Viability, purity, and genetic stability of entomopathogenic fungi species using different preservation methods

Physiological response of Metarhizium rileyi with linoleic acid supplementation

Metarhizium rileyi has a broad biocontrol spectrum but is highly sensitive to abiotic factors. A Colombian isolate M. rileyi Nm017 has shown notorious potential against Helicoverpa zea. However, it has a loss of up to 22 % of its conidial germination after drying, which limits its potential as a biocontrol agent and further commercialization. Conidial desiccation resistance can be enhanced by nutritional supplements, which promotes field adaptability and facilitates technological development as a biopesticide. In this study, the effect of culture medium supplemented with linoleic acid on desiccation tolerance in Nm017 conidia was evaluated. Results showed that using a 2 % linoleic acid-supplemented medium increased the relative germination after drying by 41 % compared to the control treatment, without affecting insecticidal activity on H. zea. Also, the fungus increased the synthesis of trehalose, glucose, and erythritol during drying, independently of linoleic acid use. Ultrastructural analyses of the cell wall-membrane showed a loss of thickness by 22 % and 25 %, in samples obtained from 2 % linoleic acid supplementation and the control, respectively. Regarding its morphological characteristics, conidia inner area from both treatments did not change after drying. However, conidia from the control had a 24 % decrease in length/width ratio, whereas there was no alteration in conidia from acid linoleic. The average value of dry conidia elasticity coefficient from linoleic acid treatment was 200 % above the control. Medium supplementation with linoleic acid is a promising fermentation strategy for obtaining more tolerant conidia without affecting production and biocontrol parameters, compatible solutes synthesis, or modifying its cell configuration.

Native fungi from Amazon with potential for control of Aedes aegypti L. (Diptera: Culicidae)

Aedes aegypti L. (Diptera: Culicidae) is the main transmitter of pathogens that cause human diseases, including dengue, chikungunya, zika and yellow fever. Faced with this problem, this study aims to select fungi with entomopathogenic potential against Ae. aegypti and develop formulations that optimize the control action of entomopathogenic fungi in the semi-field condition. 23 fungal strains native from Amazon were inoculated in Potato-Dextrose-Agar (PDA) culture medium for 14 days and then transferred by scraping to tubes containing 0.9% NaCl solution. To obtain the larvae, eggs were collected using traps in peridomestic environments for 7 days. 20 larvae of Ae. aegypti in 125 mL erlenmeyers containing 20 mL of conidial suspension at a concentration of 1x106 conidia/mL for initial selection and 1×104, 1×105, 1×106 and 1×107 conidia/mL for determination of LC50. Mortality was checked every 24 h for 5 days. The three fungi with the best virulence rates were identified using molecular techniques. The compatibility between fungi at a concentration of 1×106 conidia/mL and oily adjuvants, mineral oil and vegetable oil (andiroba, chestnut and copaiba) at concentrations of 0.1, 0.5 and 1% was evaluated. The germination capacity of 100 conidia per treatment was evaluated after incubation at 28 ºC for 24 h. To evaluate the entomopathogenic potential of the fungal formulations, conidial suspensions (1×106 conidia/mL) were added with 0.1% mineral and vegetable oil. The treatments were submitted to laboratory and semi-field conditions and mortality was verified every 24 h for 5 days. Beauveria sp. (4,458) (LC50 = 8.66× 103), Metarhizium anisopliae (4,420) (LC50 = 5.48×104) and M. anisopliae (4,910) (LC50 = 1.13×105) were significantly more effective in the larval control of Ae. aegypti, in relation to the other fungal morphospecies evaluated. Mineral oil was better compatible in all treatments evaluated. Beauveria sp. (4,458) was considerably less virulent under semi-field conditions. M. anisopliae (4,910) formulated with mineral oil increased larval mortality to 100% on the 4th day in the laboratory and on the 5th day in the semi-field. Fungal formulations developed from native Amazonian isolates represent a promising tool for the development of strategies to control Ae. aegypti.

