Plant Exudates Research Articles

Effect of Allelopathic Potential of some Plants Root Exudates Concerning Growth and Pathogenicity of some Fungus on Brassica oleracea Varplant

The study aims to evaluate the effectiveness of allelopathic exudates of five plant species, namely (maize, barley, Sunflower, chard, and mung bean) concerning the release of organic substances exudated from their roots and their effect on the germination and growth of brassica oleracea var plant, as well as their impact on the growth of two isolated fungi, Rhizoctona solani and Pythium ulitmum. According to the laboratory results, chard root exudates had the highest rate of Brassica oleracea var seed germination, reaching 100%, while maize plant exudates had the lowest rate, 23.33%, compared to the control treatment, which was 100%. Sunflower and mung bean plant exudates had germination rates of 50, 43%, respectively, while barley exudates had a germination rate of 90%. Furthermore, the laboratory results also revealed discrepancy in the above-mentioned root exudates regarding the reduction of the diagonal growth of the two fungi, R.solani and P. ulitmum. Obviously, 10-day-old chard root exudates outperformed in reducing the diagonal growth of both fungi, with an average colony diameter of 0.00 cm for P. ulitmum and 1.59 cm for R.solani when compared by the control treatment (water only), which amounted to 9.00 cm. On other hand, Sunflower root exudates had the highest diagonal growth of the two fungi was reaching to 4.86 cm for R.solani and 8.50 cm for P. ulitmum. The chemical analysis of these five plants’ exudates shows that the highest concentration of the terpene compound was obtained by separating this compound from chard root exudates, using HPLC method, which amounted 3.55 55 μg/g. While the least concentration was in the maize treatment, which reached 15 μg/g. Whereas the total phenolic compounds were more concentrated in the root exudates of mung bean, which reached 52.69 μg/g, and the lowest concentration was in the root exudates of Sunflower reaching 7.89 μg/g. With regard to, the ferulic acid compound, the highest rate was observed in the root exudates of mung bean, which amounted to 19.87 μg/g, then, chard control comes in second place in which the concentration reached 10.25 μg/g. while the least concentration of the same compound was in Sunflower root exudates reached 5.69 μg/g. Seemingly, the diversity and variation of concentrations of allopathic compounds exudated from the roots of these plants, which may either negatively or positively affect the germination of Brassica oleracea var plant and the accompanying pathogenic fungi, are considered to be the cause of the varying effect of the understudied plant roots’ metabolites on germination percentages of Brassica oleracea var plant as well as in the reduction of the diagonal growth of the both fungi, R. solani and P. ulitmum.

Plant-microbe community dynamics in rhizosphere: Reviewing the grassroots ecology towards sustainable agriculture

The interaction between microbes and plants in rhizospheric environment is evident regarding sustainable development in agriculture. Microbes are involved in various metabolic activities in plant systems, which in turn help in plant health improvement. Eventually, plant-microbe interactions are connected with biogeochemical cycles. In this context, metagenomic study helps us to survey the microbial diversity in their natural niches, especially in rhizospheric regions. Noticeably, a diverse group of bacteria, fungi, and archaea are likely to be involved in plant growth promoting (PGP) activities. Variation in microbial communities in the rhizosphere depends on various parameters, such as soil organic matter, plant genotype, plant exudates, crop rotation, soil pH, nutrient cycling, etc. Some abiotic factors and chemical fertilizers have negative impact on crop productivity, influencing sustainable development of environment. Despite having negative impacts from climate change, microbes cope with this altered scenario and try to adjust themselves successfully and consequently promote plant growth by nutrient acquisition and stress tolerance approaches. Therefore, climate change has appeared as a big threat to the agricultural sector in recent past and this might be persistent in near future. However, the conservation of microbial diversity in the rhizospheric regions appears as one of the most promising options for long-term environmental sustainability.

