showed that the pH, conductivity, salinity, and ammonium levels in the natural habitat were within the optimal range for duckweed growth. The measured orthophosphate concentrations were higher compared to previous observations, while the recorded chemical oxygen demand values were low. The study also revealed a significant effect of the culture medium composition on the morphophysiological and biochemical parameters of the duckweed. The fresh weight biomass, relative growth rate in fronds, relative growth rate in surface area, root length, protein content, carbohydrates, chlorophyll (a), chlorophyll (b), total chlorophyll, carotenoids, and the chlorophyll (a/b) ratio were all affected by the culture medium.The most accurate regression models described the growth index GI(F) based on time and in vitro culture conditions in both phases. In Phase I, the best models for MS, SIS, AAP, and SH media were linear, weighted quadratic, cubic, and weighted cubic, respectively. In Phase II, the best models for all growth media were linear. The time coefficients (in days) for Phase II were 0.321, 0.547, 1.232, 1.470, and 0.306 for AAP, HM, MS, SH, and SIS, respectively.Comparing the morphophysiological and biochemical parameters of fronds from different media and analysing the regression model results showed that the SH and MS media were the best among the tested media for the in vitro culture of L. minuta in controlled aeration conditions. However, further research is needed to develop new synthetic media that best promote the growth and maintenance of this duckweed in long-term culture.

Optimization of Molecular Methods for Detecting Duckweed-Associated Bacteria

The bacterial colonization dynamics of plants can differ between phylogenetically similar bacterial strains and in the context of complex bacterial communities. Quantitative methods that can resolve closely related bacteria within complex communities can lead to a better understanding of plant-microbe interactions. However, current methods often lack the specificity to differentiate phylogenetically similar bacterial strains. In this study, we describe molecular strategies to study duckweed-associated bacteria. We first systematically optimized a bead-beating protocol to co-isolate nucleic acids simultaneously from duckweed and bacteria. We then developed a generic fingerprinting assay to detect bacteria present in duckweed samples. To detect specific duckweed-bacterium associations, we developed a genomics-based computational pipeline to generate bacterial strain-specific primers. These strain-specific primers differentiated bacterial strains from the same genus and enabled the detection of specific duckweed-bacterium associations present in a community context. Moreover, we used these strain-specific primers to quantify the bacterial colonization of duckweed by normalization to a plant reference gene and revealed differences in colonization levels between strains from the same genus. Lastly, confocal microscopy of inoculated duckweed further supported our PCR results and showed bacterial colonization of the duckweed root-frond interface and root interior. The molecular methods introduced in this work should enable the tracking and quantification of specific plant-microbe associations within plant-microbial communities.

Potential of Predatory Bacteria to Colonize the Duckweed Microbiome and Change Its Structure: A Model Study Using the Obligate Predatory Bacterium, Bacteriovorax sp. HI3

Modifying the duckweed microbiome is a major challenge for enhancing the effectiveness of duckweed-based wastewater treatment and biomass production technologies. We herein examined the potential of the exogenous introduction of predatory bacteria to change the duckweed microbiome. Bacteriovorax sp. HI3, a model predatory bacterium, colonized the core of the Lemna microbiome, and its predatory behavior changed the microbiome structure, which correlated with colonization density. These results reveal that bacterial predatory interactions may be important drivers that shape the duckweed microbiome, suggesting their potential usefulness in modifying the microbiome.

Microbial Population Dynamics in Lemnaceae (Duckweed)-Based Wastewater Treatment System

The dynamic microflora associated within, and in the surrounding aquatic environment, has been found to be responsible for the functional properties of many aquatic plants. The aim of the current work was to evaluate the effectiveness of Lemnaceae-based wastewater treatment system under tropical conditions and investigate the changes in the aquatic microflora upon plant growth. A biological wastewater treatment system was designed and investigated using mixed Lemnaceae culture comprising Lemna minor and Spirodela polyrhiza in a batch mode. A significant reduction in total solids (31.8%), biochemical oxygen demand (93.5%), and chemical oxygen demand (73.2%) was observed after seven days of duckweed growth using a low inoculum. A preliminary study on the change in the microbial population diversity and functionality, in the wastewater before and after treatment, revealed an increase in the denitrifying microflora in wastewater post-Lemnaceae treatment. Dominance of 10 bacterial phyla, contributing for 98.3% of the total bacterial communities, was recorded, and ~ 50.6% loss of diversity post-treatment of wastewater was revealed by the Shannon Index. Among 16 bacterial families showing relative abundance of ≥ 1% in untreated wastewater, Methylobacteriaceae, Pseudomonadaceae, Brucellaceae, Rhodobacteraceae, and Acetobacteraceae prevailed in the water post-treatment by duckweeds. This is a novel work done on the dynamics of aquatic microflora associated with Lemnaceae under tropical Indian conditions. It confirms the application of Lemnaceae-based wastewater treatment system as effective biofilter and calls for further studies on the active involvement of the endophytic and aquatic microflora in the functions of these plant.

The Effects of Microbiota on the Herbivory Resistance of the Giant Duckweed Are Plant Genotype-Dependent

In nature, all plants live with microbes, which can directly affect their host plants' physiology and metabolism, as well as their interacting partners, such as herbivores. However, to what extent the microbiota shapes the adaptive evolution to herbivory is unclear. To address this challenge, it is essential to quantify the intra-specific variations of microbiota effects on plant fitness. Here, we quantified the fitness effects of microbiota on the growth, tolerance, and resistance to herbivory among six genotypes of the giant duckweed, Spirodela polyrhiza. We found that the plant genotypes differed in their intrinsic growth rate and tolerance, but not in their resistance to a native herbivore, the great pond snail. Inoculation with microbiota associated with S. polyrhiza growing outdoors reduced the growth rate and tolerance in all genotypes. Additionally, the microbiota treatment altered the herbivory resistance in a genotype-specific manner. Together, these data show the potential of microbiota in shaping the adaptive evolution of plants.

