Composition Of Root Exudates Research Articles

Root Exudates of Ginger Induced by Ralstonia solanacearum Infection Could Inhibit Bacterial Wilt.

Bacterial wilt caused by Ralstonia solanacearum (Rs) is one of the most important diseases found in ginger; however, the disease resistance mechanisms dependent on root bacteria and exudates are unclear. In the present study, we analyzed the changes in the composition of rhizobacteria, endobacteria, and root exudates during the pathogenesis of bacterial wilt using high-throughput sequencing and gas chromatography-mass spectrometry (GC-MS). Rs caused bacterial wilt in ginger with an incidence of 50.00% and changed the bacterial community composition in both endosphere and rhizosphere. It significantly reduced bacterial α-diversity but increased the abundance of beneficial and stress-tolerant bacteria, such as Lysobacter, Ramlibacter, Pseudomonas, and Azospirillum. Moreover, the change in rhizobacterial composition induced the changes in endobacterial and root exudate compositions. Moreover, the upregulated exudates inhibited ginger bacterial wilt, with the initial disease index (77.50%) being reduced to 40.00%, suggesting that ginger secretes antibacterial compounds for defense against bacterial pathogens.

Unraveling the secrets of plant roots: Simplified method for large scale root exudate sampling and analysis in Arabidopsis thaliana

Background: Plants exude a plethora of compounds to communicate with their environment. Although much is known about above-ground plant communication, we are only beginning to fathom the complexities of below ground chemical communication channels. Studying root-exuded compounds and their role in plant communication has been difficult due to a lack of standardized methodologies. Here, we develop an interdisciplinary workflow to explore the natural variation in root exudate chemical composition of the model plant Arabidopsis thaliana. We highlight key challenges associated with sampling strategies and develop a framework for analyzing both narrow and broad scale patterns of root exudate composition in a large set of natural A. thaliana accessions. Methods: Our method involves cultivating individual seedlings in vitro inside a plastic mesh, followed by a short hydroponic sampling period in small quantities of ultrapure water. The mesh makes it easy to handle plants of different sizes and allows for large-scale characterization of individual plant root exudates in axenic conditions. This setup can also be easily extended for prolonged temporal exudate collection experiments. Furthermore, the short sampling time minimizes the duration of the experiment while still providing sufficient signal even with a small volume of sampling solution. We used ultra-high performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS) for untargeted metabolic profiling, followed by tentative compound identification using MZMine3 and SIRIUS 5 software, to capture a broad overview of root exudate composition in A. thaliana accessions. Results: Based on 28 replicates of Columbia genotype (Col-0) compared with 10 random biological controls, MZMine3 identified 354 metabolites to be present only in Col-0 by negative ionization. Of these, 313 compounds could be annotated by SIRIUS 5 software. Conclusions: The methodology developed in this study can be used to broadly investigate the role of root exudates as chemical signals in plant belowground interactions.

CO2 concentration and water availability alter the organic acid composition of root exudates in native Australian species

PurposeRoot exudation of organic acids (OAs) facilitates plant P uptake from soil, playing a key role in rhizosphere nutrient availability. However, OA exudation responses to CO2 concentrations and water availability remain largely untested.MethodsWe examined the effects of CO2 and water on OA exudates in three Australian woodland species: Eucalyptus tereticornis, Hakea sericea and Microlaena stipoides. Seedlings were grown in a glasshouse in low P soil, exposed to CO2 (400 ppm [aCO2] or 540 ppm [eCO2]) and water treatments (100% water holding capacity [high-watered] or 25–50% water holding capacity [low-watered]). After six weeks, we collected OAs from rhizosphere soil (OArhizo) and trap solutions in which washed roots were immersed (OAexuded).ResultsFor E. tereticornis, the treatments changed OArhizo composition, driven by increased malic acid in plants exposed to eCO2 and increased oxalic acid in low-watered plants. For H. sericea, low-watered plants had higher OAexuded per plant (+ 116%) and lower OArhizo per unit root mass (–77%) associated with larger root mass but fewer cluster roots. For M. stipoides, eCO2 increased OAexuded per plant (+ 107%) and per unit root mass (+ 160%), while low-watered plants had higher citric and lower malic acids for OArhizo and OAexuded: changes in OA amounts and composition driven by malic acid were positively associated with soil P availability under eCO2.ConclusionWe conclude that eCO2 and altered water availability shifted OAs in root exudates, modifying plant–soil interactions and the associated carbon and nutrient economy.

