Interactions between the mycoparasite Pythium oligandrum and two types of sclerotia of plant-pathogenic fungi

Current Insights into the Role of Rhizosphere Bacteria in Disease Suppressive Soils

Disease suppressive soils offer effective protection to plants against infection by soil-borne pathogens, including fungi, oomycetes, bacteria, and nematodes. The specific disease suppression that operates in these soils is, in most cases, microbial in origin. Therefore, suppressive soils are considered as a rich resource for the discovery of beneficial microorganisms with novel antimicrobial and other plant protective traits. To date, several microbial genera have been proposed as key players in disease suppressiveness of soils, but the complexity of the microbial interactions as well as the underlying mechanisms and microbial traits remain elusive for most disease suppressive soils. Recent developments in next generation sequencing and other 'omics' technologies have provided new insights into the microbial ecology of disease suppressive soils and the identification of microbial consortia and traits involved in disease suppressiveness. Here, we review the results of recent 'omics'-based studies on the microbial basis of disease suppressive soils, with specific emphasis on the role of rhizosphere bacteria in this intriguing microbiological phenomenon.

Influence of chosen microbes and some chemical substances on the production of aflatoxins

Aflatoxins are produced as secondary metabolites by A. flavus, A. parasiticus, A. nomius and A. tamarii. The aflatoxin biosynthetic pathway involves several enzymatic steps and genes (apa-2, ver-1) that appear to be regulated by the aflR gene in these fungi. The aim of this work was the detection of aflatoxins by the HPLC method and the ascertainment of factors influencing their production. A. parasiticus CCM F-108, A. parasiticus CCF 141, A. parasiticus CCF 3137 and two isolates A. flavus were used. These toxigenic isolates were recovered from spice (strain 1) and wraps (strain 2). The gene for the production of aflatoxin B1 for each species of fungi was detected using an optimized PCR method. Rhodotorula spp.*, Lactococcus lactis subsp. lactis CCM 1881, Flavobacterium spp. and fungal strain Pythium oligandrum* were tested for inhibition of aflatoxins production and fungal growth. Having used the HPLC detection, various preservatives (propionic acid, citric acid, potassium sorbate) were tested from the viewpoint of their influence on the growth of aflatoxigenic fungi followed by the production of aflatoxins. The growth of A. flavus and A. parasiticus and aflatoxin production in Potato Dextrose Agar supplemented with propionic acid (1000-2000-3000 mg/kg), citric acid (2000-3000-4000 mg/kg) and potassium sorbate (500-800-1000 mg/kg) was tested by Agar Dilution Method. After 72 h of incubation was evaluated growth of fungi, all samples were frozen for later extraction and aflatoxins quantification by HPLC. Effect of peptone and sucrose additions were studied in yeast extract (2%) supplemented with peptone (5-10-15%) or sucrose (15%). Growth inhibition of Aspergillus by Pythium oligandrum was tested on wood surface. As shown, the highest inhibition effect on the aflatoxins production was obtained when propionic acid was applied in concentrations since 1000 mg/kg. A total inhibition of the fungi growth and aflatoxins production was observed in all samples containing peptone in the concentration range tested. Significant limitation of the growth and production of aflatoxins was also observed in the presence of other microorganisms such like Pythium oligandrum and Rhodotorula spp.

Biological control of plant pathogens: advantages and limitations seen through the case study of Pythium oligandrum

The management of certain plant beneficial microorganisms [biological control agents (BCAs)] seems to be a promising and environmental friendly method to control plant pathogens. However, applications are still limited because of the lack of consistency of BCAs when they are applied in the field. In the present paper, the advantages and limitations of BCAs are seen through the example of Pythium oligandrum, an oomycete that has received much attention in the last decade. The biological control exerted by P. oligandrum is the result of a complex process, which includes direct effects through the control of pathogens and/or indirect effects mediated by P. oligandrum, i.e. induction of resistance and growth promotion. P. oligandrum antagonism is a multifaceted and target fungus-dependent process. Interestingly, it does not seem to disrupt microflora biodiversity on the roots. P. oligandrum has an atypical relationship with the plant because it rapidly penetrates into the root tissues but it cannot stay alive in planta. After root colonisation, because of the elicitation by P. oligandrum of the plant-defence system, plants are protected from a range of pathogens. The management of BCAs, here P. oligandrum, is discussed with regard to its interactions with the incredibly complex agrosystems.

A family of small tyrosine rich proteins is essential for oogonial and oospore cell wall development of the mycoparasitic oomycete Pythium oligandrum

The mycoparasitic oomycete Pythium oligandrum is homothallic, producing an abundance of thick-walled spiny oospores in culture. After mining a cDNA sequence dataset, we identified a family of genes that code for small tyrosine rich (Pythium oligandrumsmall tyrosine rich (PoStr)) proteins. Sequence analysis identified similarity between the PoStr proteins and putative glycine-rich cell wall proteins from the related plant pathogenic oomycete Pythium ultimum, and mating-induced genes from the oomycete Phytophthora infestans. Expression analysis showed that PoStr transcripts accumulate during oospore production in culture and immunolocalisation indicates the presence of these proteins in oogonial and oospore cell walls. PoStr protein abundance correlated positively with production of oogonia as determined by antibiotic-mediated oogonia suppression. To further characterise the role of PoStr proteins in P. oligandrum oospore production, we silenced this gene family using homology-dependent gene silencing. This represents the first characterisation of genes using gene silencing in a Pythium species. Oospores from silenced strains displayed major ultrastructural changes and were sensitive to degradative enzyme treatment. Oogonia of silenced strains either appeared to be arrested at the mature oosphere stage of development or in around 40 % of the structures, showed a complete suppression of oospore formation. Suppressed oogonia were highly vacuolated and the oogonium wall was thickened by a new inner wall layer. Our data suggest PoStr proteins are probably integral structural components of the normal oospore cell wall and play a key role in oospore formation.

