Ginsenosides stimulate the growth of soilborne pathogens of American ginseng

Cell-Free Extracts of the Ginseng Soil Bacterium Pseudomonas plecoglossicida Promote Suppression of Resistance of American Ginseng (Panax quinquefolius) to Root Rot Caused by Ilyonectria mors-panacis

A prior report showed that soil previously planted with American ginseng (Panax quinquefolius) contained compound(s) which could reduce ginseng resistance to root infection by Ilyonectria mors-panacis, and this was not found in extracts from ginseng roots or soils not previously planted with ginseng. However, the origin of this ginseng-related factor in ginseng soils is unknown. An isolate of Pseudomonas plecoglossicida obtained from soil where P. quinquefolius had been harvested grew more in culture media when ginseng root extract was included, indicating the use of compounds in the extract as nutrients. Treatment with cell-free extracts from media containing ginseng root extracts where P. plecoglossicida had been cultured resulted in root lesions caused by I. mors-panacis being significantly larger than roots treated with fresh media containing root extract or with cell-free media inoculated with the same bacterial isolate without root extract. Levels of ginsenosides in the media decreased over time with incubation. Genome sequencing revealed that the bacterium had genes homologous to those reported for ginsenoside metabolism, which can release sugars for microbial growth. Thus, a ginseng soil bacterium, P. plecoglossicida, can create compound(s) suppressive to root rot resistance, similar to that found in soils previously planted with ginseng, indicating that the activity suppressing root rot resistance in soil previously planted with ginseng may be of microbial origin, utilizing compounds from ginseng roots.

Analysis Transcriptome and Phytohormone Changes Associated with the Allelopathic Effects of Ginseng Hairy Roots Induced by Different-Polarity Ginsenoside Components

The allelopathic autotoxicity of ginsenosides is an important cause of continuous cropping obstacles in ginseng planting. There is no report on the potential molecular mechanism of the correlation between polarity of ginsenoside components and their allelopathic autotoxicity. This study applied a combination of metabolomics and transcriptomics analysis techniques, combined with apparent morphology, physiological indexes, and cell vitality detection of the ginseng hairy roots, through which the molecular mechanism of correlation between polarity and allelopathic autotoxicity of ginsenosides were comprehensively studied. The hairy roots of ginseng presented more severe cell apoptosis under the stress of low-polarity ginsenoside components (ZG70). ZG70 exerted allelopathic autotoxicity by regulating the key enzyme genes of cis-zeatin (cZ) synthesis pathway, indole-3-acetic acid (IAA) synthesis pathway, and jasmonates (JAs) signaling transduction pathway. The common pathway for high-polarity ginsenoside components (ZG50) and ZG70 to induce the development of allelopathic autotoxicity was through the expression of key enzymes in the gibberellin (GA) signal transduction pathway, thereby inhibiting the growth of ginseng hairy roots. cZ, indole-3-acetamid (IAM), gibberellin A1 (GA1), and jasmonoyl-L-isoleucine (JA-ILE) were the key response factors in this process. It could be concluded that the polarity of ginsenoside components were negatively correlated with their allelopathic autotoxicity.

Effect of Soil and Root Extracts on the Innate Immune Response of American Ginseng (Panax quinquefolius) to Root Rot Caused by Ilyonectria mors-panacis

Panax quinquefolius shows much higher mortality to Ilyonectria mors-panacis root rot when grown in soil previously planted with ginseng than in soil not previously planted with ginseng, which is known as ginseng replant disease. Treatment of ginseng roots with methanol extracts of previous ginseng soils significantly increased root lesion sizes due to I. mors-panacis compared to roots treated with water or methanol extracts of ginseng roots or non-ginseng soils. Inoculation of water-treated roots with I. mors-panacis increased expression of a basic chitinase 1 gene (PqChi-1), neutral pathogenesis-related protein 5 gene (PqPR5) and pathogenesis-related protein 10-2 gene (PqPR10-2), which are related to jasmonic acid (JA), ethylene (ET) or necrotrophic infection, and also increased expression of an acidic β-1-3-glucanase gene (PqGlu), which is related to salicylic acid (SA). Infection did not affect expression of a cysteine protease inhibitor gene (PqCPI). Following infection, roots treated with ginseng root extract mostly showed similar expression patterns as roots treated with water, but roots treated with previous ginseng soil extract showed reduced expression of PqChi-1, PqPR5, PqPR10-2 and PqCPI, but increased expression of PqGlu. Methanol-soluble compound(s) in soil previously planted with ginseng are able to increase root lesion size, suppress JA/ET-related gene expression and trigger SA-related gene expression in ginseng roots during I. mors-panacis infection, and may be a factor contributing to ginseng replant disease.

Application of UPLC-Orbitrap-HRMS targeted metabolomics in screening of allelochemicals and model plants of ginseng

Continuous cropping of ginseng leads to serious declines in yield and quality because of self-toxicity of allelochemicals and other factors in soil. However, because of the long growth cycle and low survival rate of ginseng, rapid screening of autotoxic activity is difficult. Therefore, it is important to analyze the allelochemicals and identify a model plant with autotoxic responses similar to those of ginseng. In this study, UPLC-Orbitrap-HRMS targeted metabolomics and verification of autotoxic activity were used to analyze a problem soil from continuously cropped ginseng. Allelochemical markers were screened by OPLS-DA. Seeds and seedlings of maize, Chinese cabbage, cucumber, green beans, wheat, sunflower, and oats were selected to identify potential model plants. Model plants with autotoxic responses similar to those of ginseng were evaluated by comparing morphological, physiological, and biochemical characteristics. The n-butanol extract of the continuously cropped problem soil had the most significant autotoxic activity. Twenty-three ginsenosides and the contributions to autotoxic effects were screened and evaluated. Of potential model plants, seeds and seedlings of cucumber showed similar growth inhibition to that of ginseng under the action of allelochemicals. Thus, metabolomics can be used to screen allelochemicals in soil and predict the autotoxic effects, and the cucumber plant model can be used to rapidly screen allelopathic activity of ginseng. The study will provide reference for methodology in allelopathy research on ginseng.

Ginsenosides and Biotic Stress Responses of Ginseng

Ginsenosides are saponins that possess a sugar moiety attached to a hydrophobic aglycone triterpenoid. They have been widely studied for their various medicinal benefits, such as their neuroprotective and anti-cancer activities, but their role in the biology of ginseng plants has been much less widely documented. In the wild, ginsengs are slow-growing perennials with roots that can survive for approximately 30 years; thus, they need to defend themselves against many potential biotic stresses over many decades. Biotic stresses would be a major natural selection pressure and may at least partially explain why ginseng roots expend considerable resources in order to accumulate relatively large amounts of ginsenosides. Ginsenosides may provide ginseng with antimicrobial activity against pathogens, antifeedant activity against insects and other herbivores, and allelopathic activity against other plants. In addition, the interaction of ginseng with pathogenic and non-pathogenic microorganisms and their elicitors may trigger increases in different root ginsenosides and associated gene expression, although some pathogens may be able to suppress this behavior. While not covered in this review, ginsenosides also have roles in ginseng development and abiotic stress tolerance. This review shows that there is considerable evidence supporting ginsenosides as important elements of ginseng's defense against a variety of biotic stresses.