Cadmium tolerance and accumulation from the perspective of metal ion absorption and root exudates in broomcorn millet

Cadmium (Cd) is a persistent heavy metal that poses environmental and public health concerns. This study aimed to identify the potential biomarkers responsible for Cd tolerance and accumulation by investigating the response of the content of essential metal elements, transporter gene expression, and root exudates to Cd stress in broomcorn millet (Panicum miliaceum). A hydroponics experiment was conducted using two broomcorn millet cultivars with distinct Cd tolerance levels and accumulation phenotypes (Cd-tolerant and Cd-sensitive cultivars). Cd stress inhibited lateral root growth, especially in the Cd-sensitive cultivar. Furthermore, Cd accumulation was significantly greater in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. Cd stress significantly inhibited the absorption of essential metal elements and significantly increased the calcium concentration. Differentially expressed genes involved in metal ion transport were identified via transcriptome analysis. Cd stress altered the composition of root exudates, thus increasing lipid species and decreasing alkaloid, lignan, sugar, and alcohol species. Moreover, Cd stress significantly reduced most alkaloid, organic acid, and phenolic acid exudates in the Cd-tolerant cultivar, while it increased most lipid and phenolic acid exudates in the Cd-sensitive cultivar. Some significantly changed root exudates (ferulic acid, O-coumaric acid, and spermine) are involved in the phenylalanine biosynthesis, and arginine and proline metabolic pathways, thus, may be potential biomarkers of Cd stress response. Overall, metal ion absorption and root exudates are critical for Cd tolerance and accumulation in broomcorn millet. These findings provide valuable insights into improving Cd phytoremediation by applying mineral elements or metabolites.

The effect of root hairs on exudate composition: a comparative non-targeted metabolomics approach

Root exudation is a major pathway of organic carbon input into soils. It affects soil physical properties, element solubility as well as speciation, and impacts the microbial community in the rhizosphere. Root exudates contain a large number of primary and secondary plant metabolites, and the amount and composition are highly variable depending on plant species and developmental stage. Detailed information about exudate composition will allow for a better understanding of exudate-driven rhizosphere processes and their feedback loops. Although non-targeted metabolomics by high-resolution mass spectrometry is an established tool to characterize root exudate composition, the extent and depth of the information obtained depends strongly on the analytical approach applied. Here, two genotypes of Zea mays L., differing in root hair development, were used to compare six mass spectrometric approaches for the analysis of root exudates. Reversed-phase liquid chromatography and hydrophilic interaction liquid chromatography combined with time-of-flight mass spectrometry (LC-TOF-MS), as well as direct infusion Fourier-transform ion cyclotron resonance mass spectrometry (DI-FT-ICR-MS), were applied with positive and negative ionization mode. By using the same statistical workflow, the six approaches resulted in different numbers of detected molecular features, ranging from 176 to 889, with a fraction of 48 to 69% of significant features (fold change between the two genotypes of > 2 and p-value < 0.05). All approaches revealed the same trend between genotypes, namely up-regulation of most metabolites in the root hair defective mutant (rth3). These results were in agreement with the higher total carbon and nitrogen exudation rate of the rth3-mutant as compared to the corresponding wild-type maize (WT). However, only a small fraction of features were commonly found across the different analytical approaches (20–79 features, 13–31% of the rth3-mutant up-regulated molecular formulas), highlighting the need for different mass spectrometric approaches to obtain a more comprehensive view into the composition of root exudates. In summary, 111 rth3-mutant up-regulated compounds (92 different molecular formulas) were detected with at least two different analytical approaches, while no WT up-regulated compound was found by both, LC-TOF-MS and DI-FT-ICR-MS. Zea mays L. exudate features obtained with multiple analytical approaches in our study were matched against the metabolome database of Zea mays L. (KEGG) and revealed 49 putative metabolites based on their molecular formula.Graphical

Arbuscular mycorrhizal colonization alters biochemical, molecular defense responses and root exudate composition against Phytophthora capsici infection in black pepper