Molecular Detection and Quantification of Pythium Species: Evolving Taxonomy, New Tools, and Challenges

The genus Pythium is one of the most important groups of soilborne plant pathogens, present in almost every agricultural soil and attacking the roots of thousands of hosts, reducing crop yield and quality. Most species are generalists, necrotrophic pathogens that infect young juvenile tissue. In fact, Cook and Veseth have called Pythium the "common cold" of wheat, because of its chronic nature and ubiquitous distribution. Where Pythium spp. are the cause of seedling damping-off or emergence reduction, the causal agent can easily be identified based on symptoms and culturing. In more mature plants, however, infection by Pythium spp. is more difficult to diagnose, because of the nonspecific symptoms that could have abiotic causes such as nutrient deficiencies or be due to other root rotting pathogens. Molecular methods that can accurately identify and quantify this important group are needed for disease diagnosis and management recommendations and to better understand the epidemiology and ecology of this important group. The purpose of this article is to outline the current state-of-the-art in the detection and quantification of this important genus. In addition, we will introduce the reader to new changes in the taxonomy of this group.

Influence of Pythium oligandrum on the bacterial communities that colonize the nutrient solutions and the rhizosphere of tomato plants

The influence exerted by the biocontrol oomycete Pythium oligandrum on the bacterial populations proliferating in the rhizosphere of tomato plants grown in a hydroponic system and in the circulating solutions is studied in the present experiment. Quantitative PCR and single-strand conformation polymorphism were used to investigate the genetic structure and dynamics of the bacterial communities colonizing the root systems and the various circulating solutions. Quantitative PCR assays showed that bacteria heavily colonized the rhizosphere of tomato plants with, however, no significant density changes throughout the cultural season (April–September). Single strand conformation polymorphism fingerprints revealed the occurrence of transient perturbations in the rhizospheric indigenous bacterial communities following P. oligandrum introduction in the root system of plants. This effect was, however, transient and did not persist until the end of the cropping season. Interestingly, the genetic structure of the bacterial microflora colonizing either the roots or the nutrient solutions evolved throughout the cropping season. This temporal evolution occurred whatever the presence and persistence of P. oligandrum in the rhizosphere. Evidence is also provided that bacterial microflora that colonize the root system are different from the ones colonizing the circulating solutions. The relationships between these 2 microflora (at the root and solution levels) are discussed.

Use of a non-homologous end-joining-deficient strain (delta-ku70) of the biocontrol fungus Trichoderma virens to investigate the function of the laccase gene lcc1 in sclerotia degradation

The aim of this study was to apply a generated Δtku70 strain with increased homologous recombination efficiency from the mycoparasitic fungus Trichoderma virens for studying the involvement of laccases in the degradation of sclerotia of plant pathogenic fungi. Inactivation of the non-homologous end-joining pathway has become a successful tool in filamentous fungi to overcome poor targeting efficiencies for genetic engineering. Here, we applied this principle to the biocontrol fungus T. virens, strain I10, by deleting its tku70 gene. This strain was subsequently used to delete the laccase gene lcc1, which we found to be expressed after interaction of T. virens with sclerotia of the plant pathogenic fungi Botrytis cinerea and Sclerotinia sclerotiorum. Lcc1 was strongly upregulated at early colonization of B. cinerea sclerotia and steadily induced during colonization of S. sclerotiorum sclerotia. The Δtku70Δlcc1 mutant was altered in its ability to degrade the sclerotia of B. cinerea and S. sclerotiorum. Interestingly, while the decaying ability for B. cinerea sclerotia was significantly decreased, that to degrade S. sclerotiorum sclerotia was even enhanced, suggesting the operation of different mechanisms in the mycoparasitism of these two types of sclerotia by the laccase LCC1.

Influence of Pythium oligandrum biocontrol on fungal and oomycete population dynamics in the rhizosphere

Fungal and oomycete populations and their dynamics were investigated following the introduction of the biocontrol agent Pythium oligandrum into the rhizosphere of tomato plants grown in soilless culture. Three strains of P. oligandrum were selected on the basis of their ability to form oospores (resting structures) and to produce tryptamine (an auxin-like compound) and oligandrin (a glycoprotein elicitor). Real-time PCR and plate counting demonstrated the persistence of large amounts of the antagonistic oomycete in the rhizosphere throughout the cropping season (April to September). Inter-simple-sequence-repeat analysis of the P. oligandrum strains collected from root samples at the end of the cropping season showed that among the three strains used for inoculation, the one producing the smallest amount of oospores was detected at 90%. Single-strand conformational polymorphism analysis revealed increases in the number of members and the complexity of the fungal community over time. There were no significant differences between the microbial ecosystems inoculated with P. oligandrum and those that were not treated, except for a reduction of Pythium dissotocum (ubiquitous tomato root minor pathogen) populations in inoculated systems during the last 3 months of culture. These findings raise interesting issues concerning the use of P. oligandrum strains producing elicitor and auxin molecules for plant protection and the development of biocontrol.