Effect of ginsenosides on microbial community and enzyme activity in continuous cropping soil of ginseng

Root exudates contain plant metabolites secreted by the roots into the soil, such as ginsenosides secreted by the ginseng root. However, little is known about ginseng root exudate and its impact on the chemical and microbial properties of soil. In this study, the effect of increasing concentrations of ginsenosides on the chemical and microbial properties of soil was tested. Chemical analysis and high-throughput sequencing techniques were used to evaluate the soil chemical properties and microbial characteristics following exogenous application of 0.1 mg·L^-1, 1 mg·L^-1, and 10 mg·L^-1 ginsenosides. Ginsenosides application significantly altered soil enzyme activities; SOM-dominated physicochemical properties were significantly reduced which altered the composition and structure of the soil microbial community. In particular, treatment with 10 mg∙L^-1 ginsenosides significantly increased the relative abundance of pathogenic fungi such as Fusarium, Gibberella and Neocosmospora. These findings indicate that ginsenosides in root exudates are important factors that may lead to increased deterioration of soil during ginseng cultivation and provided new research direction for the subsequent study on the mechanism of interaction between ginsenosides and soil microbial communities.

Foliar Pathogen Infection Manipulates Soil Health through Root Exudate-Modified Rhizosphere Microbiome

Negative plant-soil feedback (NPSF) due to the buildup of soilborne pathogens in soil is a major obstacle in sustainable agricultural systems. Beneficial rhizosphere microfloras are recruited by plants, and mediating this has become a strategic priority to manipulate plant health. Here, we found that foliar infection of Panax notoginseng by Alternaria panax changed plant-soil feedback from negative to positive. Foliar infection modified the rhizosphere soil microbial community and reversed the direction of the buildup of the soilborne pathogen Ilyonectria destructans and beneficial microbes, including Trichoderma, Bacillus, and Streptomyces, in rhizosphere soil. These beneficial microbes not only showed antagonistic ability against the pathogen I. destructans but also enhanced the resistance of plants to A. panax. Foliar infection enhanced the exudation of short- and long-chain organic acids, sugars, and amino acids from roots. In vitro and in vivo experiments validated that short- and long-chain organic acids and sugars play dual roles in simultaneously suppressing pathogens but enriching beneficial microbes. In summary, foliar infection could change root secretion to drive shifts in the rhizosphere microbial community to enhance soil health, providing a new strategy to alleviate belowground disease in plants through aboveground inducement. IMPORTANCE Belowground soilborne disease is the main factor limiting sustainable agricultural production and is difficult to manage due to the complexity of the soil environment. Here, we found that aboveground parts of plants infected by foliar pathogens could enhance the secretion of organic acids, sugars, and amino acids in root exudates to suppress soilborne pathogens and enrich beneficial microbes, eventually changing the plant and soil feedback from negative to positive and alleviating belowground soilborne disease. This is an exciting strategy by which to achieve belowground soilborne disease management by manipulating the aboveground state through aboveground stimulation.

Deciphering the transcriptomic response of Ilyonectria robusta in relation to ginsenoside Rg1 treatment and the development of Ginseng rusty root rot

Rusty root rot is a severe disease in Ginseng (Panax ginseng C.A.Mey.) production caused by Ilyonectria robusta. The severity of the disease may be related to the residual ginsenosides in soil. In order to elucidate the response mechanism between Rg1 treatment and occurrence of ginseng rust, we performed growth, reproduction and transcriptome analysis on Rg1-treated. The results showed that Rg1 significantly promoted the mycelial growth, and sporulation compared to the control, and aggravated the disease symptoms of Panax ginseng. A total of 6708 transcripts out of 213 131 annotated genes identified from global transcriptomic analysis were differentially expressed in Ilyonectria robusta grown in the Rg1 treatment. These genes were found to be related to the carbon-nitrogen metabolism, transport, and assimilation. Many of these genes were also associated with pathogenicity based on the Phi-base database. Several transcription factors were related to specific biological processes, such as nitrogen utilization. The current results revealed that Rg1 played a major role in the development of rusty root rot by promoting fungal cell growth and affected the expression of genes required for pathogenesis. Rg1 could aggravate the invasion of Ilyonectria robusta on ginseng root, which preliminarily revealed the reason for the aggravation of rusty root rot in ginseng soil-borne.

The Rhizosphere Microbiome of Ginseng

The rhizosphere of ginseng contains a wide range of microorganisms that can have beneficial or harmful effects on the plant. Root exudates of ginseng, particularly ginsenosides and phenolic acids, appear to select for particular microbial populations through their stimulatory and inhibitory activities, which may account for the similarities between the rhizosphere microbiomes of different cultivated species of Panax. Many practices of cultivation attempt to mimic the natural conditions of ginseng as an understory plant in hilly forested areas. However, these practices are often disruptive to soil, and thus the soil microbiome differs between wild and cultivated ginseng. Changes in the microbiome during cultivation can be harmful as they have been associated with negative changes of the soil physiochemistry as well as the promotion of plant diseases. However, isolation of a number of beneficial microbes from the ginseng rhizosphere indicates that many have the potential to improve ginseng production. The application of high-throughput sequencing to study the rhizosphere microbiome of ginseng grown under a variety of conditions continues to greatly expand our knowledge of the diversity and abundance of those organisms as well as their impacts of cultivation. While there is much more to be learnt, many aspects of the ginseng rhizosphere microbiome have already been revealed.

Autotoxin Rg1 Induces Degradation of Root Cell Walls and Aggravates Root Rot by Modifying the Rhizospheric Microbiome

Management of crop root rot disease is one of the key factors in ensuring sustainable development in agricultural production. The accumulation of autotoxins and pathogens in soil has been reported as a primary driver of root rot diseases; however, less is known about the correlation of plants, their associated pathogens and microbiome mediated by autotoxins as well as the contributions autotoxins make to the occurrence of root rot disease. Here, we integrated metabolomic, transcriptomic, and rhizosphere microbiome analyses to identify the root cell wall degradants cellobiose and d-galacturonic acid as being induced by the autotoxic ginsenoside Rg₁ of Panax notoginseng, and we found that exogenous cellobiose and d-galacturonic acid in addition to Rg₁ could aggravate root rot disease by modifying the rhizosphere microbiome. Microorganisms that correlated positively with root rot disease were enriched and those that correlated negatively were suppressed by exogenous cellobiose, d-galacturonic acid, and Rg₁. In particular, they promoted the growth and infection of the soilborne pathogen Ilyonectria destructans by upregulating pathogenicity-related genes. Cellobiose showed the highest ability to modify the microbiome and enhance pathogenicity, followed by Rg₁ and then d-galacturonic acid. Collectively, autotoxins damaged root systems to release a series of cell wall degradants, some of which modified the rhizosphere microbiome so that the host plant became more susceptible to root rot disease.