Harnessing plant-microbiome interactions for bioremediation across a freshwater urbanization gradient

Urbanization impacts land, air, and water, creating environmental gradients between cities and rural areas. Urban stormwater delivers myriad co-occurring, understudied, and mostly unregulated contaminants to aquatic ecosystems, causing a pollution gradient. Recipient ecosystems host interacting species that can affect each others' growth and responses to these contaminants. For example, plants and their microbiomes often reciprocally increase growth and contaminant tolerance. Here, we identified ecological variables affecting contaminant fate across an urban-rural gradient using 50 sources of the aquatic plant Lemna minor (duckweed) and associated microbes, and two co-occurring winter contaminants of temperate cities, benzotriazole and salt. We conducted experiments totalling >2,500 independent host-microbe-contaminant microcosms. Benzotriazole and salt negatively affected duckweed growth, but not microbial growth, and duckweeds maintained faster growth with their local, rather than disrupted, microbiota. Benzotriazole transformation products of plant, microbial, and phototransformation pathways were linked to duckweed and microbial growth, and were affected by salt co-contamination, microbiome disruption, and source sites of duckweeds and microbes. Duckweeds from urban sites grew faster and enhanced phytotransformation, but supported less total transformation of benzotriazole. Increasing microbial community diversity correlated with greater removal of benzotriazole, but taxonomic groups may explain shifts across transformation pathways: the genus Aeromonas was linked to increasing phototransformation. Because benzotriazole toxicity could depend on amount and type of in situ transformation, this variation across duckweeds and microbes could be harnessed for better management of urban stormwater. Broadly, our results demonstrate that plant-microbiome interactions harbour manipulable variation for bioremediation applications.

Isolation of Aquatic Plant Growth-Promoting Bacteria for the Floating Plant Duckweed (Lemna minor)

Plant growth-promoting bacteria (PGPB) can exert beneficial growth effects on their host plants. Little is known about the phylogeny and growth-promoting mechanisms of PGPB associated with aquatic plants, although those of terrestrial PGPB have been well-studied. Here, we report four novel aquatic PGPB strains, MRB1–4 (NITE P-01645–P-01648), for duckweed Lemna minor from our rhizobacterial collection isolated from Lythrum anceps. The number of L. minor fronds during 14 days co-culture with the strains MRB1–4 increased by 2.1–3.8-fold, compared with an uninoculated control; the plant biomass and chlorophyll content in co-cultures also increased. Moreover, all strains possessed an indole-3-acetic acid production trait in common with a plant growth-promoting trait of terrestrial PGPB. Phylogenetic analysis showed that three strains, MRB-1, -3, and -4, were affiliated with known proteobacterial genera (Bradyrhizobium and Pelomonas); this report is the first to describe a plant-growth promoting activity of Pelomonas members. The gammaproteobacterial strain MRB2 was suggested to be phylogenetically novel at the genus level. Under microscopic observation, the Pelomonas strain MRB3 was epiphytic and adhered to both the root surfaces and fronds of duckweed. The duckweed PGPB obtained here could serve as a new model for understanding unforeseen mechanisms behind aquatic plant-microbe interactions.

Microbial Metabolites Beneficial to Plant Hosts Across Ecosystems

Plants are intimately connected with their associated microorganisms. Chemical interactions via natural products between plants and their microbial symbionts form an important aspect in host health and development, both in aquatic and terrestrial ecosystems. These interactions range from negative to beneficial for microbial symbionts as well as their hosts. Symbiotic microbes synchronize their metabolism with their hosts, thus suggesting a possible coevolution among them. Metabolites, synthesized from plants and microbes due to their association and coaction, supplement the already present metabolites, thus promoting plant growth, maintaining physiological status, and countering various biotic and abiotic stress factors. However, environmental changes, such as pollution and temperature variations, as well as anthropogenic-induced monoculture settings, have a significant influence on plant-associated microbial community and its interaction with the host. In this review, we put the prominent microbial metabolites participating in plant-microbe interactions in the natural terrestrial and aquatic ecosystems in a single perspective and have discussed commonalities and differences in these interactions for adaptation to surrounding environment and how environmental changes can alter the same. We also present the status and further possibilities of employing chemical interactions for environment remediation. Our review thus underlines the importance of ecosystem-driven functional adaptations of plant-microbe interactions in natural and anthropogenically influenced ecosystems and their possible applications.