Mycorrhizal symbiosis benefits the host plants in several aspects. Among the soil microbiota, AM fungi have established a substantial potential to decrease crop damage from plant pathogens in several crops, yet the mechanism of its action in black pepper has not yet been studied. In this study, we investigated the effect of mycorrhizal colonization on biochemical, molecular defense responses, and root exudates composition in the Phytophthora capsici -black pepper host-pathosystem. Four biochemical parameters such as total phenols, orthodihydroxy (OD) phenols, peroxidase activity, and lignin were studied to delineate the effect of mycorrhizal colonization upon pathogen inoculation. Total phenol, OD phenol, and lignin content increased consistently upon challenge inoculation with P. capsici as well as in AM alone inoculated plants in both leaves as well as roots. Similar peroxidase activity was noticed in AM alone and challenged inoculated plants. The quantitative polymerase chain reaction analysis showed that AM pre-inoculation led to up regulation of pathogenesis related genes viz., cAPX, Osmotin and β-1,3-glucanase, phenylalanine ammonia-lyase and NPR 1 in black pepper leaves and roots upon of P. capsici inoculation. Sole inoculation of P. capsici also positively influenced the copies of most genes studied. Root exudates of AM colonized plants showed higher fatty acid/fatty acyl compounds viz., tetradecanoic acid, n-hexadecanoic acid, octadecanoic acid, and nonacos-1-ene compared to P. capsici inoculated plants. On the other hand, most of the alkane hydrocarbons such as (z)-3-tetradecene, octylcyclohexane, 4-cyclohexylundecane, and decylcyclohexane were observed majorly in the root exudates of P. capsici inoculated plants. Multivariate analysis of the root exudate data revealed that AM colonization in black pepper roots normalized the content of root exudates during Phytophthora infection comparable to control plants. Our findings indicates that the AM colonization enhances the resistance of P. capsici in black pepper plants by alterations in defense enzymes and gene expression and root exudates composition.

Root exudate composition from different plant species influences the growth of rhizosphere bacteria

Plant roots release exudates that fuel microbial activities and can structure rhizosphere microbial communities, but how different plant species use their root exudate to potentially select for different soil microbes in the rhizosphere is not well understood. Here, we investigated how root exudate from plants of three diverging lineages, Lactuca sativa (lettuce), Brassica juncea (mustard cabbage), and Zea mays (maize) influence the growth of their own rhizosphere bacteria (host) and those from other plant species (non-host) in growth bioassays. We found that on the community level, lettuce rhizosphere bacteria grew better in non-host exudate, but mustard cabbage and maize rhizosphere bacteria grew similarly well in both host and non-host exudate. However, individual bacteria taxa showed strong preferences for exudate from different plant species. The bacterial growth patterns were independent of C and N quantity, suggesting that certain exometabolic compounds may drive the growth patterns. Our results demonstrate that root exudate from a given plant species have the potential to stimulate or suppress soil bacteria and hint at a mechanism that different plant species use to select for their specific suite of rhizosphere bacteria. These findings contribute to our broader understanding of how root exudate composition could be a mechanism that plants use to select for distinct microbial communities in the rhizosphere.

Metabolomics of plant root exudates: From sample preparation to data analysis.

Plants release a set of chemical compounds, called exudates, into the rhizosphere, under normal conditions and in response to environmental stimuli and surrounding soil organisms. Plant root exudates play indispensable roles in inhibiting the growth of harmful microorganisms, while also promoting the growth of beneficial microbes and attracting symbiotic partners. Root exudates contain a complex array of primary and specialized metabolites. Some of these chemicals are only found in certain plant species for shaping the microbial community in the rhizosphere. Comprehensive understanding of plant root exudates has numerous applications from basic sciences to enhancing crop yield, production of stress-tolerant crops, and phytoremediation. This review summarizes the metabolomics workflow for determining the composition of root exudates, from sample preparation to data acquisition and analysis. We also discuss recent advances in the existing analytical methods and future perspectives of metabolite analysis.

Response of root exudates and bacterial community to N fertilization and termination methods in Avena sativa L. as a winter cover crop model