IMPORTANCE The accumulation of autotoxins and pathogens in soil has been reported as a primary driver of root rot disease and one of the key factors limiting sustainable development in agricultural production. However, less is known about the correlation of plants, their associated pathogens, and the microbiome mediated by autotoxins, as well as the contributions autotoxins make to the occurrence of root rot disease. In our study, we found that autotoxins can damage root systems, thus releasing a series of cell wall degradants, and both autotoxins and the cell wall degradants they induce could aggravate root rot disease by reassembling the rhizosphere microbiome, resulting in the enrichment of pathogens and microorganisms positively related to the disease but the suppression of beneficial microorganisms. Deciphering this mechanism among plants, their associated pathogens, and the microbiome mediated by autotoxins will advance our fundamental knowledge of and ability to degrade autotoxins or employ microbiome to alleviate root rot disease in agricultural systems.

Enrichment of Burkholderia in the Rhizosphere by Autotoxic Ginsenosides to Alleviate Negative Plant-Soil Feedback

The accumulation of autotoxins and soilborne pathogens in soil was shown to be the primary driver of negative plant-soil feedback (NPSF). There is a concerted understanding that plants could enhance their adaptability to biotic or abiotic stress by modifying the rhizosphere microbiome. However, it is not clear whether autotoxins could enrich microbes to degrade themselves or antagonize soilborne pathogens. Here, we found that the microbiome degraded autotoxic ginsenosides, belonging to triterpenoid glycosides, and antagonized pathogens in the rhizosphere soil of Panax notoginseng (sanqi). Deep analysis by 16S rRNA sequencing showed that the bacterial community was obviously changed in the rhizosphere soil and identified the Burkholderia-Caballeronia-Paraburkholderia (BCP) group as the main ginsenoside-enriched bacteria in the rhizosphere soil. Eight strains belonging to the BCP group were isolated, and Burkholderia isolate B36 showed a high ability to simultaneously degrade autotoxic ginsenosides (Rb₁, Rg₁, and Rd) and antagonize the soilborne pathogen Ilyonectria destructans. Interestingly, ginsenosides could stimulate the growth and biofilm formation of B36, eventually enhancing the antagonistic ability of B36 to I. destructans and the colonization ability in the rhizosphere soil. In summary, autotoxic ginsenosides secreted by P. notoginseng could enrich beneficial microbes in the rhizosphere to simultaneously degrade autotoxins and antagonize pathogen, providing a novel ecological strategy to alleviate NPSF.

IMPORTANCE Autotoxic ginsenosides, secreted by sanqi into soil, could enrich Burkholderia sp. to alleviate negative plant-soil feedback (NPSF) by degrading autotoxins and antagonizing the root rot pathogen. In detail, ginsenosides could stimulate the growth and biofilm formation of Burkholderia sp. B36, eventually enhancing the antagonistic ability of Burkholderia sp. B36 to a soilborne pathogen and the colonization of B36 in soil. This ecological strategy could alleviate NPSF by manipulating the rhizosphere microbiome to simultaneously degrade autotoxins and antagonize pathogen.

Autotoxic Ginsenoside Disrupts Soil Fungal Microbiomes by Stimulating Potentially Pathogenic Microbes

Autotoxic ginsenosides have been implicated as one of the major causes for replant failure of Sanqi ginseng (Panax notoginseng); however, the impact of autotoxic ginsenosides on the fungal microbiome, especially on soilborne fungal pathogens, remains poorly understood. In this study, we aimed to investigate the influence of the ginsenoside monomers Rg₁, Rb₁, and Rh₁, and that of their mixture (Mix), on the composition and diversity of the soil fungal community, as well as on the abundance and growth of the soilborne pathogen Fusarium oxysporum in pure culture. The addition of autotoxic ginsenosides altered the composition of the total fungal microbiome, as well as the taxa within the shared and unique treatment-based components, but did not alter alpha diversity (α-diversity). In particular, autotoxic ginsenosides enriched potentially pathogenic taxa, such as Alternaria, Cylindrocarpon, Gibberella, Phoma, and Fusarium, and decreased the abundances of beneficial taxa such as Acremonium, Mucor, and Ochroconis Relative abundances of pathogenic taxa were significantly and negatively correlated with those of beneficial taxa. Among the pathogenic fungi, the genus Fusarium was most responsive to ginsenoside addition, with the abundance of Fusarium oxysporum consistently enhanced in the ginsenoside-treated soils. Validation tests confirmed that autotoxic ginsenosides promoted mycelial growth and conidial germination of the root rot pathogen F. oxysporum In addition, the autotoxic ginsenoside mixture exhibited synergistic effects on pathogen proliferation. Collectively, these results highlight that autotoxic ginsenosides are capable of disrupting the equilibrium of fungal microbiomes through the stimulation of potential soilborne pathogens, which presents a significant hurdle in remediating replant failure of Sanqi ginseng.IMPORTANCE Sanqi ginseng [Panax notoginseng (Burk.) F. H. Chen] is geoauthentically produced in a restricted area of southwest China, and successful replanting requires a rotation cycle of more than 15 to 30 years. The increasing demand for Sanqi ginseng and diminishing arable land resources drive farmers to employ consecutive monoculture systems. Replant failure has severely threatened the sustainable production of Sanqi ginseng and causes great economic losses annually. Worse still, the acreage and severity of replant failure are increased yearly, which may destroy the Sanqi ginseng industry in the near future. The significance of this work is to decipher the mechanism of how autotoxic ginsenosides promote the accumulation of soilborne pathogens and disrupt the equilibrium of soil fungal microbiomes. This result may help us to develop effective approaches to successfully conquer the replant failure of Sanqi ginseng.

Negative Plant-Soil Feedback Driven by Re-assemblage of the Rhizosphere Microbiome With the Growth of Panax notoginseng

There is a concerted understanding of the accumulation of soil pathogens as the major driving factor of negative plant-soil feedback (NPSF). However, our knowledge of the connection between plant growth, pathogen build-up and soil microbiome assemblage is limited. In this study, significant negative feedback between the soil and sanqi (Panax notoginseng) was found, which were caused by the build-up of the soil-borne pathogens Fusarium oxysporum, F. solani, and Monographella cucumerina. Soil microbiome analysis revealed that the rhizospheric fungal and bacterial communities were changed with the growth of sanqi. Deep analysis of the phylum and genus levels corroborated that rhizospheric fungal Ascomycota, including the soil-borne pathogens F. oxysporum, F. solani, and especially M. cucumerina, were significantly enriched with the growth of sanqi. However, the bacteria Firmicutes and Acidobacteria, including the genera Pseudomonas, Bacillus, Acinetobacter and Burkholderia, were significantly suppressed with the growth of sanqi. Using microbial isolation and in vitro dual culture tests, we found that most isolates derived from the suppressed bacterial genera showed strong antagonistic ability against the growth of sanqi soil-borne pathogens. Interestingly, inoculation of these suppressed isolates in consecutively cultivated soil could significantly alleviate NPSF. In summary, sanqi growth can suppress antagonistic bacteria through re-assemblage of the rhizosphere microbiome and cause the accumulation of soil-borne pathogens, eventually building negative feedback loops between the soil and plants.