Impact of Hydrophobic and Electrostatic Forces on the Adsorption of Acacia Gum on Oxide Surfaces Revealed by QCM-D

The adsorption of Acacia gum from two plant exudates, A. senegal and A. seyal, at the solid-liquid interface on oxide surfaces was studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). The impact of the hydrophobic and electrostatic forces on the adsorption capacity was investigated by different surface, hydrophobicity, and charge properties, and by varying the ionic strength or the pH. The results highlight that hydrophobic forces have higher impacts than electrostatic forces on the Acacia gum adsorption on the oxide surface. The Acacia gum adsorption capacity is higher on hydrophobic surfaces compared to hydrophilic ones and presents a higher stability with negatively charged surfaces. The structural configuration and charge of Acacia gum in the first part of the adsorption process are important parameters. Acacia gum displays an extraordinary ability to adapt to surface properties through rearrangements, conformational changes, and/or dehydration processes in order to reach the steadiest state on the solid surface. Rheological analysis from QCM-D data shows that the A. senegal layers present a viscous behavior on the hydrophilic surface and a viscoelastic behavior on more hydrophobic ones. On the contrary, A. seyal layers show elastic behavior on all surfaces according to the Voigt model or a viscous behavior on the hydrophobic surface when considering the power-law model.

Plant-exuded chemical signals induce surface attachment of the bacterial pathogen Pseudomonas syringae.

Many plant pathogenic bacteria suppress host defenses by secreting small molecule toxins or immune-suppressing proteins into host cells, processes that likely require close physical contact between pathogen and host. Yet, in most cases, little is known about whether phytopathogenic bacteria physically attach to host surfaces during infection. Here we report that Pseudomonas syringae pv. tomato strain DC3000, a Gram-negative bacterial pathogen of tomato and Arabidopsis, attaches to polystyrene and glass surfaces in response to chemical signals exuded from Arabidopsis seedlings and tomato leaves. We characterized the molecular nature of these attachment-inducing signals and discovered that multiple hydrophilic metabolites found in plant exudates, including citric acid, glutamic acid, and aspartic acid, are potent inducers of surface attachment. These same compounds were previously identified as inducers of P. syringae genes encoding a type III secretion system (T3SS), indicating that both attachment and T3SS deployment are induced by the same plant signals. To test if surface attachment and T3SS are regulated by the same signaling pathways, we assessed the attachment phenotypes of several previously characterized DC3000 mutants, and found that the T3SS master regulator HrpL was partially required for maximal levels of surface attachment, whereas the response regulator GacA, a negative regulator of T3SS, negatively regulated DC3000 surface attachment. Together, our data indicate that T3SS deployment and surface attachment by P. syringae may be co-regulated by the same host signals during infection, possibly to ensure close contact necessary to facilitate delivery of T3SS effectors into host cells.

Tomato defence against Meloidogyne incognita by jasmonic acid-mediated fine-tuning of kaempferol homeostasis.

Jasmonic acid (JA) is involved in the modulation of defence and growth activities in plants. The best-characterized growth-defence trade-offs stem from antagonistic crosstalk among hormones. In this study, we first confirmed that JA negatively regulates root-knot nematode (RKN) susceptibility via the root exudates (REs) of tomato plants. Omics and toxicological analyses implied that kaempferol, a type of flavonol, from REs has a negative effect on RKN infection. We demonstrated that SlMYB57 negatively regulated kaempferol contents in tomato roots, whereas SlMYB108/112 had the opposite effect. We revealed that JA fine-tuned the homeostasis of kaempferol via SlMYB-mediated transcriptional regulation and the interaction between SlJAZs and SlMYBs, thus ensuring a balance between lateral root (LR) development and RKN susceptibility. Overall, this work provides novel insights into JA-modulated LR development and RKN susceptibility mechanisms and elucidates a trade-off model mediated by JA in plants encountering stress.

Brazilian Brown Propolis: an Overview About Its Chemical Composition, Botanical Sources, Quality Control, and Pharmacological Properties.

The online version contains supplementary material available at 10.1007/s43450-023-00374-x.

Chaenothecopsis (Mycocaliciales, Ascomycota) from exudates of endemic New Zealand Podocarpaceae.