Agricultural practices associated with winter cover crops (WCCs) and nitrogen (N) fertilization could change the amount and composition of root exudates by altering the plant metabolism, ultimately affecting their interaction with microorganisms. The goal of this study was to evaluate the combined effect of N fertilization (N0 and N100, kg N ha−1), and spring growth termination methods (M: DQ, chemical suppression; CM, mechanical cut; SS: no suppression) of oat (Avena sativa L.) as a WCC model, on the chemistry of root exudates and the microbial community structure in a greenhouse. The chemical characterization of root exudates was performed by Fourier transform infrared spectroscopy (FTIR), while total phenols (TPC) and phosphorus (P) contents were measured in aqueous extracts of root exudates. Bacterial community structure and composition were assessed through sequencing in Illumina MiSeq followed by QIIME2 analysis. The root exudation pattern of oat and P content in exudates showed significant differences among M, while a significant interaction was observed between N fertilization and the M for TPC. At the rhizospheric bacterial community level, differences in composition were detected between M. Overall, both termination methods (DQ and CM) negatively affected genera considered beneficial for the plants, while the chemical suppression of oat increased the abundance of some other genera with species identified as soil-borne pathogens in the rhizosphere. FTIR spectral profiles in exudates of oat were different in plants terminated by CM compared to SS and DQ, regardless of N fertilization, partially explaining the shifts found in the composition of the rhizospheric bacterial community. Considering that decaying roots of the terminated WCC remain in soil, future studies should address whether the termination methods have effect on the next crop and its microbiome.

Drought, heat, and their combination impact the root exudation patterns and rhizosphere microbiome in maize roots

Root exudates play an essential role in helping plants to cope with abiotic stress. However, the modulation of root exudation profiles under multiple stress conditions is still poorly understood. Using targeted and untargeted metabolomics , here we tested the effect of drought, heat stress, and their combination on maize root exudates, also considering the differences that might exist between root types (seminal and primary) and root zones (apical and sub-apical). In addition, we built an analytical framework that relates the root exudation profile with the rhizosphere bacterial community , enabling us to dissect the interactions between specific root exudates and bacterial taxa. The composition of root exudates undergoes distinct modulation according to the single or combined stress and to the root zone, but not according to the root type. In addition, we found that stress-specific exudates can influence the relative abundance of specific bacterial taxa, some of which are known to be beneficial microorganisms . Our results contribute to the understanding of plant-soil interactions under the influence of abiotic stressors, which is key in paving the way towards an increased resilience to abiotic stresses, representing a powerful tool to craft the next generation of agricultural practices. • Drought, heat, and their combination have a specific influence on root exudation. • The combination of stresses influences exudates differently from the single stresses. • Changes in root exudation are associated with changes of specific microbial taxa.

Root exudates contribute to belowground ecosystem hotspots: A review.

Root exudates are an essential carrier for material cycling, energy exchange, and information transfer between the belowground parts of plants and the soil. We synthesize current properties and regulators of root exudates and their role in the belowground ecosystem as substances cycle and signal regulation. We discussed the composition and amount of root exudates and their production mechanism, indicating that plant species, growth stage, environmental factors, and microorganisms are primary influence factors. The specific mechanisms by which root secretions mobilize the soil nutrients were summarized. First, plants improve the nutrient status of the soil by releasing organic acids for acidification and chelation. Then, root exudates accelerated the SOC turnover due to their dual impacts, forming and destabilizing aggregates and MASOC. Eventually, root exudates mediate the plant–plant interaction and plant–microbe interaction. Additionally, a summary of the current collection methods of root exudates is presented.

Dysbiosis of the rhizosphere microbiome caused by γ-irradiation alters the composition of root exudates and reduces phosphorus uptake by rice in flooded soils

Dysbiosis of the rhizosphere microbiome caused by γ-irradiation alters the composition of root exudates and reduces phosphorus uptake by rice in flooded soils

Exudates from Miscanthus x giganteus change the response of a root-associated Pseudomonas putida strain towards heavy metals

The composition of root exudates is modulated by several environmental factors, and it remains unclear how that affects beneficial rhizosphere or inoculated microorganisms under heavy metal (HM) contamination. Therefore, we evaluated the transcriptional response of Pseudomonas putida E36 (a Miscanthus x giganteus isolate with plant growth promotion-related properties) to Cd, Pb and Zn in an in vitro study implementing root exudates from M. x giganteus. To collect root exudates and analyse their composition plants were grown in a pot experiment under HM and control conditions.Our results indicated higher exudation rate for plants challenged with HM. Further, out of 29 organic acids identified and quantified in the root exudates, 8 of them were significantly influenced by HM (e.g., salicylic and terephthalic acid). The transcriptional response of P. putida E36 was significantly affected by the HM addition to the growth medium, increasing the expression of several efflux pumps and stress response-related functional units. The additional supplementation of the growth medium with root exudates from HM-challenged plants resulted in a downregulation of 29% of the functional units upregulated in P. putida E36 as a result of HM addition to the growth medium. Surprisingly, root exudates + HM downregulated the expression of P. putida E36 functional units related to plant colonization (e.g., chemotaxis, motility, biofilm formation) but upregulated its antibiotic and biocide resistance compared to the control treatment without HM.Our findings suggest that HM-induced changes in root exudation pattern may attract beneficial bacteria that are in turn awarded with organic nutrients, helping them cope with HM stress. However, it might affect the ability of these bacteria to colonize plants growing in HM polluted areas. Those findings may offer an insight for future in vivo studies contributing to improvements in phytoremediation measures.