Reductive soil disinfestation effectively alleviates the replant failure of Sanqi ginseng through allelochemical degradation and pathogen suppression

Replant failure has threatened the production of Sanqi ginseng (Panax notoginseng) mainly due to the accumulation of soil-borne pathogens and allelochemicals. Reductive soil disinfestation (RSD) is an effective practice used to eliminate soil-borne pathogens; however, the potential impact of RSD on the degradation of allelochemicals and the growth of replant Sanqi ginseng seedlings remain poorly understood. In this study, RSD was conducted on a Sanqi ginseng monoculture system (SGMS) and a maize-Sanqi ginseng system (MSGS), defined as SGMS_RSD and MSGS_RSD, respectively. The aim was to investigate the impact of RSD on allelochemicals, soil microbiomes, and survival rates of replant seedlings. Both short-term maize planting and RSD treatment significantly degraded the ginsenosides in Sanqi ginseng-cultivated soils, with the degradation rate being higher in the RSD treatment. The population of Fusarium oxysporum and the relative abundance of genus Fusarium were dramatically suppressed by RSD treatment. Furthermore, the RSD treatment, but not maize planting, markedly alleviated the replant failure of Sanqi ginseng, with the seedling survival rate being 52.7-70.7% 6 months after transplanting. Interestingly, RSD followed by short-term maize planting promoted microbial activity restoration, ginsenoside degradation, and ultimately alleviated the replant failure much better than RSD treatment alone (70.7% vs. 52.7%). Collectively, these results indicate that RSD treatment could considerably reduce the obstacles and might also act as a potential agriculture regime for overcoming the replant failure of Sanqi ginseng. Additional practices, such as crop rotation, beneficial microorganism inoculation, etc. may also still be needed to ensure the long-term efficacy of seedling survival.

Development of a single-tube nested PCR-lateral flow biosensor assay for rapid and accurate detection of Alternaria panax Whetz

Alternaria panax Whetz causes one of the most commonly occurring and serious diseases in ginseng cultivation, and may cause significant production and economic losses in the ginseng industry. Rapid, early, and accurate identification of Alternaria panax Whetz is an essential prerequisite for the effective prevention and control of further infection spread. In this work, a rapid and accurate molecular diagnostic method, a single-tube nested PCR-lateral flow biosensor assay (STNPCR-LFBA), was developed for rapid identification of Alternaria panax Whetz. The STNPCR-LFBA was 100 times more sensitive than the traditional PCR-LFBA. Besides that, the PCR product was checked by a lateral flow biosensor assay, which provided a basis for the migration of the detection technology to a point-of-care test (POCT) format. STNPCR-LFBA was specific to Alternaria panax Whetz, and no cross-reactions were observed in other non-target samples; the limit of detection was up to 0.01 pg of Alternaria panax Whetz genomic DNA. STNPCR-LFBA could also be used for specific identification of Alternaria panax Whetz in real samples. STNPCR-LFBA is useful for identifying Alternaria panax Whetz due to its rapidity, accuracy, and simple manipulation.

Fertilizer N application rate impacts plant-soil feedback in a sanqi production system

Replant failure caused by negative plant-soil feedback (NPFS) in agricultural ecosystems is a critical factor restricting the development of sustainable agriculture. Soil nutrient availability has the capacity to affect plant-soil feedback. Here, we used sanqi (Panax notoginseng), which is severely threatened by NPSF, as a model plant to decipher the overall effects of nitrogen (N) rates on NPSF and the underlying mechanism. We found that a high rate of N at 450kgNha^-1 (450N) aggravated the NPSF through the accumulation of pathogens in the soil compared with the optimal 250N. The increased N rates resulted in a significant increase in the soil electrical conductivity and available nitrogen but a decrease in the soil pH and C/N ratio. GeoChip 5.0 data demonstrated that these changed soil properties caused the soil to undergo stress (acidification, salinization and carbon starvation), as indicated by the enriched soil microbial gene abundances related to stress response and nutrition cycling (N, C and S). Accordingly, increased N rates reduced the richness and diversity of soil fungi and bacteria and eventually caused a shift in soil microbes from a bacterial-dominant community to a fungal-dominant community. In particular, the high 450N treatment significantly suppressed the abundance of copiotrophic bacteria, including beneficial genera Bacillus and Pseudomonas, thus weakening the antagonistic activity of these bacteria against fungal pathogens. Moreover, 450N application significantly enriched the abundance of pathogen pathogenicity-related genes. Once sanqi plants were grown in this N-stressed soil, their host-specific fungal pathogen Fusarium oxysporum significantly accumulated, which aggravated the process of NPSF. This study suggested that over-application of nitrogen is not beneficial for disease management or the reduction of fungicide application in agricultural production.

Panax notoginseng Root Cell Death Caused by the Autotoxic Ginsenoside Rg1 Is Due to Over-Accumulation of ROS, as Revealed by Transcriptomic and Cellular Approaches

Panax notoginseng is a highly valuable medicinal herb, but its culture is strongly hindered by replant failure, mainly due to autotoxicity. Deciphering the response mechanisms of plants to autotoxins is critical for overcoming the observed autotoxicity. Here, we elucidated the response of P. notoginseng to the autotoxic ginsenoside Rg1 via transcriptomic and cellular approaches. Cellular analyses demonstrated that Rg1 inhibited root growth by disrupting the cell membrane and wall. Transcriptomic analyses confirmed that genes related to the cell membrane, cell wall decomposition and reactive oxygen species (ROS) metabolism were up-regulated by Rg1 stress. Further cellular analyses revealed that Rg1 induced ROS ([Formula: see text] and H2O2) accumulation in root cells by suppressing ascorbate peroxidase (APX) and the activities of enzymes involved in the ascorbate-glutathione (ASC-GSH) cycle. Exogenous antioxidants (ASC and gentiobiose) helped cells scavenge over-accumulated ROS by promoting superoxide dismutase (SOD) activity and the ASC-GSH cycle. Collectively, the autotoxin Rg1 caused root cell death by inducing the over-accumulation of ROS, and the use of exogenous antioxidants could represent a strategy for overcoming autotoxicity.