The order Mycocaliciales (Ascomycota) comprises fungal species with diverse, often highly specialized substrate ecologies. Particularly within the genus Chaenothecopsis, many species exclusively occur on fresh and solidified resins or other exudates of vascular plants. In New Zealand, the only previously known species growing on plant exudate is Chaenothecopsisschefflerae, found on several endemic angiosperms in the family Araliaceae. Here we describe three new species; Chaenothecopsismatai Rikkinen, Beimforde, Tuovila & A.R. Schmidt, C.nodosa Beimforde, Tuovila, Rikkinen & A.R. Schmidt, and C.novae-zelandiae Rikkinen, Beimforde, Tuovila & A.R. Schmidt, all growing on exudates of endemic New Zealand conifers of the Podocarpaceae family, particularly on Prumnopitystaxifolia. Phylogenetic analyses based on ribosomal DNA regions (ITS and LSU) grouped them into a distinct, monophyletic clade. This, as well as the restricted host range, suggests that all three taxa are endemic to New Zealand. Copious insect frass between the ascomata contain ascospores or show an early stage of ascomata development, indicating that the fungi are spread by insects. The three new species represent the first evidence of Chaenothecopsis from any Podocarpaceae species and the first from any gymnosperm exudates in New Zealand.

Rhizobiome Signature and Its Alteration Due to Watering in the Wild Plant Moringa oleifera

Metagenomic approach was used to detect microbial gene abundance and relative abundance in the rhizosphere of Moringa oleifera and surrounding bulk soil and to detect the response of soil microbes to watering. Expectedly, the number and abundance of non-redundant genes were extremely higher in bacteria followed by archaea, eukaryota and viruses. Results demonstrated unexpected high abundance of some microbes (ex., endophyte genus Nocardioides) in the rhizosphere that are supposed to exist mainly in other rhizocompartments. We suggest this differential distribution of microbes is due to the specific pattern of host-microbe interaction. Other endosymbiont microbes, ex., fungi Mucoromycota and Ascomycota, were highly abundant in the bulk soil possibly because they are phytopathogens where plant exudates might inhibit their growth or force these fungi to approach reverse chemotaxis. Our data indicated high abundance of other symbiont microbes in the rhizosphere of M. oleifera at phylum (ex., Actinobacteria) and genus (ex., Streptomyces) levels. Watering experiment indicated that phylum Actinobacteria and the descending genus Streptomyces are among the highest. Rhizobiome of M. oleifera seems to harbor a wealth of new species of the genus Streptomyces that are required to be deciphered for function in order to be eventually utilized in pharmaceutical and agricultural applications.

Extraction kinetics and physicochemical characteristics of Colombian propolis

AbstractPropolis, a resinous matter collected by Apis mellifera bees from exudates of plants, exhibits different biological properties, such as antimicrobial, antifungal, antiviral, and antioxidant activities. Standardization of the extraction of their bioactive compounds is a complex issue in particular for rural regions in developing countries due to the need for specialized equipment and personnel. Herein, kinetic extractions by maceration over 7 days were compared with ultrasound and supercritical fluids extractions in terms of the polyphenolic content obtained. Ultrasound showed the fastest kinetic extraction for each propolis without affecting the antioxidant activity. Supercritical fluids showed the lowest polyphenolic content recovery and no selectivity for antioxidant activity, in agreement with the solubility predictions. The main difference among the extraction methods was the extraction time required to reach the equilibrium concentration. No significant differences in selectivity were observed for the different methods. The antioxidant activity remained almost constant with the extraction method and extraction time, indicating similar extraction kinetics for the extracted polyphenols. We found that the physicochemical characteristics and UV‐vis spectra could be used as predictors of the polyphenolic contents for the studied samples.Practical ApplicationEven though most of the beneficial properties to the human health of the propolis extracts depend largely on the compounds present in the matrix and their extractability, the evaluation of these compounds and their biological activities might be costly and time‐consuming for beekeepers, particularly in developing countries. Also, the extraction method and the extraction time are important decisions that significantly affect the economics of the propolis production process. The main difference among extraction methods was the extraction time required to reach equilibrium and no significant differences in selectivity were observed for the different methods. Moreover, it was found that simple parameters such as ash and moisture can be used as predictors of the polyphenolic contents and antioxidant activity of the propolis, and they can provide important information about the handling during harvest, transport, and storage of the propolis.