Nanomaterials and environmental antimicrobial resistance: Propagation and inhibition of antibiotic resistance gene flow in the soil-plant system

<p indent="0mm">The propagation of antibiotic resistance genes (ARGs), one of the emerging contaminants threatening global sanitation and health, has called for the necessity to overcome the spread of antimicrobial resistance (AMR). ARGs derived from manure is the main source of AMR in agricultural soil that can be propagated rapidly in soil-plant system and finally into food chain via horizontal gene transfer (HGT), which mainly includes three pathways mediated by mobile genetic elements (MGEs), namely, plasmid-mediated conjugation, extracellular DNA-mediated transformation, and phage- mediated transduction. The massive production and wide application of engineered nanomaterials (ENMs) and micro(nano) plastics have resulted in increasing concentrations of ENMs and micro(nano) plastics in the environment, which will eventually enter the soil and lead to migration and propagation of ARGs in soil-plant system. Bacterial pure culture studies reported that ENMs can exacerbate the dissemination of ARGs via changing intracellular reactive oxygen species (ROS), cell membrane permeability, and expression of genes related to conjugation. Moreover, ENMs and micro(nano) plastics in soil can change the soil antibiotic resistome and plant root morphology, structure and root exudates, which may affect the migration of ARB and ARGs from the rhizosphere to plants. The interactions between ENMs/micro(nano) plastics and ARGs might drive the development of multidrug resistance in soil-plant system. Although research targeting the effects of ENMs/micro(nano) plastics on the fate of ARGs is growing, an overview of biological impact and related mechanism is somehow lacking. In this review article, we systematically reviewed the interaction between ENMs/micro(nano) plastics and ARGs, especially in soil-plant system and attempted to explore the possibilities of nanotechnology to inhibit the transfer of antibiotic resistance gene flow. This review mainly covers: (1) Molecular mechanisms of ENMs and micro(nano) plastics regulating ARGs propagation. ENMs (i.e., Ag ENMs, CuO ENMs, TiO₂/Ag/GO ENMs and nano-graphene oxide) can promote ARGs propagation by accumulating intracellular reactive oxygen species (ROS), increasing cell membrane permeability and up-regulating of conjugation-related gene expression. However, CeO₂ ENMs and Fe₂O₃@MoS₂ ENMs have the potential to inhibit the propagation of ARGs via ROS elimination and down-regulating expression of HGT-related genes. (2) Influence of ENMs/micro(nano) plastics on ARGs in soil-plant system. ENMs/micro(nano) plastics can alter soil bacterial community and affect plant growth which will likely reduce the horizontal transfer of ARGs in soil by inhibiting the growth of soil microorganisms, reducing the abundance and diversity of ARB in soil, or changing the amount and composition of root exudates. (3) Application of nanotechnology on inhibiting antibiotic resistance gene flow. The regulatory mechanisms of nanotechnology on ARGs can be classified as: Inhibition of HGT, change of bacterial community composition, adsorption/degradation on ARGs and antimicrobial performance. Nanotechnology is promising for eliminating ARGs with high accuracy while its high cost and instability remain to be solved. For future study, more attention should be paid on the management of antibiotics, ARB and ARGs and the establishment of AMR risk assessment system to comprehensively assess the potential risk of AMR propagation via soil-plant-human pathways. Besides, the propagation of ARGs regulated by ENMs might be a challenge to distinguish and balance the advantages and disadvantages of ENMs in soil-plant system. Thus, it is urgent to comprehensively and systematically assess the environmental behavior and ecological risks of ENMs and micro(nano) plastics, contributing to the safe and efficient nano-based reduction of AMR.

Autotoxin affects the rhizosphere microbial community structure by influencing the secretory characteristics of grapevine roots.