Ginsenosides and ginsenosidases in the pathobiology of ginseng-Cylindrocarpon destructans (Zinss) Scholten

To investigate the role that ginsenosides (and some of their metabolites) play in interactions between plants and phytopathogenic fungi (e.g. Cylindrocarpon destructans (Zinss) Scholten), we systematically determined the anti-fungal activities of six major ginsenosides (Rb₁, Rb₂, Rc, Rd, Re and Rg₁), along with the metabolites of ginsenoside Rb₁ (Gypenoside XVII (G-XVII) and F₂), against the ginseng root pathogen C. destructans (Zinss) Scholten and non-ginseng pathogens Fusarium graminearum Schw., Exserohilum turcicum (Pass.) Leonard et Suggs, Phytophthora megasperma Drech. and Pyricularia oryzae Cav. Our results showed that the growth of both ginseng pathogens and non-pathogens could be inhibited by using the proto-panaxatriol (PPT) ginsenosides Re and Rg₁. In addition, the growth of the non-pathogens could also be inhibited by using proto-panaxadiol (PPD) ginsenosides Rb₁, Rb₂, Rc and Rd, whereas the growth of ginseng pathogen C. destructans (Zinss) Scholten was enhanced by ginsenosides Rb₁ and Rb₂. In contrast, ginsenoside G-XVII and F₂ strongly inhibited the hyphal growth of both C. destructans (Zinss) Scholten and the non-pathogens tested. Furthermore, addition of sucrose to the media increased the growth of C. destructans (Zinss) Scholten, whereas glucose did not affect the growth. Moreover, C. destructans (Zinss) Scholten and all four non-pathogens were able to deglycosylate PPD ginsenosides using a similar transformation pathway, albeit with different sensitivities. We also discussed the anti-fungal structure-activity relationships of the ginsenosides. Our results suggest that the pathogenicity of C. destructans (Zinss) Scholten against ginseng root is independent of its ability to deglycosylate ginsenosides.

Cylindrocarpon destructans/Ilyonectria radicicola-species complex: Causative agent of ginseng root-rot disease and rusty symptoms

Cylindrocarpon destructans/Ilyonectria radicicola is thought to cause both rusty symptom and root-rot disease of American and Korean ginseng. Root-rot disease poses a more serious threat to ginseng roots than rusty symptoms, which we argue result from the plant defense response to pathogen attack. Therefore, strains causing rotten root are characterized as more aggressive than strains causing rusty symptoms. In this review, we state 1- the molecular evidence indicating that the root-rot causing strains are genetically distinct considering them as a separate species of Ilyonectria, namely I. mors-panacis and 2- the physiological and biochemical differences between the weakly and highly aggressive species as well as those between rusty and rotten ginseng plants. Eventually, we postulated that rusty symptom occurs on ginseng roots due to incompatible interactions with the weakly aggressive species of Ilyonectria, by the established iron-phenolic compound complexes while root-rot is developed by I. mors-panacis infection due to the production of high quantities of hydrolytic and oxidative fungal enzymes which destroy the plant defensive barriers, in parallel with the pathogen growth stimulation by utilizing the available iron. Furthermore, we highlight future areas for study that will help elucidate the complete mechanism of root-rot disease development.

Soil microbial community structure and catabolic activity are significantly degenerated in successive rotations of Chinese fir plantations

This study examined the hypotheses that soil microbial community composition and catabolic activity would significantly degenerated by consecutive monoculture in Chinese fir plantations. The phospholipid fatty acids (PLFA) and community level physiological profiles (CLPP) methods were used to assess the variations of soil microbial community among the first rotation Chinese fir plantation (FCP), the second rotation plantation (SCP) and the third rotation plantation (TCP). The total content of PLFA biomarkers was highest in FCP, followed by SCP, and TCP was the least detected. Conversely, the fungi/bacteria ratio significantly increased in the SCP and TCP soils. The average well-color development (AWCD) values significantly decreased (FCP > SCP > TCP). However, the sum of AWCD values of amino acids, carboxylic acids and phenolic compounds were higher significantly in the SCP and TCP soils than FCP soils, suggesting that the microflora feeding on acids gradually became predominant in the continuous monoculture plantation soils. Soil C/N ratio was one of the most important factors to soil microbial diversity. Both the PLFA and CLPP results illustrated the long-term pure plantation pattern exacerbated the microecological imbalance in the rhizospheric soils of Chinese fir, and markedly decreased the soil microbial community diversity and metabolic activity.

Heterologous production of a ginsenoside saponin (compound K) and its precursors in transgenic tobacco impairs the vegetative and reproductive growth

MAIN CONCLUSION

Production of compound K (a ginsenoside saponin) and its precursors in transgenic tobacco resulted in stunted growth and seed set failure, which may be caused by strong autotoxicity of heterologously produced phytochemicals against the tobacco itself. Panax ginseng roots contain various saponins (ginsenosides), which are major bioactive compounds. A monoglucosylated saponin, compound K (20-O-(β-D-glucopyranosyl)-20(S)-protopanaxadiol), has high medicinal and cosmetic values but is present in undetectable amounts in naturally grown ginseng roots. The production of compound K (CK) requires complicated deglycosylation of ginsenosides using physicochemical and/or enzymatic degradation. In this work, we report the production of CK in transgenic tobacco by co-overexpressing three genes (PgDDS, CYP716A47 and UGT71A28) isolated from P. ginseng. Introduction and expression of the transgenes in tobacco lines were confirmed by genomic PCR and RT-PCR. All the lines of transgenic tobacco produced CK including its precursors, protopanaxadiol and dammarenediol-II (DD). The concentrations of CK in the leaves ranged from 1.55 to 2.64 µg/g dry weight, depending on the transgenic line. Interestingly, production of CK in tobacco brought stunted plant growth and gave rise to seed set failure. This seed set failure was caused by both long-styled flowers and abnormal pollen development in transgenic tobacco. Both CK and DD treatments highly suppressed in vitro germination and tube growth in wild-type pollens. Based on these results, metabolic engineering for CK production in transgenic tobacco was successfully achieved, but the production of CK and its precursors in tobacco severely affects vegetative and reproductive growth due to the cytotoxicity of phytochemicals that are heterologously produced in transgenic tobacco.

Chemotactic response of Ginseng bacterial soft-rot to Ginseng root exudates

Our purpose was to evaluate chemotactic response of Ginseng bacterial soft-rot to ginseng root exudates. The exudates of plant roots has a significant influence on the population changes of rhizosphere microorganisms and chemotaxis is an important way in which many pathogens sense the signals of host plants and invade the host plants. In this study, with the capillary method, we tested the chemotactic responses of Ginseng bacterial soft-rot for three ginseng roots exudates under four chemotactic parameters (concentration, temperature, pH and time). The results showed that the chemotatic response of the Ginseng bacterial soft-rot for the ginseng roots exudates at the water layer where pH = 7 and the concentration was 0.0125 mg/L reached its peak value under the circumstance that the exudates was cultivated for 60 min at 25 °C. The chemotatic ratios were respectively 124.89% and 89.44%. For the butanol extract layer and the petroleum ether faction at the concentration of 0.125 mg/L and the pH value at 7, the ginseng roots exudatess reached peak values at 25 °C and 30 °C and 60 min and 75 min respectively, and the chemotatic ratios were respectively 139.64% and101.87%, and 115.29% and 81.36%. The three ginseng roots exudates had positive effects for the chemotaxis of the Ginseng soft-rot bacteria, but the effect declined as the concentration increased.