РОЛЬ АУКСИНОВ И ИХ ПЕРЕНОСЧИКОВ ПРИ НИЗКОЙ ИНТЕНСИВНОСТИ СВЕТА В РЕГУЛЯЦИИ ВЕТВЛЕНИЯ КОРНЕЙ ПШЕНИЦЫ

Light is vital for any plant, so there are many mechanisms for adapting to its deficiency. For example, when shading by other plants, the stems are elongated due to the resources going to other organs, in particular, roots, the branching of which may decrease. The decrease in root branching under the influence of low light is explained by a decrease in auxin transport (IAA) from shoots to roots. However, data on the effect of low light on auxins is rather contradictory. The data obtained in this work indicate a decrease in the concentration of auxins in the roots and phloem exudate of plants at low light intensity, which indicates the role of auxins in the transmission of a signal from the shoot about this external factor and its effect on the level of auxins in roots. The root growth response to low light was manifested as a decrease in branching, and since auxins are known to be required to induce lateral root growth, this low light effect appears to be related to the long-range signal from the leaves in the form of reduced delivery of auxins through the phloem.

Determination of Inhibition Effect of Propolis Extract on Watermelon mosaic virus in Edible Seed Squash

Edible seed squash (Cucurbita pepo L, ESS), which is grown for its seeds to consume as a snack, has a very important place in vegetable production in Türkiye. Viruses are one of the most problems in the cultivation of ESS in the country. One of these diseases is Watermelon mosaic virus (WMV, Potyvirus, Potyviridae). Also, it’s very common and devastating on other cucurbits worldwide. Propolis, which is composed mainly by the plant resins and exudates that honey bees gather, showed antiviral activity against plant and human viruses. In this study, the potential of propolis to control WMV was investigated by taking advantage of these antimicrobial properties of propolis. For this purpose, propolis extracted with 95% ethanol and diluted in distilled water to obtain different concentrations as 2, 4, 6, 8 and 10% were used. Effects of propolis against WMV were determined by in vitro and in vivo studies. Except of 2, and 4%, all concentrations caused to symptoms reductions of the infection, in all studies. In the result of in vitro studies, the ratios of healthy plants were calculated as 10, 20, 40% and 20, 30, 50% after one and two hours, respectively. In vivo, by spraying concentrations of 6, 8 and 10% before inoculation obtained healthy plants as 20, 40 and 60% after one-hour period, and as 20, 30 and 50% after two-hour period, while the extracts sprayed after inoculation, ratios of healthy plants as 10, 20 and 30% after one-hour period, and as 10, 20 and 20% after two-hour period. According to the results of the study, it was determined that applications of different propolis concentrations had the potential to reduce WMV infection, and these results should be supported by field trials.

A Comparative Study Between Froth Flotation And Gravity Separation Method To Recover Fossil Resins From Bukan Kwato Coal, Lafia, Nassarawa State, Nigeria

Background: Fossil resins are sticky plants secretions that harden on atmospheric exposure overtime, and has been found within coal and other sedimentary rocks. They are formed tens of millions of years ago through the diagenetic or catagenic transformations of plant exudates induced by various geochemical processes. The recovery of fossil resins from Bukan Kwato using froth flotation method and gravity separation method is the focus of this research. Results: The two methods were then compared in order to see which has the highest recovery. The obtained samples were analyzed using sieve analysis. It was also analyzed using XRF analysis which reveals that the samples have 2.09% of carbon and 2.72% of sulphur from froth flotation method. It also has 2.08% of carbon and 2.41% of sulphur from gravity separation method. Conclusively: it was observed that froth flotation method has the highest recovery and is more efficient.

Mobility of oxathiapiprolin in and between tomato plants.