Cyanobacteria as Candidates to Support Mars Colonization: Growth and Biofertilization Potential Using Mars Regolith as a Resource

Cyanobacteria are indicated as organisms that can possibly support Mars colonization, contributing to the production of oxygen and other commodities therein. In this general context, the aim of this work was to evaluate the ability of three species of cyanobacteria (Anabaena cylindrica, Nostoc muscorum, and Arthrospira platensis) and a green microalga (Chlorella vulgaris) to grow using only the resources existing in Mars, i.e., water and Martian regolith stimulant (MGS-1), under an Earth-like atmosphere. A Martian regolith extract was produced and used as a culture medium to grow these species. Their growth was assessed during a period of 25 days, using optical density and fluorometric parameters. After this period, the possible contribution of end-of-life cyanobacteria/microalga as biofertilizing agents was also assessed, using the macrophyte Lemna minor as a vegetable model. Among the three species, N. muscorum showed the best growth performance when compared to the other species, while A. platensis and C. vulgaris were not able to thrive on Mars regolith extract. Therefore, N. muscorum should be the target of future studies not only due to their role in oxygen production but also due to their possible use as a food source, as many members of the Nostoc genus. Cyanobacteria and microalgae (A. platensis and C. vulgaris) showed good abilities as biofertilizing agents, i.e., they stimulated biomass (i.e., dry weight) production at levels comparable to the plants that grew on standard synthetic medium. The highest yield was reached with A. platensis, while the lowest was achieved using the media with N. muscorum. FTIR-ATR (Fourier transform infrared with attenuated total reflectance) spectroscopy showed that the differences between the plants grown on media with or without Martian regolith seem to be related mainly to polysaccharides.

Auxin-Producing Bacteria from Duckweeds Have Different Colonization Patterns and Effects on Plant Morphology

The role of auxin in plant-microbe interaction has primarily been studied using indole-3-acetic acid (IAA)-producing pathogenic or plant-growth-promoting bacteria. However, the IAA biosynthesis pathway in bacteria involves indole-related compounds (IRCs) and intermediates with less known functions. Here, we seek to understand changes in plant response to multiple plant-associated bacteria taxa and strains that differ in their ability to produce IRCs. We had previously studied 47 bacterial strains isolated from several duckweed species and determined that 79% of these strains produced IRCs in culture, such as IAA, indole lactic acid (ILA), and indole. Using Arabidopsis thaliana as our model plant with excellent genetic tools, we performed binary association assays on a subset of these strains to evaluate morphological responses in the plant host and the mode of bacterial colonization. Of the 21 tested strains, only four high-quantity IAA-producing Microbacterium strains caused an auxin root phenotype. Compared to the commonly used colorimetric Salkowski assay, auxin concentration determined by LC-MS was a superior indicator of a bacteria's ability to cause an auxin root phenotype. Studies with the auxin response mutant axr1-3 provided further genetic support for the role of auxin signaling in mediating the root morphology response to IAA-producing bacteria strains. Interestingly, our microscopy results also revealed new evidence for the role of the conserved AXR1 gene in endophytic colonization of IAA-producing Azospirillum baldaniorum Sp245 via the guard cells.

Genome-wide identification of bacterial colonization and fitness determinants on the floating macrophyte, duckweed

Bacterial communities associated with aquatic macrophytes largely influence host primary production and nutrient cycling in freshwater environments; however, little is known about how specific bacteria migrate to and proliferate at this unique habitat. Here, we separately identified bacterial genes involved in the initial colonization and overall fitness on plant surface, using the genome-wide transposon sequencing (Tn-seq) of Aquitalea magnusonii H3, a plant growth-promoting bacterium of the floating macrophyte, duckweed. Functional annotation of identified genes indicated that initial colonization efficiency might be simply explained by motility and cell surface structure, while overall fitness was associated with diverse metabolic and regulatory functions. Genes involved in lipopolysaccharides and type-IV pili biosynthesis showed different contributions to colonization and fitness, reflecting their metabolic cost and profound roles in host association. These results provide a comprehensive genetic perspective on aquatic-plant-bacterial interactions, and highlight the potential trade-off between bacterial colonization and proliferation abilities on plant surface.

Growth and Nutritional Quality of Lemnaceae Viewed Comparatively in an Ecological and Evolutionary Context

This review focuses on recently characterized traits of the aquatic floating plant Lemna with an emphasis on its capacity to combine rapid growth with the accumulation of high levels of the essential human micronutrient zeaxanthin due to an unusual pigment composition not seen in other fast-growing plants. In addition, Lemna's response to elevated CO₂ was evaluated in the context of the source-sink balance between plant sugar production and consumption. These and other traits of Lemnaceae are compared with those of other floating aquatic plants as well as terrestrial plants adapted to different environments. It was concluded that the unique features of aquatic plants reflect adaptations to the freshwater environment, including rapid growth, high productivity, and exceptionally strong accumulation of high-quality vegetative storage protein and human antioxidant micronutrients. It was further concluded that the insensitivity of growth rate to environmental conditions and plant source-sink imbalance may allow duckweeds to take advantage of elevated atmospheric CO₂ levels via particularly strong stimulation of biomass production and only minor declines in the growth of new tissue. It is proposed that declines in nutritional quality under elevated CO₂ (due to regulatory adjustments in photosynthetic metabolism) may be mitigated by plant-microbe interaction, for which duckweeds have a high propensity.

Growth Promotion of Giant Duckweed Spirodela polyrhiza (Lemnaceae) by Ensifer sp. SP4 Through Enhancement of Nitrogen Metabolism and Photosynthesis