Autotoxins secreted by roots into the soil can trigger rhizosphere microecological imbalances and affect root secretory properties resulting in conditions such as replanting disease. However, information on the effect of autotoxins on root secretion characteristics and regulation of the composition of rhizosphere microorganisms by altered root exudates is limited. In this study, autotoxin ρ-hydroxybenzoic acid (4-HBA) was added to the soil of potted grapevine seedlings, CO2 pulse-labeling, and DNA stable isotope probing were used to track the rhizosphere microbiome that assimilates root exudates. Bacterial and fungal microbiomes that assimilated plant-derived carbon were identified by high-throughput sequencing. Results showed that 4-HBA treatment altered bacterial and fungal communities in 13C-labeled organisms, with a lower abundance of beneficial bacteria (e.g., Gemmatimonas, Streptomyces, and Bacillus) and a higher abundance of potential pathogen fungi (e.g., Fusarium, Neocosmospora, Gibberella, and Fusicolla) by changing the composition of root exudates. The exogenous addition of upregulated compound mixtures of root exudates reduced the abundance of beneficial bacterial Bacillus and increased the abundance of potential pathogen fungi Gibberella. These results suggest that 4-HBA can alter root secretion properties and altered root exudates may enrich certain potential pathogens and reduce certain beneficial bacteria, thereby unbalancing the structure of the rhizosphere microbial community.

Root exudate composition reflects drought severity gradient in blue grama (Bouteloua gracilis)

Plant survival during environmental stress greatly affects ecosystem carbon (C) cycling, and plant–microbe interactions are central to plant stress survival. The release of C-rich root exudates is a key mechanism plants use to manage their microbiome, attracting beneficial microbes and/or suppressing harmful microbes to help plants withstand environmental stress. However, a critical knowledge gap is how plants alter root exudate concentration and composition under varying stress levels. In a greenhouse study, we imposed three drought treatments (control, mild, severe) on blue grama (Bouteloua gracilis Kunth Lag. Ex Griffiths), and measured plant physiology and root exudate concentration and composition using GC–MS, NMR, and FTICR. With increasing drought severity, root exudate total C and organic C increased concurrently with declining predawn leaf water potential and photosynthesis. Root exudate composition mirrored the physiological gradient of drought severity treatments. Specific compounds that are known to alter plant drought responses and the rhizosphere microbiome mirrored the drought severity-induced root exudate compositional gradient. Despite reducing C uptake, these plants actively invested C to root exudates with increasing drought severity. Patterns of plant physiology and root exudate concentration and composition co-varied along a gradient of drought severity.

Maize root exudate composition alters rhizosphere bacterial community to control hotspots of hydrolase activity in response to nitrogen supply

Improving nitrogen (N) acquisition by crops from soil is essential to reduce fertilization rates whilst maintaining yields. Plants can adapt their nutrient acquisition strategies according to N availability, which also affects soil microbial community structure, functions and activities and relies on the supply of carbon (C) for energy. We hypothesized that N deprivation would create hotspots of N- and C-acquiring hydrolase activities in maize rhizosphere through the effects of altered root exudation on the rhizosphere bacterial community. We grew maize under three N fertilization rates and combined soil zymography with the identification of rhizosphere microbial communities and non-targeted metabolic profiling of root exudates to explore enzyme hotspot formation. The rhizosphere extents of β-1,4-glucosidase (BG) and β-N-acetylglucosaminidase (NAG) activities decreased after N fertilization, narrowing by 48% and 39%, respectively, under typical field N application rates compared to zero application. Rhizosphere extents of enzyme activities were more sensitive to altered N supply than changes in the rates of enzyme activities: BG activity decreased by ∼10%, while NAG activity was unaffected. Decreases in the activities of both hydrolases and their rhizosphere extents caused by N addition correlated with reduced abundances of oligotrophs. The relative abundances of oligotrophic bacteria (e.g., Acidobacteria) decreased, while copiotrophs (e.g., Pseudomonadota and Patescibacteria) increased under the highest N application rate. Co-occurrence networks of the rhizosphere bacterial community revealed that functional units increased with BG activity, while an efficient and denser co-occurrence network supported expansion of its rhizosphere extent. The metabolic profiles of root exudates changed according to the N application rate, suggesting that their chemistry was regulated by the plant in response to N supply. The composition of root exudates and dissolved organic C and nitrate contents explained the largest variations in NAG hotspots in the rhizosphere. In summary, maize actively adjusts the composition of root exudates to increase interactions with rhizosphere bacteria, thereby stimulating hydrolase production and activities, and altering their rhizosphere extents to mobilize N and energy (C) in a larger soil volume, under conditions of N deficiency.