Oxathiapiprolin (OXPT; FRAC code 49) is a new piperidinyl-thiazole isooxazoline anti-oomycete fungicide that targets oxysterol-binding proteins. The fungicide is known to translocate acropetally from root to shoot to protect plants against fungal attack. OXPT is ambimobile. It can also translocate basipetally from shoot to root. OXPT exhibits an unprecedented capacity for trans-plant protection. When two tomato plants are grown in one pot, and one is treated with OXPT (on the stem, leaves or apex), while the other plant and soil surface are adequately covered, both plants become protected against late blight caused by Phytophthora infestans. Trans-plant systemic protection induced by OXPT involves translocation of the fungicide from the shoot of the treated plant to its root, exudation into the soil and uptake by the root of the neighboring untreated plant to protect it against the disease. Liquid chromatography-tandem mass spectrometry analyses confirmed the occurrence of OXPT in root exudates of OXPT-treated tomato plants in quantities sufficient to protect detached tomato leaves and intact plants against P. infestans. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Phytochemical Profile, Plant Precursors and Some Properties of Georgian Propolis.

Propolis (bee glue) is a resinous substance produced by different species of bees i.a. from available plant resins, balsams, and exudates. It is characterized by significant biological activity (e.g., antimicrobial and antioxidant) and phytochemical diversity related to the available plant sources in specific geographical regions. The available scientific literature on propolis is quite extensive; however, there are only a few reports about propolis originating from Georgia. Therefore, our research was focused on the characterization of Georgian propolis in terms of phytochemical composition and antimicrobial/antioxidant activity. Performed research included UHPLC-DAD-MS/MS phytochemical profiling, determination of total phenolic and flavonoid content, antiradical and antioxidant activity (DPPH and FRAP assays) as well as antibacterial activity of propolis extracts obtained using 70% ethanol (70EE). Georgian propolis extracts exhibited strong activity against Gram-positive bacteria (22 mm-disc assay/64 µg/mL-MIC for S. aureus, sample from Imereti) and weaker against Gram-negative strains as well as strong antioxidant properties (up to 117.71 ± 1.04 mgGAE/g in DPPH assay, up to 16.83 ± 1.02 mmol Fe2+/g in FRAP assay for samples from Orgora and Qvakhreli, respectively). The phytochemical profile of Georgian propolis was characterized by the presence of flavonoids, free phenolic acids, and their esters. In most of the samples, flavonoids were the main chemical group (52 compounds), represented mainly by 3-O-pinobanksin acetate, pinocembrin, chrysin, galangin, and pinobanksin. The primary plant precursor of the Georgian bee glue is black poplar (Populus nigra L.) while the secondary is aspen poplar (P. tremula L.).

Histological, Immunohistochemical and Antioxidant Analysis of Skin Wound Healing Influenced by the Topical Application of Brazilian Red Propolis.

Skin wound healing is a complex process that requires the mutual work of cellular and molecular agents to promote tissue restoration. In order to improve such a process, especially in cases of impaired healing (e.g., diabetic ulcer, chronic wounds), there is a search for substances with healing properties and low toxicity: two features that some natural products-such as the bee product named propolis-exhibit. Propolis is a resinous substance obtained from plant resins and exudates with antioxidant, anti-inflammatory, and antitumoral activities, among other biological ones. Based on the previously reported healing actions of different types of propolis, the Brazilian red propolis (BRP) was tested for this matter. A skin wound excision model in male Wistar rats was performed using two topical formulations with 1% red propolis as treatments: hydroalcoholic extract and Paste. Macroscopical, histological and immunohistochemical analysis were performed, revealing that red propolis enhanced wound contraction, epithelialization, reduced crust formation, and modulated the distribution of healing associated factors, mainly collagen I, collagen III, MMP-9, TGF-β3 and VEGF. Biochemical analysis with the antioxidants SOD, MPO, GSH and GR showed that propolis acts similarly to the positive control, collagenase, increasing these molecules' activity. These results suggest that BRP promotes enhanced wound healing by modulating growth factors and antioxidant molecules related to cutaneous wound healing.