Duckweeds (Lemnaceae) are representative producers in fresh aquatic ecosystems and also yield sustainable biomass for animal feeds, human foods, and biofuels, and contribute toward effective wastewater treatment; thus, enhancing duckweed productivity is a critical challenge. Plant-growth-promoting bacteria (PGPB) can improve the productivity of terrestrial plants; however, duckweed-PGPB interactions remain unclear and no previous study has investigated the molecular mechanisms underlying duckweed-PGPB interaction. Herein, a PGPB, Ensifer sp. strain SP4, was newly isolated from giant duckweed (Spirodela polyrhiza), and the interactions between S. polyrhiza and SP4 were investigated through physiological, biochemical, and metabolomic analyses. In S. polyrhiza and SP4 coculture, SP4 increased the nitrogen (N), chlorophyll, and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) contents and the photosynthesis rate of S. polyrhiza by 2.5-, 2.5-, 2.7-, and 2.4-fold, respectively. Elevated photosynthesis increased the relative growth rate and biomass productivity of S. polyrhiza by 1.5- and 2.7-fold, respectively. Strain SP4 significantly altered the metabolomic profile of S. polyrhiza, especially its amino acid profile. N stable isotope analysis revealed that organic N compounds were transferred from SP4 to S. polyrhiza. These N compounds, particularly glutamic acid, possibly triggered the increase in photosynthetic and growth activities. Accordingly, we propose a new model for the molecular mechanism underlying S. polyrhiza growth promotion by its associated bacteria Ensifer sp. SP4, which occurs through enhanced N compound metabolism and photosynthesis. Our findings show that Ensifer sp. SP4 is a promising PGPB for increasing biomass yield, wastewater purification activity, and CO₂ capture of S. polyrhiza.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Production of poly (l-lactide)-degrading enzyme by Actinomadura keratinilytica strain T16-1 under solid state fermentation using agricultural wastes as substrate

Poly (l-lactide) (PLLA) is an aliphatic polyester that can be obtained from renewable resources and degraded by various microorganisms. In previous reports, Actinomadura keratinilytica strain T16-1 demonstrated high ability to degrade PLLA under various conditions. PLLA-degrading enzyme production under solid state fermentation has been sparsely studied. PLLA-degrading enzyme production by A. keratinilytica strain T16-1 was investigated using agricultural wastes as substrate under solid state fermentation (SSF). Three agricultural wastes as soybean meal, cassava chips and duckweed were tested as substrates for PLLA-degrading enzyme production by statistical methods using mixture design. Results revealed that using duckweed as the substrate gave the highest enzyme production (138.66 ± 13.57 U/g dry substrate). Maximum enzyme activity of 391.24 ± 15.57 U/g dry substrate was obtained under 10 g duckweed, 10% inoculum size, 7 days of cultivation time, pH 7.0, 2.8% PLLA powder, and 60% moisture content at 45 °C. It can be concluded that duckweed is an inexpensive substrate, which reduces the costs of PLLA-degrading enzyme production, as an alternative to effective water weed management.

Return of the Lemnaceae: duckweed as a model plant system in the genomics and postgenomics era

The aquatic Lemnaceae family, commonly called duckweed, comprises some of the smallest and fastest growing angiosperms known on Earth. Their tiny size, rapid growth by clonal propagation, and facile uptake of labeled compounds from the media were attractive features that made them a well-known model for plant biology from 1950 to 1990. Interest in duckweed has steadily regained momentum over the past decade, driven in part by the growing need to identify alternative plants from traditional agricultural crops that can help tackle urgent societal challenges, such as climate change and rapid population expansion. Propelled by rapid advances in genomic technologies, recent studies with duckweed again highlight the potential of these small plants to enable discoveries in diverse fields from ecology to chronobiology. Building on established community resources, duckweed is reemerging as a platform to study plant processes at the systems level and to translate knowledge gained for field deployment to address some of society's pressing needs. This review details the anatomy, development, physiology, and molecular characteristics of the Lemnaceae to introduce them to the broader plant research community. We highlight recent research enabled by Lemnaceae to demonstrate how these plants can be used for quantitative studies of complex processes and for revealing potentially novel strategies in plant defense and genome maintenance.

Insights into endophytic bacterial diversity of rice grown across the different agro-ecological regions of West Bengal, India

Endophytes have recently garnered importance worldwide and multiple studies are being conducted to understand their important role and mechanism of interaction inside plants. But before we indulge in their functions it is necessary to dig into the microbiome. This will help to get a complete picture of the microbes intrinsic to their host and understand changes in community composition with respect to their habitats. To fulfil this requirement in our study we have attempted to dissect the endophytic diversity in roots of rice plant grown across the various agro-ecological zones of West Bengal by undergoing amplicon analysis of their 16S rRNA gene. Based on the measured environmental parameters agro-ecological zones can be divided into two groups: nutrient dense groups, representing zones like Gangetic, Northern hill and Terai-Teesta zone characterised by soil with higher levels of nitrogen (N) and total organic carbon and nutrient low groups representing Coastal saline, Red-laterite and Vindhyan zone mainly characterised by high electroconductivity and pH. Gammaproteobacteria, Alphaproteobacteria, Bacilli and Bacteroidetes were mostly abundant in nutrient dense sites whereas Clostridia and Planctomycetes were concentrated in nutrient low sites. Few genera (Aeromonas, Sulfurospirillum, Uliginosibacterium and Acidaminococcus) are present in samples cultivated in all the zones representing the core microbiome of rice in West Bengal, while some other genera like Lactococcus, Dickeya, Azonexus and Pectobacterium are unique to specific zone. Hence it can be concluded that this study has provided some insight in to the endophytic status of rice grown across the state of West Bengal.