Cross Interaction Between Ilyonectria mors-panacis Isolates Infecting Korean Ginseng and Ginseng Saponins in Correlation with Their Pathogenicity

Ilyonectria mors-panacis belongs to I. radicicola species complex and causes root rot and replant failure of ginseng in Asia and North America. The aims of this work were to identify I. mors-panacis that infect Korean ginseng using molecular approaches and to investigate whether their aggressiveness depends on their ability to metabolize ginseng saponins (ginsenosides) by their β-glucosidases, in comparison with other identified Ilyonectria species. Fourteen isolates were collected from culture collections or directly isolated from infected roots and mainly identified based on histone H3 (HIS H3) sequence. Among them, six isolates were identified as I. mors-panacis while others were identified as I. robusta and I. leucospermi. The pathogenicity tests confirmed that the isolates of I. mors-panacis were significantly more aggressive than I. robusta and I. leucospermi. The major ginsenosides in I. mors-panacis-infected roots were significantly reduced while significantly increased in those infected with other species. In vitro, the isolates were tested for their sensitivity and ability to metabolize the total major ginsenosides (Total MaG), protopanaxadiol-type major ginsenosides (PPD-type MaG), and protopanaxatriol-type major ginsenosides (PPT-type MaG). Unexpectedly, the growth rate and metabolic ability of I. mors-panacis isolates were significantly low on the three different ginsenoside fractions while those of I. robusta and I. leucospermi were significantly reduced on PPT-type MaG and Total MaG fractions and not affected on PPD-type MaG fraction. Our results indicate that major ginsenosides, especially PPT-type, have an antifungal effect and may intervene in ginseng defense during Ilyonectria species invasion, in particular the weak species. Also, the pathogenicity of I. mors-panacis may rely on its ability to reduce saponin content; however, whether this reduction is caused by detoxification or another method remains unclear.

The chemoattractant potential of ginsenosides in the ginseng - Pythium irregulare pathosystem

Ginsenosides produced by ginseng (Panax quinquefolius L.) are mildly fungitoxic saponins; however, exposure of the ginseng root pathogen Pythium irregulare Buisman to ginsenosides enhances its growth in a dose dependent manner, leading to speculation that ginsenosides may function as chemoattractants and/or growth regulators in the context of the ginseng - P. irregulare pathosystem. In the present work, it was demonstrated that the treatment of ginseng plants with a relatively high dose of ginsenosides by dipping their roots into a solution of ginsenosides prior to planting results in delayed infection by P. irregulare in pot experiments, as monitored by non-invasive chlorophyll fluorescence imaging. In an attempt to determine whether this observation results from a protective effect of the ginsenosides, or from a modification of P. irregulare growth habit in response to ginsenosides present in the soil, standard in vitro disk diffusion assays were conducted. Here, exposure of P. irregulare to crude ginsenosides or pure ginsenoside Rb1, resulted in delayed hyphal progression, while enhancing aerial hyphae build-up around ginsenoside-treated disks. By contrast, assays with pure ginsenoside F2 resulted in clear zones of inhibition around treated disks. While it remains unclear whether ginsenosides act as chemoattractants for P. irregulare in vivo, the results here suggest that these saponins serve to alter the growth habit of this organism, both in vivo and in vitro.

Comprehensive characterization of a time-course transcriptional response induced by autotoxins in Panax ginseng using RNA-Seq

BACKGROUND

As a valuable medicinal plant, the yield of Panax ginseng is seriously affected by autotoxicity, which is a common phenomenon due to continuous cropping. However, the mechanism of autotoxicity in P. ginseng is still unknown.

RESULTS

In total, high throughput sequencing of 18 RNA-Seq libraries produced 996,000,000 100-nt reads that were assembled into 72,732 contigs. Compared with control, 3697 and 2828 genes were significantly up- and down-regulated across different tissues and time points, respectively. Gene Ontology enrichment analysis showed that 'enzyme inhibitor activity', 'carboxylesterase activity', 'pectinesterase activity', 'centrosome cycle and duplication' and 'mitotic spindle elongation' were enriched for the up-regulated genes. Transcription factors including AP2s/ERFs, MYBs, and WRKYs were up-regulated in roots after benzoic acid treatment. Moreover, reactive oxygen species, peroxidases and superoxide dismutase contigs were up-regulated in roots after benzoic acid treatment. Physiological and biochemical indexes showed that the proline and malondialdehyde content were restored to lower levels at a later stage after benzoic acid treatment. Benzoic acid inhibited the root hair development in a dose-dependent manner, and several differential expressed genes potentially involved in hair development were identified. Several key contigs in the flavonoid and ginsenoside biosynthesis pathways were repressed. Finally, 58,518 alternative splicing (AS) events from 12,950 genes were found after benzoic acid treatment. Interestingly, contigs in the ginsenoside biosynthetic pathway underwent AS, providing useful information about post-transcriptional regulation in P. ginseng.

CONCLUSIONS

This study revealed the stress-response molecular mechanisms in P. ginseng induced by benzoic acid.

Autotoxic ginsenosides in the rhizosphere contribute to the replant failure of Panax notoginseng

BACKGROUND AND AIMS

Sanqi ginseng (Panax notoginseng) growth is often hampered by replant failure. In this study, we aimed to examine the role of autotoxicity in Sanqi replant failures and assess the role of ginsenosides in autotoxicity.

METHODS

The autotoxicities were measured using seedling emergence bioassays and root cell vigor staining. The ginsenosides in the roots, soils, and root exudates were identified with HPLC-MS.

RESULTS

The seedling emergence and survival rate decreased significantly with the continuous number of planting years from one to three years. The root exudates, root extracts, and extracts from consecutively cultivated soils also showed significant autotoxicity against seedling emergence and growth. Ginsenosides, including R1, Rg1, Re, Rb1, Rb3, Rg2, and Rd, were identified in the roots and consecutively cultivated soil. The ginsenosides, Rg1, Re, Rg2, and Rd, were identified in the root exudates. Furthermore, the ginsenosides, R1, Rg1, Re, Rg2, and Rd, caused autotoxicity against seedling emergence and growth and root cell vigor at a concentration of 1.0 µg/mL.

CONCLUSION

Our results demonstrated that autotoxicity results in replant failure of Sanqi ginseng. While Sanqi ginseng consecutively cultivated, some ginsenosides can accumulate in rhizosphere soils through root exudates or root decomposition, which impedes seedling emergence and growth.

Bioactive molecules in soil ecosystems: masters of the underground

Complex biological and ecological processes occur in the rhizosphere through ecosystem-level interactions between roots, microorganisms and soil fauna. Over the past decade, studies of the rhizosphere have revealed that when roots, microorganisms and soil fauna physically contact one another, bioactive molecular exchanges often mediate these interactions as intercellular signal, which prepare the partners for successful interactions. Despite the importance of bioactive molecules in sustainable agriculture, little is known of their numerous functions, and improving plant health and productivity by altering ecological processes remains difficult. In this review, we describe the major bioactive molecules present in below-ground ecosystems (i.e., flavonoids, exopolysaccharides, antibiotics and quorum-sensing signals), and we discuss how these molecules affect microbial communities, nutrient availability and plant defense responses.