Melanin Induction Restores the Pathogenicity of Gaeumannomyces graminis var. tritici in Wheat Plants

One of the most challenging aspects of long-term research based on microorganisms is the maintenance of isolates under ex situ conditions, particularly the conservation of phytopathological characteristics. Our research group has worked for more than 10 years with Gaumannomyces graminis var. tritici (Ggt), the main biotic factor affecting wheat. In this sense we preserved the microorganisms in oil overlaid. However, several strains preserved for a long time lost their pathogenicity. These strains show white and non-infective mycelia. In this sense, we hypothesized that this is attributable to low melanin content. Melanin is a natural pigment mainly involved in UV protection, desiccation, salinity, oxidation, and fungal pathogenicity. Therefore, understanding the melanin role on Ggt pathogenicity is fundamental to developing melanin activation strategies under laboratory studies. In this study, we induce melanin activation by UV-A light chamber, 320 to 400 nm (T1) and temperature changes of 30 °C, 15 °C, and 20 °C (T2). Fungal pathogenicity was evaluated by determination of blackening roots and Ggt was quantified by real-time PCR in inoculated wheat plants. Results revealed that Ggt grown under UV-A (T1) conditions showed around 40% higher melanin level with a concomitant effect on root infection (98% of blackened roots) and 4-fold more Ggt genome copy number compared with the control (non-infective mycelia) being T1, a more inductor factor compared with T2. These findings would support the role of melanin in pathogenicity in darkly pigmented fungi such as Ggt and could serve as a basis for activating pathogenicity under laboratory conditions.

Successive mycelial subculturing decreased lignocellulase activity and increased ROS accumulation in Volvariella volvacea

Strain degradation is a common problem in many artificially-cultivated edible mushrooms. As a fungus with poor tolerance to low-temperature, Volvariella volvacea cannot delay its degradation by long-term low temperature storage like other fungi, so its degradation is particularly severe, which hinders industrial applications. Periodic mycelial subculture is a common storage method for V. volvacea, but excessive subculturing can also lead to strain degeneration. After 20 months of continuous subculturing every 3 days, V. volvacea strains S1–S20 were obtained, and their characteristics throughout the subculture process were analyzed. With increasing number of subculture, the growth rate, mycelial biomass, the number of fruiting bodies and biological efficiency gradually decreased while the production cycle and the time to primordium formation was lengthened. Strains S13–S20, obtained after 13–20 months of mycelial subculturing, also lacked the ability to produce fruiting bodies during cultivation experiments. Determination of reactive oxygen species (ROS) content as well as enzyme activity showed that decreased lignocellulase activity, along with excessive accumulation of ROS, was concomitant with the subculture-associated degeneration of V. volvacea. Reverse transcription polymerase chain reaction (RT-PCR) was eventually used to analyze the gene expression for lignocellulase and antioxidant enzymes in subcultured V. volvacea strains, with the results found to be consistent with prior observations regarding enzyme activities. These findings could form the basis of further studies on the degeneration mechanism of V. volvacea and other fungi.

Providing inoculum for Didymella bryoniae studies: the effect of light spectrum and storing at low temperature

The in vitro sporulation of Didymella bryoniae is of great importance for studies that require pure inoculum and in large quantities. Thus, the objectives of this study were to identify the best condition for D. bryoniae sporulation combining different light spectra (UV-A or UV-B light, white light, and continuous dark), with distinct culture media (PDA, V8, ML, and PDAB) and, to evaluate fungus' survivability stored at -20°C over time. The fungus samples were only able to sporulate when subjected to the UV-B light treatment, regardless of the culture medium. The highest appearance of spores conidium type was observed in the PDAB medium, and the lowest production occurred in the ML medium. Reproductive structures, such as perithecia and pycnidia, were observed in all culture media. However, there was considerable variation in the amount of each structure between the different culture media. The ML and V8 media showed a greater number of perithecia and the PDA and PDAB media presented a greater proportion of pycnidia compared to perithecia. The storage duration at -20°C did not affect mycelial growth or mycelial growth rate. In conclusion, the UV-B light is essential for D. bryoniae in vitro sporulation. Moreover, the culture medium composition influences the type of fungal structure produced, as well as spores' size and quantity. Freezing at -20°C is an efficient technique that can be used to store D. bryoniae for at least five months without loss of viability.