Spatial patterns in phage-Rhizobium coevolutionary interactions across regions of common bean domestication

Bacteriophages play significant roles in the composition, diversity, and evolution of bacterial communities. Despite their importance, it remains unclear how phage diversity and phage-host interactions are spatially structured. Local adaptation may play a key role. Nitrogen-fixing symbiotic bacteria, known as rhizobia, have been shown to locally adapt to domesticated common bean at its Mesoamerican and Andean sites of origin. This may affect phage-rhizobium interactions. However, knowledge about the diversity and coevolution of phages with their respective Rhizobium populations is lacking. Here, through the study of four phage-Rhizobium communities in Mexico and Argentina, we show that both phage and host diversity is spatially structured. Cross-infection experiments demonstrated that phage infection rates were higher overall in sympatric rhizobia than in allopatric rhizobia except for one Argentinean community, indicating phage local adaptation and host maladaptation. Phage-host interactions were shaped by the genetic identity and geographic origin of both the phage and the host. The phages ranged from specialists to generalists, revealing a nested network of interactions. Our results suggest a key role of local adaptation to resident host bacterial communities in shaping the phage genetic and phenotypic composition, following a similar spatial pattern of diversity and coevolution to that in the host.

Novel Plant-Associated Acidobacteria Promotes Growth of Common Floating Aquatic Plants, Duckweeds

Duckweeds are small, fast growing, and starch- and protein-rich aquatic plants expected to be a next generation energy crop and an excellent biomaterial for phytoremediation. Despite such an importance, very little is known about duckweed-microbe interactions that would be a key biological factor for efficient industrial utilization of duckweeds. Here we first report the duckweed growth promoting ability of bacterial strains belonging to the phylum Acidobacteria, the members of which are known to inhabit soils and terrestrial plants, but their ecological roles and plant-microbe interactions remain largely unclear. Two novel Acidobacteria strains, F-183 and TBR-22, were successfully isolated from wild duckweeds and phylogenetically affiliated with subdivision 3 and 6 of the phylum, respectively, based on 16S rRNA gene sequence analysis. In the co-culture experiments with aseptic host plants, the F-183 and TBR-22 strains visibly enhanced growth (frond number) of six duckweed species (subfamily Lemnoideae) up to 1.8-5.1 times and 1.6-3.9 times, respectively, compared with uninoculated controls. Intriguingly, both strains also increased the chlorophyll content of the duckweed (Lemna aequinoctialis) up to 2.4-2.5 times. Under SEM observation, the F-183 and TBR-22 strains were epiphytic and attached to the surface of duckweed. Taken together, our findings suggest that indigenous plant associated Acidobacteria contribute to a healthy growth of their host aquatic plants.

Synthetic Bacterial Community of Duckweed: A Simple and Stable System to Study Plant-microbe Interactions

A complete understanding of the plant microbiome has not yet been achieved due to its complexity and temporal shifts in the community structure. To overcome these issues, we created a synthetic bacterial community of the aquatic plant, duckweed. The synthetic community established with six bacterial strains showed a stable composition for 50 days, which may have been because duckweed maintains a similar physiological status through its clonal reproduction. Additionally, the synthetic community reflected the taxonomic structure of the natural duckweed microbiome at the family level. These results suggest the potential of a duckweed-based synthetic community as a useful model system for examining the community assembly mechanisms of the plant microbiome.

Eco-evolutionary interaction between microbiome presence and rapid biofilm evolution determines plant host fitness

Microbiomes are important to the survival and reproduction of their hosts. Although ecological and evolutionary processes can happen simultaneously in microbiomes, little is known about how microbiome eco-evolutionary dynamics determine host fitness. Here we show, using experimental evolution, that fitness of the aquatic plant Lemna minor is modified by interactions between the microbiome and the evolution of one member, Pseudomonas fluorescens. Microbiome presence promotes P. fluorescens' rapid evolution to form biofilm, which reciprocally alters the microbiome's species composition. These eco-evolutionary dynamics modify the host's multigenerational fitness. The microbiome and non-evolving P. fluorescens together promote host fitness, whereas the microbiome with P. fluorescens that evolves biofilm reduces the beneficial impact on host fitness. Additional experiments suggest that the microbial effect on host fitness may occur through changes in microbiome production of auxin, a plant growth hormone. Our study, therefore, experimentally demonstrates the importance of the eco-evolutionary dynamics in microbiomes for host-microbiome interactions.

Indigenous bacteria, an excellent reservoir of functional plant growth promoters for enhancing duckweed biomass yield on site

The advantages of aquatic biomass production using wastewater as a cost-free fertilizer have recently been highlighted. Here, we report a successful study in which duckweed, Lemna gibba, biomass production in a food factory effluent containing low nitrogen and high salts was enhanced by employing customized plant growth-promoting bacteria (PGPB). Two common PGPB strains previously obtained from natural pond water, Acinetobacter calcoaceticus P23 and Pseudomonas fulva Ps6, hardly promoted the growth of duckweed; on the contrary, they inhibited its growth in treated factory wastewater, far different water conditions. Then, we asked if some indigenous wastewater bacteria could promote the growth of duckweed. We found that Chryseobacterium strains, a group of bacteria with limited nitrogen metabolism, were dominantly selected as effective PGPB. Moreover, we demonstrated that nitrogen limitation is the crucial environmental factor that induces the plant growth-inhibiting behavior of A. calcoaceticus P23 through competition for mineral nutrients with the host duckweed. This study uncovered points to be considered in PGPB technology to achieve efficient production of duckweed biomass in a factory effluent with unbalanced content of mineral nutrients.