Gum Arabic protects the rat heart from ischemia/reperfusion injury through anti-inflammatory and antioxidant pathways

Gum Arabic (GA) is a plant exudate with antioxidant and anti-inflammatory effects. GA has shown promise in protection from and treatment of kidney failure, however, its role in the protection of the heart from ischemia and reperfusion (I/R) has not been investigated. This study investigated the antioxidant and anti-inflammatory effects of Gum Arabic (GA) in the protection of the heart against ischemia/reperfusion (I/R) injury. Langendorff-perfused Wistar rat hearts were divided into seven groups. One group which was subjected to I/R with no other treatment served as the control group. The second group was subjected to buffer perfusion with no ischemia (sham group). The third group was perfused with GA in the absence of ischemia (sham + GA). The rest of the hearts were isolated from rats that had been treated with GA for 4 or 2 weeks in the drinking water, or GA that had been infused intravenously 2 h before sacrifice or added to perfusion buffer at reperfusion. Hemodynamics data were digitally computed; infarct size was measured using 2,3,5-triphenyltetrazolium chloride (TTC) staining and cardiomyocyte injury was assessed by quantifying creatine kinase (CK) and lactate dehydrogenase (LDH) enzymes. The total oxidants (TOS) and antioxidants (TAS), superoxide dismutase (SOD) and pro- and anti-inflammatory cytokines levels were estimated by ELISA. GA treatment for 2 weeks, 4 weeks or 2 hours before sacrifice resulted in a significant (P < 0.05) improvement in cardiac hemodynamics and reduction in infarct size and cardiac enzyme levels compared to respective controls. However, GA administration at the time of reperfusion did not protect the hearts against I/R injury. Furthermore, GA treatment decreased the pro-inflammatory and anti-inflammatory cytokines levels. The levels of TOS in the effluent were significantly decreased (P < 0.05) and SOD levels were significantly (P < 0.05) increased by GA administration. GA protected the heart against I/R injury when administered for 2 or 4 weeks or when infused 2 hours before sacrifice. GA treatment decreased the total oxidants levels, the pro-inflammatory cytokines TNF-α, IL-1β and IL-6 protein levels and increases SOD and anti-inflammatory cytokine IL-10 protein levels.

&lt;i&gt;Trichoderma spp.&lt;/i&gt; strains: growth and interaction at different temperatures

The paper examines the growth of Trichoderma spp. strains and their interaction at different temperatures. Trichoderma spp. is a genus of microscopic ascomycetes; these fungi belong to different ecosystems and are found in soil (rhizosphere). The study aims to analyze and compare the characteristics pertaining to the growth of Trichoderma species and the interaction between them. The experiments used the following media: a Czapek medium and a medium simulating plant root exudates (Cucumis sativus). The growth rate of Trichoderma spp. strains was determined at different temperatures. The use of a medium containing plant exudate models provided a different growth rate as compared to that obtained when using a rich growth medium; some variations in the macromorphology of colonies were also observed. The growth of Trichoderma longibrachiatum F2124 and Trichoderma viride F2001 strains was detected at 9 °С on the Czapek medium, while only the Trichoderma longibrachiatum F2124 strain was observed to grow on the medium simulating exudates. A significant growth inhibition was noted at 40 °С for all strains, except for Trichoderma longibrachiatum species. No colony growth was observed at 50 °С. All the strains grew within the temperature range of 12 to 28 °С. It was assumed that Trichoderma species growing in various media may interact differently with each other. Interaction between the three strains of Trichoderma spp. was examined via the surface culture method under different temperature conditions. The study found no significant differences in the interaction between the Trichoderma species.

Plant Hormone and Inorganic Ion Concentrations in the Xylem Exudate of Grafted Plants Depend on the Scion-Rootstock Combination.