Ambiguities of PGPR-Induced Plant Signaling and Stress Management.

The growth and stress responses developed by the plant in virtue of the action of PGPR are dictated by the changes in hormone levels and related signaling pathways. Each plant possesses its specific type of microbiota that is shaped by the composition of root exudates and the signal molecules produced by the plant and microbes. Plants convey signals through diverse and complex signaling pathways. The signaling pathways are also controlled by phytohormones wherein they regulate and coordinate various defense responses and developmental stages. On account of improved growth and stress tolerance provided by the PGPR to plants, there exist crosstalk of signaling events between phytohormones and other signaling molecules secreted by the plants and the PGPR. This review discusses some of the important aspects related to the ambiguities of signaling events occurring in plants, allowing the interaction of PGPR with plants and providing stress tolerance to the plant.

Variation in Root Exudate Composition Influences Soil Microbiome Membership and Function

Root exudation is one of the primary processes that mediate interactions between plant roots, microorganisms, and the soil matrix, yet the mechanisms by which exudation alters microbial metabolism in soils have been challenging to unravel. Here, utilizing distinct sorghum genotypes, we characterized the chemical heterogeneity between root exudates and the effects of that variability on soil microbial membership and metabolism. Distinct exudate chemical profiles were quantified and used to formulate synthetic root exudate treatments: a high-organic-acid treatment (HOT) and a high-sugar treatment (HST). To parse the response of the soil microbiome to different exudate regimens, laboratory soil reactors were amended with these root exudate treatments as well as a nonexudate control. Amplicon sequencing of the 16S rRNA gene illustrated distinct microbial diversity patterns and membership in response to HST, HOT, or control amendments. Exometabolite changes reflected these microbial community changes, and we observed enrichment of organic and amino acids, as well as possible phytohormones in the HST relative to the HOT and control. Linking the metabolic capacity of metagenome-assembled genomes in the HST to the exometabolite patterns, we identified microorganisms that could produce these phytohormones. Our findings emphasize the tractability of high-resolution multiomics tools to investigate soil microbiomes, opening the possibility of manipulating native microbial communities to improve specific soil microbial functions and enhance crop production. IMPORTANCE Decrypting the chemical interactions between plant roots and the soil microbiome is a gateway for future manipulation and management of the rhizosphere, a soil compartment critical to promoting plant fitness and yields. Our experimental results demonstrate how soil microbial community and genomic diversity is influenced by root exudates of differing chemical compositions and how changes in this microbiome result in altered production of plant-relevant metabolites. Together, these findings demonstrate the tractability of high-resolution multiomics tools to investigate soil microbiomes and provide new information on plant-soil environments useful for the development of efficient and precise microbiota management strategies in agricultural systems.

Uncovering the phytochemicals of root exudates and extracts of lead (Pb) tolerant Chrysopogon zizanioides (L.) Roberty in response to lead contamination and their effect on the chemotactic behavior of rhizospheric bacteria.

The chemical composition of root exudates and root extracts from Chrysopogon zizanioides (L.) Roberty cv KS-1 was determined in the presence of lead [Pb(II)]. Hitherto, no information is available in the literature concerning the phytochemical components of root exudates of C. zizanioides. Significantly higher concentrations of total carbohydrates (26.75 and 42.62% in root exudates and root extract, respectively), reducing sugars (21.46 and 56.11% in root exudates and root extract, respectively), total proteins (9.22 and 23.70% in root exudates and root extract, respectively), total phenolic acids (14.69 and 8.33% in root exudates and root extract, respectively), total flavonoids (14.30 and 12.28% in root exudates and root extract, respectively), and total alkaloids (12.48 and 7.96% in root exudates and root extract, respectively) were observed in samples from plants growing under Pb(II) stress in comparison to the respective controls. GC-MS profiling showed the presence of a diverse group of compounds in root exudates and extracts, including terpenes, alkaloids, flavonoids, carotenoids, plant hormones, carboxylic/organic acids, and fatty acids. Among the detected compounds, many have an important role in plant development, regulating rhizosphere microbiota and allelopathy. Furthermore, the results indicated that C. zizanioides exudates possess a chemotactic response for rhizospheric bacterial strains Bacillus licheniformis, Bacillus subtilis, and Acinetobacter junii Pb1.