Flowering and Seed Production across the Lemnaceae

Plants in the family Lemnaceae are aquatic monocots and the smallest, simplest, and fastest growing angiosperms. Their small size, the smallest family member is 0.5 mm and the largest is 2.0 cm, as well as their diverse morphologies make these plants ideal for laboratory studies. Their rapid growth rate is partially due to the family's neotenous lifestyle, where instead of maturing and producing flowers, the plants remain in a juvenile state and continuously bud asexually. Maturation and flowering in the wild are rare in most family members. To promote further research on these unique plants, we have optimized laboratory flowering protocols for 3 of the 5 genera: Spirodela; Lemna; and Wolffia in the Lemnaceae. Duckweeds were widely used in the past for research on flowering, hormone and amino acid biosynthesis, the photosynthetic apparatus, and phytoremediation due to their aqueous lifestyle and ease of aseptic culture. There is a recent renaissance in interest in growing these plants as non-lignified biomass sources for fuel production, and as a resource-efficient complete protein source. The genome sequences of several Lemnaceae family members have become available, providing a foundation for genetic improvement of these plants as crops. The protocols for maximizing flowering described herein are based on screens testing daylength, a variety of media, supplementation with salicylic acid or ethylenediamine-N,N'-bis(2-hydroxyphenylacetic acid) (EDDHA), as well as various culture vessels for effects on flowering of verified Lemnaceae strains available from the Rutgers Duckweed Stock Cooperative.

Intraspecific variations in cadmium tolerance and phytoaccumulation in giant duckweed (Spirodela polyrhiza)

Duckweeds are widely recognized for the heavy metal phytoremediation. However, the intraspecific variations in biological responses of duckweeds to heavy metal remain largely unknown. Here, the toxicity and phytoaccumulation of cadmium (Cd) were synchronously evaluated in 30 accessions of giant duckweed (Spirodela polyrhiza) collected from different provenances in Southern China. Exposure to 1 μM Cd decreased relative growth rates of dry weight, fronds number and fronds area, as well as photosynthetic pigment contents, while it increased H₂O₂ accumulation, lipid peroxidation and activities of anti-oxidant enzymes in the majority of accessions. Cd treatment led to remarkable Cd accumulation but little changes in the starch content in giant duckweed. The biological responses to Cd varied among the accessions. Further correlation analysis indicated that growth traits and Cd concentration were positively correlated with Cd accumulation, while the contents of chlorophyll, H₂O₂ and MDA were negatively associated with Cd accumulation. Our results proved the great intraspecific variation in Cd tolerance of giant duckweed, suggesting a valuable natural resource for Cd phytoremediation. Moreover, different mechanisms may be exploited by S. polyrhiza for phytoaccumulation, but growth maintenance, Cd uptake and antioxidative enzyme-independent ROS-scavenging under Cd exposure are the common mechanisms contributing to Cd accumulation ability.

Mutualistic Outcomes Across Plant Populations, Microbes, and Environments in the Duckweed Lemna minor

The picture emerging from the rapidly growing literature on host-associated microbiota is that host traits and fitness often depend on interactive effects of host genotype, microbiota, and abiotic environment. However, testing interactive effects typically requires large, multi-factorial experiments and thus remains challenging in many systems. Furthermore, most studies of plant microbiomes focus on terrestrial hosts and microbes. Aquatic habitats may confer unique properties to microbiomes. We grew different populations of duckweed (Lemna minor), a floating aquatic plant, in three microbial treatments (adding no, "home", or "away" microbes) at two levels of zinc, a common water contaminant in urban areas, and measured both plant and microbial performance. Thus, we simultaneously manipulated plant source population, microbial community, and abiotic environment. We found strong effects of plant source, microbial treatment, and zinc on duckweed and microbial growth, with significant variation among duckweed genotypes and microbial communities. However, we found little evidence of interactive effects: zinc did not alter effects of host genotype or microbial community, and host genotype did not alter effects of microbial communities. Despite strong positive correlations between duckweed and microbe growth, zinc consistently decreased plant growth, but increased microbial growth. Furthermore, as in recent studies of terrestrial plants, microbial interactions altered a duckweed phenotype (frond aggregation). Our results suggest that duckweed source population, associated microbiome, and contaminant environment should all be considered for duckweed applications, such as phytoremediation. Lastly, we propose that duckweed microbes offer a robust experimental system for study of host-microbiota interactions under a range of environmental stresses.

Effect of treated sewage characteristics on duckweed biomass production and microbial communities

Duckweed biomass production in a duckweed pond fed with three differently treated sewage (i.e. sewage treated by primary sedimentation (PS); conventional activated sludge process (CAS); and downflow hanging sponge process (DHS)) was evaluated in order to assess the effects of water quality on biomass yield. Higher and stable biomass production was observed when the duckweed pond was fed with PS or DHS-effluent than with CAS-effluent, evidently due to the difference in nutrient loads. Availability of nutrients, especially phosphorus, affected the biomass production rate: higher the nutrient, faster the production. Microbial community analysis revealed that the members of Rhizobiales were likely to contribute to stable and high biomass growth. From the results of the study, a sewage treatment system consisting of a primary sedimentation followed by a duckweed pond and a tertiary treatment unit can be proposed to maximize biomass production without compromising treatment objectives. Size and operational parameters of the duckweed pond should be determined primarily based on the nutrient availability in the influent water to maximize duckweed growth.