In grafted plants, inorganic ions and plant hormones in the xylem exudate transported from the rootstock to the scion directly or indirectly affect the scion, thereby improving the traits. Therefore, the concentration of these components in the xylem exudate of grafted plants may be an indicator for rootstock selection. On the other hand, few reports have presented a comprehensive analysis of substances transferred from the rootstock to the scion in plants grafted onto different rootstocks, primarily commercial cultivars. In this study, we measured inorganic ions and plant hormones in the xylem exudate from the rootstock to the scion in various grafted plants of tomato and eggplant. The results revealed that the concentrations of inorganic ions and plant hormones in the xylem exudate significantly differed depending on the type of rootstock. In addition, we confirmed the concentration of the inorganic ions and plant hormones in the xylem exudate of plants grafted onto the same tomato rootstock cultivars as rootstock with tomato or eggplant as the scions. As a result, the concentrations of inorganic ions and plant hormones in the xylem exudate were significantly different in the grafted plants with eggplant compared with tomato as the scion. These results suggest that signals from the scion (shoot) control the inorganic ions and plant hormones transported from the rootstock (root).

Root exudates enhanced rhizobacteria complexity and microbial carbon metabolism of toxic plants

SummaryRoot exudates and rhizosphere microorganisms play key roles in the colonization of toxic plants under climate change and land degradation. However, how root exudates affect the rhizosphere microorganisms and soil nutrients of toxic plants in degraded grasslands remains unknown. We compared the interaction of soil microbial communities, root exudates, microbial carbon metabolism, and environmental factors in the rhizosphere of toxic and non-toxic plants. Deterministic processes had a greater effect on toxic than non-toxic plants, as root exudates affected rhizosphere microorganisms directly. The 328 up-regulated compounds in root exudates of toxic plants affected the diversity of rhizosphere microorganisms. Rhizosphere bacteria-enriched enzymes were involved in the phenylpropanoid biosynthesis pathway. Root exudates of toxic plants form complex networks of rhizosphere microorganisms, provide high rhizosphere nutrients, and increase microbial carbon metabolism. The interaction between root exudates and rhizosphere microorganisms is the key mechanism that enables toxic plants to spread in degraded grassland habitats.

Citric acid secretion from rice roots contributes to reduction and immobilization of Cr(VI) by driving microbial sulfur and iron cycle in paddy soil

Root exudates released by plants can promote microbial growth and activity, thereby affecting the transformation and availability of soil pollutants. However, the effects of the root exudates of rice plants on chromium (Cr) transformation in paddy soils and the underlying mechanisms are yet to be elucidated properly. The present study investigated how rice root exudates interact with rhizosphere microorganisms to influence the transformation of Cr and explored the key components in root exudates that affect Cr(VI) reduction. The results showed that the addition of root exudate and citric acid markedly decreased soil pH and increased dissolved organic carbon content that created favorable conditions and provided sufficient electron donors for Cr(VI) reduction, thereby greatly facilitating the reduction of Cr(VI) and the transformation of HOAc-extractable Cr into more stable oxidizable and residual Cr. Additionally, Desulfovibrio-related sulfate-reducing bacteria, Thiobacillus-related sulfide-oxidizing bacteria, and Geobacter-related Fe(III)-reducing bacteria were enriched with the addition of root exudate and citric acid. Among them, sulfate would be reduced by Desulfovibrio to sulfide, which would be further utilized by Thiobacillus to reduce Cr(VI), thereby enabling the continuous reduction of Cr(VI); simultaneously, Geobacter would sustain the reduction of Cr(VI) by reducing Fe(III) to Fe(II). Furthermore, based on the high-level secretion of citric acid in response to Cr(VI) exposure and the similar effects of root exudates and citric acid on Cr(VI) reduction, it is proposed that citric acid is the key component in rice root exudates that affects Cr(VI) reduction. These results suggest that root exudates (citric acid as the key component) contribute to the reduction and immobilization of Cr(VI) by driving microbial S and Fe cycles, with Desulfovibrio, Thiobacillus, and Geobacter being the keystone genera. The study provides a novel insight into the Fe/S/Cr co-transformation processes with microbial involvement, and the artificial root exudate mixtures designed to reduce Cr(VI).