Age-dependent Distribution of Fungal Endophytes in Panax ginseng Roots Cultivated in Korea

Fungal endophytes were isolated from 1-, 2-, 3-, and 4-year-old ginseng roots (Panax ginseng Meyer) cultivated in Korea. The isolated fungal endophytes were identified based on sequence analysis of the internal transcribed spacer and morphological characterization by microscopic observations. A total of 81 fungal endophytes were isolated from 24 ginseng roots. Fungal endophytes were classified into 9 different fungal species and 2 unknown species. Ginseng roots that were 1-, 2-, 3-, and 4-years old were colonized by 2, 6, 8, and 5 species of fungal endophytes, respectively. While Phoma radicina was the most frequent fungal endophyte in 2-, 3-, and 4-year-old ginseng roots, Fusarium solani was the dominant endophyte in 1-year-old ginseng roots. The colonization frequencies (CF) varied with the host age. The CF were 12%, 40%, 31%, and 40% for 1-, 2-, 3-, and 4-year-old ginseng roots, respectively. We found a variety of fungal endophytes that were distributed depending on the age of ginseng plants.

Ginsenosidases and the pathogenicity of Pythium irregulare

American ginseng (Panax quinquefolius L.) produces triterpenoid saponins, ginsenosides, that possess mild fungitoxic activity toward some common ginseng leaf pathogens. However, numerous oomycete root pathogens of ginseng, most notably Pythium irregulare Buisman, are able to partially deglycosylate 20 (S)-protopanaxadiol ginsenosides Rb1, Rd and gypenoside XVII via extracellular glycosidases, leading to a common product, ginsenoside F2. Conversion of the common 20 (S)-protopanaxadiols into F2 requires both β (1→6) and β (1→2) glucosidase activity. In the present study, the ability of nine distinct isolates of P. irregulare, as well as a P. ultimum Trow isolate and two isolates of Trichoderma hamatum (Bonord.) Bainier, to deglycosylate 20 (S)-protopanaxadiols, in vitro was examined. The pathogenicity of each isolate was also examined by scoring the severity of disease symptoms caused by each in separate inoculations of one- and two-year old ginseng seedlings. Disease severity was scored using a disease severity index, as well as by taking F(v)/F(m) measurements of leaves during a 14-day infection period. Based on these measurements, it was concluded that (1) the use of direct F(v)/F(m) measurements correlates strongly with observations of disease severity (R(2)=0.79), and that (2) the pathogenicity of P. irregulare isolates correlates with their ability to deglycosylate ginsenosides (R(2)=0.57). These results further support the hypothesis that the pathogenicity of P. irregulare on ginseng roots is dependent, in part, on the ability of this organism to deglycosylate ginsenosides.

Fungal sensitivity to and enzymatic deglycosylation of ginsenosides

A ginseng pathogen, Cylindrocarpon destructans, and five nonpathogens were tested for their sensitivity to a total ginsenoside fraction (T-GF), a protopanaxadiol-type ginsenoside fraction (PPD-GF) and a protopanaxatriol-type ginsenoside fraction (PPT-GF) from the roots of Panax ginseng C.A. Meyer. The results showed that T-GF inhibited growth of the five ginseng nonpathogens, while it promoted growth of the ginseng pathogen C. destructans. PPT-GF and PPD-GF both inhibited the growth of the five ginseng nonpathogens, although the activity of PPT-GF was higher than that of PPD-GF. PPT-GF and PPD-GF exhibited different activities on C. destructans: PPT-GF inhibited its growth, whereas PPD-GF significantly enhanced its growth. The subsequent analysis of enzymatic degradation of ginsenosides by the test fungi showed that C. destructans can consecutively hydrolyze the terminal monosaccharide units from the sugar chains attached at C3 and C20 in PPD-type ginsenosides by extracellular glycosidase activity to yield four major products, gypenoside XVII (G-XVII), compound O, compound Mb and the ginsenoside F(2). By contrast, the ginseng nonpathogens Aspergillus nidulans and Cladosporium fulvum have no extracellular glycosidase activity toward sugar chains attached to C3 in PPD-type ginsenosides. These results indicated that ginsenosides might act as host chemical defenses, while the ginseng root pathogenic fungi might counter their toxicity by converting PPD-type ginsenosides into growth or host recognition factors. The ability of ginseng root pathogens to deglycosylate PPD-type ginsenosides may be a pathogenicity factor.

Ginseng aqueous extract attenuates the production of virulence factors, stimulates twitching and adhesion, and eradicates biofilms of Pseudomonas aeruginosa

This study was carried out to examine the antimicrobial activity of the aqueous extract of Panax quinquefolius from North American ginseng (NAGE) root against Pseudomonas aeruginosa . The minimum inhibitory concentrations of reference and clinical isolates of Pseudomonas aeruginosa were measured by a standard agar-dilution method. At subinhibitory NAGE concentrations, the secretion of virulence factors, motility on agar, and adhesion to 96-well microplates were studied on the nonmucoid Pseudomonas aeruginosa O1 strain. At suprainhibitory concentrations, the activity of NAGE against mature biofilm complexes formed in the Calgary Biofilm Device and the Stovall flow cell were assessed. NAGE possessed an antibacterial activity against all the Pseudomonas aeruginosa strains at 1.25%-2.5% w/v. NAGE also significantly attenuated pyocyanin, pyoverdine, and lipase concentrations, stimulated twitching, and attenuated swarming and swimming motility. At 1.25% w/v, NAGE augmented adhesion, and at 5% w/v detached 1-day-old biofilms in microplates. The extract also eradicated 6-day-old mature biofilms (5% w/v), and fluorescence microscopy displayed a reduction of live cells and biofilm complexes compared with nontreated biofilms. These data suggest that the aqueous extract from North American ginseng possesses antimicrobial activities in vitro.

Impact of constitutive plant natural products on herbivores and pathogensThe present review is one in the special series of reviews on animal–plant interactions

Plants defend themselves from pests with deterrent or toxic phytochemicals. In addition to the development of preformed mechanical barriers such as cutin and suberin, the first line of defense for plants against pathogens and herbivores is constitutive (preformed) biologically active inhibitors. Because of the adaptation of insects and pathogens to these inhibitors, plants have evolved a stunning diversity of new and different bioactive molecules to combat pests. Some representative mechanisms of plant defense include the use of antimicrobial, anitfeedant, and phototoxic molecules. Examples of natural product defenses of specific plant families are also described. Diversity and redundancy in plant defenses is key to slowing pest resistance to host-plant defenses.