Community dynamics of duckweed-associated bacteria upon inoculation of plant growth-promoting bacteria

Plant growth-promoting bacteria (PGPB) have recently been demonstrated as a promising agent to improve wastewater treatment and biomass production efficiency of duckweed hydrocultures. With a view to their reliable use in aqueous environments, this study analysed the plant colonization dynamics of PGPB and the ecological consequences for the entire duckweed-associated bacterial community. A PGPB strain, Aquitalea magnusonii H3, was inoculated to duckweed at different cell densities or timings in the presence of three environmental bacterial communities. The results showed that strain H3 improved duckweed growth by 11.7–32.1% in five out of nine experiments. Quantitative-PCR and amplicon sequencing analyses showed that strain H3 successfully colonized duckweed after 1 and 3 d of inoculation in all cultivation tests. However, it significantly decreased in number after 7 d, and similar bacterial communities were observed on duckweed regardless of H3 inoculation. Predicted metagenome analysis suggested that genes related to bacterial chemotactic motility and surface attachment systems are consistently enriched through community assembly on duckweed. Taken together, strain H3 dominantly colonized duckweed for a short period and improved duckweed growth. However, the inoculation of the PGPB did not have a lasting impact due to the strong resilience of the natural duckweed microbiome.

Co-occurrence analysis reveal that biotic and abiotic factors influence soil fungistasis against Fusarium graminearum

The current study determined the levels of soil fungistasis against a soil-borne pathogen inoculum, Fusarium graminearum (Fg, a major causal agent of Fusarium Head Blight (FHB)), in 31 wheat fields by quantifying Fg growth after a 15-day incubation period using qPCR in autoclaved versus non-autoclaved soils. The results were used to define the six most Fg-resistant and the six most Fg-conducive soils. By using a metabarcoding approach, the diversity of the bacterial communities was significantly higher in Fg-resistant soils than in Fg-conducive soils. Microbial taxa potentially contributing to Fg-fungistasis of soil were selected if they were significantly more prevalent and/or abundant in Fg-resistant soils than in Fg-conducive soils. Some of these candidate indicators, e.g. Pseudomonas spp. and Bacillus spp., have been reported previously as effective biocontrol agents against plant pathogens. Correlation-based network analysis further showed that the members of the bacterial communities in Fg-resistant soils were more connected than in Fg-conducive soils. Moreover, network modules was found significantly correlated with certain edaphic abiotics factors (such as the soil manganese and nitrogen content) and Fg-fungistasis. Such observations may suggest and emphasize, although conceptual, the importance of synergistic rather than individual effects of network members, and the nutrient use efficiency in contributing to Fg-resistance of soils in wheat fields in France.

Enhanced biomass production and nutrient removal capacity of duckweed via two-step cultivation process with a plant growth-promoting bacterium, Acinetobacter calcoaceticus P23

Plant growth-promoting bacteria (PGPB) are considered a promising tool to improve biomass production and water remediation by the aquatic plant, duckweed; however, no effective methodology is available to utilize PGPB in large hydroponic systems. In this study, we proposed a two-step cultivation process, which comprised of a "colonization step" and a "mass cultivation step," and examined its efficacy in both bucket-scale and flask-scale cultivation experiments. We showed that in the outdoor bucket-scale experiments using three kinds of environmental water, plants cultured through the two-step cultivation method with the PGPB strain, Acinetobacter calcoaceticus P23, yielded 1.9 to 2.3 times more biomass than the control (without PGPB inoculation). The greater nitrogen and phosphorus removals compared to control were also attained, indicating that this strategy is useful for accelerating nutrient removal by duckweed. Flask-scale experiments using non-sterile pond water revealed that inoculation of strain P23 altered duckweed surface microbial community structures, and the beneficial effects of the inoculated strain P23 could last for 5-10 d. The loss of the duckweed growth-promoting effect was noticeable when the colonization of strain P23 decreased in the plant. These observations suggest that the stable colonization of the plant with PGPB is the key for maintaining the accelerated duckweed growth and nutrient removal in this cultivation method. Overall, our results suggest the possibility of an improved duckweed production using a two-step cultivation process with PGPB.

Exploration of Root-associated Bacteria from the Medicinal Plant Platycodon grandiflorum

The present study was performed to investigate root-associated bacteria from Platycodon grandiflorum, a medicinal plant commonly grown in East Asia. Isolates were obtained from the rhizosphere or root interior with various culture media, and phylogenetic analyses were performed based on their 16S rDNA sequences. In consideration of practical applications, traits related to plant growth promotion and niche adaptation were assessed in several endophytic strains with fewer biosafety concerns. The effects of a bacterial inoculation on seedling and mature plant growth were evaluated. Seventeen genera that encompassed more than 30 bacterial lineages were successfully retrieved from the roots, the majority of which have not been reported as P. grandiflorum-associated bacteria, particularly for non-negligible Proteobacteria. Although nitrogen-fixing or phosphate-solubilizing and indole acetic acid-producing activities were recorded in all of the strains selected, these strains were beneficial or detrimental to plant growth as evidenced by their influence on the length of seedlings and biomass of mature plants. Among the 4 endophytic Rhizobium species tested in the present study, the potentially novel Rhizobium sp. BF-E16, which was more compatible with the non-leguminous medicinal plant P. grandiflorum, was identified. Other than plant growth-promoting traits, characteristics such as plant constituent-hydrolyzing activities need to be taken into consideration and their roles clarified when investigating plant growth-promoting rhizobacteria.

Colonization and Competition Dynamics of Plant Growth-Promoting/Inhibiting Bacteria in the Phytosphere of the Duckweed Lemna minor

Despite the considerable role of aquatic plant-associated bacteria in host plant growth and nutrient cycling in aquatic environments, the mode of their plant colonization has hardly been understood. This study examined the colonization and competition dynamics of a plant growth-promoting bacterium (PGPB) and two plant growth-inhibiting bacteria (PGIB) in the aquatic plant Lemna minor (common duckweed). When inoculated separately to L. minor, each bacterial strain quickly colonized at approximately 10⁶ cells per milligram (plant fresh weight) and kept similar populations throughout the 7-day cultivation time. The results of two-membered co-inoculation assays revealed that the PGPB strain Aquitalea magnusonii H3 consistently competitively excluded the PGIB strain Acinetobacter ursingii M3, and strain H3 co-existed at almost 1:1 proportion with another PGIB strain, Asticcacaulis excentricus M6, regardless of the inoculation ratios (99:1-1:99) and inoculation order. We also found that A. magnusonii H3 exerted its growth-promoting effect over the negative effects of the two PGIB strains even when only a small amount was inoculated, probably due to its excellent competitive colonization ability. These experimental results demonstrate that there is a constant ecological equilibrium state involved in the bacterial colonization of aquatic plants.

Complete Genome Sequences of Two Plant Growth-Inhibiting Bacteria, Acinetobacter ursingii M3 and Asticcacaulis excentricus M6, Isolated from Duckweed (Lemna minor)

Acinetobacter ursingii M3 and Asticcacaulis excentricus M6 are plant growth-inhibiting bacteria that reduce the yield of the duckweed Lemna minor. We report here the complete genome sequences of A. ursingii M3 and A. excentricus M6, sequenced using the PacBio RS II platform.

Acinetobacter ursingii M3 and Asticcacaulis excentricus M6 are plant growth-inhibiting bacteria that reduce the yield of the duckweed Lemna minor. We report here the complete genome sequences of A. ursingii M3 and A. excentricus M6, sequenced using the PacBio RS II platform.

Duckweed diversity decreases heavy metal toxicity by altering the metabolic function of associated microbial communities

Mono-cultured and mix-cultured duckweed species were investigated with respect to the function of their associated microbial communities in heavy metal contaminated wastewater. Results show that the carbon source utilization patterns of the L. aequinoctialis- and S. polyrhiza-associated microbial communities were different. The relationships between microbial activity, antioxidant enzyme activity (CAT, GSH, and SOD) and growth was positive and significant. The microbial activity of L. aequinoctialis and S. polyrhiza in mixture was higher than in monoculture in low and high heavy metal, respectively, thereby altering the utilization of specific carbon source types and increasing duckweed growth and antioxidant enzyme activity, when compared to the monocultured duckweed. Furthermore, results indicate that duckweed species in mixture are protected from damage through regulation of the associated bacterial communities.

Inter- and intra-specific competition of duckweed under multiple heavy metal contaminated water

The influences of intra- and inter-species competition on ecosystems are poorly understood. Lemna aequinoctialis and Spirodela polyrhiza were used to assess the effects of exposure to different concentrations of multiple heavy metals (copper-cadmium-zinc), when the plants were grown in mixed- or mono-culture. Parameters assessed included relative growth rate (RGR), content of chlorophyll, glutathione (GSH), malondialdehyde (MDA), as well as the activity of catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD). Inter-specific competition was affected by metal concentration, with results indicating that inter-specific competition significantly affected duckweed growth and metal uptake in different heavy metal exposure conditions. Inter-specific competition increased growth rate of duckweed under high metal concentrations, although when compared with intra-specific competition, it caused no obvious differences under low metal concentrations. The growth of L. aequinoctialis was further increased in mixed culture when exposed to high metal concentrations, with inter-specific competition increasing the content of cadmium and zinc, while decreasing copper content of L. aequinoctialis compared with under intra-specific conditions. Conversely, inter-specific competition increased the content of copper and cadmium of S. polyrhiza, without causing obvious differences in zinc accumulation under high ambient concentrations. Under high metal conditions, inter-specific competition increased antioxidant enzyme activities in duckweed species, increasing resistance to heavy metals. Results show that inter-specific competition makes duckweed develop mechanisms to increase fitness and survival, such as enhancement of antioxidant enzyme activities, rather than limiting metal uptake when exposed to high concentrations of multiple metals.

Differential oxidative and antioxidative response of duckweed Lemna minor toward plant growth promoting/inhibiting bacteria

Bacteria colonizing the plant rhizosphere are believed to positively or negatively affect the host plant productivity. This feature has inspired researchers to engineer such interactions to enhance crop production. However, it remains to be elucidated whether rhizobacteria influences plant oxidative stress vis-a-vis other environmental stressors, and whether such influence is associated with their growth promoting/inhibiting ability. In this study, two plant growth-promoting bacteria (PGPB) and two plant growth-inhibiting bacteria (PGIB) were separately inoculated into axenic duckweed (Lemna minor) culture under laboratory conditions for 4 and 8 days in order to investigate their effects on plant oxidative stress and antioxidant activities. As previously characterized, the inoculation of PGPB and PGIB strains accelerated and reduced the growth of L. minor, respectively. After 4 and 8 days of cultivation, compared to the PGPB strains, the PGIB strains induced larger amounts of O₂^•-, H₂O₂, and malondialdehyde (MDA) in duckweed, although all bacterial strains consistently increased O₂^•- content by two times more than that in the aseptic control plants. Activities of five antioxidant enzymes were also elevated by the inoculation of PGIB, confirming the severe oxidative stress condition in plants. These results suggest that the surface attached bacteria affect differently on host oxidative stress and its response, which degree correlates negatively to their effects on plant growth.

Draft Genome Sequence of Aquitalea magnusonii Strain H3, a Plant Growth-Promoting Bacterium of Duckweed (Lemna minor)

Aquitalea magnusonii strain H3 is a promising plant growth-promoting bacterium for duckweed. Here, we report the draft genome sequence of strain H3 comprising 4,750,601 bp in 73 contigs. Several genes associated with plant root colonization were identified.