Partial purification and characterization of three ginsenoside-metabolizing beta-glucosidases from Pythium irregulare

The ginseng pathogen Pythium irregulare is able to selectively metabolize the 20(S) protopanaxadiol ginsenosides Rb1, Rb2, Rc, Rd, and gypenoside XVII via extracellular glycosidases, leading to the formation and partial assimilation of ginsenoside F2. Herein we have partially purified three ginsenoside-deglycosylating enzymes from P. irregulare culture filtrates, and provide preliminary characterization. A protocol involving acetone precipitation, chromatofocusing on PBE 94, gel filtration on Sephacryl S-200 HR and ion-exchange on Q Sepharose Fast Flow resulted in a 13-25-fold purification. The three enzymes were induced in cultures grown in the presence of ginsenosides, and found to be acidic proteins (pI of 4.5-5.0), consisting of an apparent high molecular weight (approximately 160 kDa) homodimer of 78 kDa subunits, with beta(1-->6) activity, and two monomeric enzymes of 61 and 57 kDa, with beta(1-->2) activity. Primary sequence analysis identified them as beta-glucosidases, with no homology to other saponin-deglycosylating enzymes. These are the first glycosidases purified from a Pythium species. We speculate that their role is likely to help Pythium find its host, and/or obtain nutrients/growth factors from its environment.

Physiological and biochemical responses of in vitro Fusarium oxysporum f.sp. niveum to benzoic acid

The allelopathic potential of an artificially applied allelochemical, benzoic acid, on in vitro Fusarium oxysporum f.sp. niveum (a soil-borne pathogen causing watermelon wilt) was evaluated. Benzoic acid strongly inhibited its growth, sporulation and conidia germination, whereas it stimulated virulence factors of this pathogen. The biomass was reduced by 83-96 % and the conidia germinating rate and conidia production rate were decreased by 100 % at a concentration of >200 mg/L. However, phytopathogenic enzyme activities and mycotoxin production were stimulated with an increase of 10.2-1250 % for enzyme activities and 610-2630 % for mycotoxin yield.

Evidence for alteration of fungal endophyte community assembly by host defense compounds

* Plant defense compounds are common stressors encountered by endophytes. Fungi readily evolve tolerance to these compounds, yet few studies have addressed the influence of intraspecific variation in defense compound production on endophyte colonization. We compared the influence of defense compound production on the composition of fungal endophyte communities in replicated field experiments. * Maize (Zea mays) produces benzoxazinoids (BXs), compounds with antifungal byproducts persistent in the environment. Fungi were isolated from leaf and root tissue of two maize genotypes that produce BXs, and a natural mutant that does not. Isolates representing the species recovered were tested for tolerance to 2-benzoxazolinone (BOA), a toxic BX byproduct. * In seedling roots and mature leaves, the community proportion with low BOA tolerance was significantly greater in BX nonproducers than producers. Mean isolation frequency of Fusarium species was up to 35 times higher in mature leaves of BX producers than nonproducers. * Fungal species with relatively high tolerance to BOA are more abundant in BX producing than BX nonproducing maize. Production of BXs may increase colonization by Fusarium species in maize, including agents of animal toxicosis and yield-reducing disease in maize. Overall, results indicate that production of defense compounds can significantly alter endophyte community assembly.

Antibiotic effect of exogenously applied salicylic acid on in vitro soilborne pathogen, Fusarium oxysporum f.sp.niveum

Salicylic acid, which is biosynthesized inside plant and is often found and accumulated in soil due to plant debris decaying, is considered as a signaling substance during plant-microbe interactions. It is involved in the cycling of biogeochemistry and related to plant resistance to biotic and abiotic stress. The antibiotic effect of salicylic acid on Fusarium oxysporum f.sp.niveum (FON) was studied to investigate the relationships between the salicylic acid and the fungus in the ecological interaction of plant-microbe. Results showed that the biomass, colony diameter, number of conidium germination and conidium production of FON were decreased by 52.0%, 25.7%, 100% and 100% at concentrations of 800 mg L(-1). However, mycotoxin yield was increased by 233%, pectinase activity raised by 168.0% and cellulase activity increased by 1325% compared to control at higher concentrations. It was concluded that salicylic acid as an allelochemical greatly inhibited FON growth and conidia formation and germination, though stimulated mycotoxin production and activities of hydrolytic enzymes by FON.

Cinnamic acid inhibits growth but stimulates production of pathogenesis factors by in vitro cultures of Fusarium oxysporum f.sp. niveum

Long-term monoculture of watermelon leads to frequent occurrence of watermelon fusarium wilt caused by Fusarium oxysporum f.sp. niveum (FON). Some allelochemicals contained in watermelon root exudates and decaying residues are possibly responsible for promoting the wilt disease. The purpose of this study was to evaluate the allelopathic effect of artificially applied cinnamic acid on FON. Results demonstrated that hyphal growth of FON was strongly inhibited by cinnamic acid. At the highest concentration of cinnamic acid, the biomass in liquid culture was decreased by 63.3%, while colony diameter, conidial germination on plates, and conidial production in liquid culture were completely inhibited. However, mycotoxin production and activity of phytopathogenic enzymes were greatly stimulated. Mycotoxin yield, pectinase activity, proteinase activity, cellulase activity, and amylase activity were increased by 490, 590, 760, 2006, and 27.0%, respectively. It was concluded that cinnamic acid dramatically stimulated mycotoxin production and activities of hydrolytic enzymes by FON but inhibited growth and germination of FON. The findings presented here indicate that cinnamic acid is involved in promoting watermelon fusarium wilt.

Root exudates regulate soil fungal community composition and diversity

Plants are in constant contact with a community of soil biota that contains fungi ranging from pathogenic to symbiotic. A few studies have demonstrated a critical role of chemical communication in establishing highly specialized relationships, but the general role for root exudates in structuring the soil fungal community is poorly described. This study demonstrates that two model plant species (Arabidopsis thaliana and Medicago truncatula) are able to maintain resident soil fungal populations but unable to maintain nonresident soil fungal populations. This is mediated largely through root exudates: the effects of adding in vitro-generated root exudates to the soil fungal community were qualitatively and quantitatively similar to the results observed for plants grown in those same soils. This effect is observed for total fungal biomass, phylotype diversity, and overall community similarity to the starting community. Nonresident plants and root exudates influenced the fungal community by both positively and negatively impacting the relative abundance of individual phylotypes. A net increase in fungal biomass was observed when nonresident root exudates were added to resident plant treatments, suggesting that increases in specific carbon substrates and/or signaling compounds support an increased soil fungal population load. This study establishes root exudates as a mechanism through which a plant is able to regulate soil fungal community composition.

Isolation and determination of ginsenosides in American ginseng leaves and root extracts by LC-MS

Ginseng saponins (ginsenosides) were extracted from the root and leaves of locally cultivated American ginseng (Panax quinquefolium L.). For the isolation of compounds from plant samples three different extraction methods were utilized: accelerated solvent extraction, the ultrasound-assisted solvent extraction and mechanical shaking assisted solvent extraction. The separation of compounds was achieved with a water-acetonitrile gradient system using a C18 reversed-phase column. Target compounds were identified in MS(2) and MS(3) experiments. The relative distribution of these ginsenosides in each root and leaf extract was established. The limit of detection of the method was less than 30 ng/ml. Recovery of ginseng saponins in spiked samples exceeded 80%, while the relative standard deviation ranged from 7.1 to 9.1%. The total concentrations of ginsenosides were 41 and 13 mg/g in root and leaves.