Rice recycling: a simple strategy to improve conidia production in solid-state cultures

Here we studied at a laboratory scale a potential strategy to revalorize the residual rice remaining at the end of a conventional conidia production process in solid-state culture. The conidia production of Trichoderma asperellum Th-T4 (3) and Metarhizium robertsii Xoch-8.1 started with the use of fresh rice (unrecycled rice) as the substrate (cycle one), and continued with the use of recycled rice in successive cycles of conidia production. The rice remaining at the end of the first cycle was reused without any further sterilization or reinoculation. As a result, it was observed that the conidia production and productivity significantly increased in both fungi. Conidia production in T. asperellum Th-T4 (3) increased from 1 × 10⁹ (first cycle) to 2·9 × 10⁹ conidia per gram of initial dry substrate (con⋅gds^-1 ) (second cycle using recycled rice), while in M. robertsii Xoch-8.1, this parameter increased form 5·7 × 10⁸ to 1·4 × 10⁹ con⋅gds^-1 . Both fungi grew faster and conidiated earlier when recycled rice was used as the substrate, therefore, conidia productivity was also significantly improved. Furthermore, the use of recycled rice did not affect conidia viability. This is the first report about a recycling methodology completely free of extra-processing steps, and useful to increase conidia production and productivity.

Principal component analysis of the biological characteristics of entomopathogenic fungi in nutrient-limited and cuticle-based media

Media formulated with insect cuticle (0.5% and 1%; Sphequit Sph®), with a reduction in nutrients (¼ Sabouraud dextrose agar + yeast [SDAY]) and commercial media (potato dextrose agar, Sabouraud dextrose agar) were evaluated for the cultivation of Beauveria bassiana, Cordyceps javanica (Isaria javanica [Bally] Samson & Hywel-Jones), and Metarhizium robertsii. By using principal component analysis, it was determined that the ¼ SDAY and Sph formulations have greater advantages than commercial media for the development of fungi. The ¼ SDAY and Sph (0.5% and 1%) improved hydrophobicity, radial growth rate, germination, conidia yield, and virulence in B. bassiana; in M. robertsii, they favored conidia yield, germination, and virulence, and in C. javanica, the ¼ SDAY and Sph 0.5% media enhanced conidia yield, germination, radial growth rate, and virulence. We suggest that these formulations are an alternative to commercial culture media as they are cheaper and appropriate to improve the growth characteristics and virulence of the three strains evaluated. Some applications of culture media are suggested, and the importance of multivariate analysis as an exploratory tool to carry out the choice of culture media in a suitable way for the development of mycoinsecticides is also discussed.

Analysis of genetic variations of heat shock proteins Hsp70 and Hsp90 in Isaria farinosa strains from the Yunnan province of China

In the present study, the cDNA sequences of Hsp70 and Hsp90 genes of Isaria farinosa (designated IFHSP70 and IFHSP90) were cloned and characterized using multiple techniques of molecular biology and bioinformatics. The genetic differentiation of the two genes was investigated among 10 geographically separated populations distributed in the Yunnan province. The complete sequence of the IFHSP70 cDNA had a length of 2158 bp, and contained an open reading frame (ORF) of 1962 bp, encoding a 71-kDa polypeptide comprising of 653 amino acids. IFHSP90 cDNA had a length of 2144 bp, and contained an ORF of 2103 bp, encoding a polypeptide of 79.23 kDa, comprising of 700 amino acids. The deduced amino acid sequences of IFHSP70 and IFHSP90 shared high sequence identities with other fungi. Fundamental information pertaining to the protein families, signatures, and conserved motifs of Hsp70 and Hsp90 were also identified. Analysis of molecular variances (AMOVA) from the Hsp70 and Hsp90 genes showed that the genetic variation within-population (83.26%, 83.08%) was greater than among the populations (16.74%, 16.92%). The values of nucleotide diversity (Pi), haplotype diversity (Hd), coefficient of genetic differentiation (Fst), and gene flow (Nm) were calculated. For Hsp70, Pi = 0.0425, Hd = 0.888, Fst = 0.167, Nm = 1.24; For Hsp90, Pi = 0.0420, Hd = 0.894, Fst = 0.169, and Nm = 1.22. These data indicated that the genetic differentiation among 10 different geographical populations of I. farinosa was limited. This study describes, for the first time, cloning, characterization and identification of Isaria farinosa Hsp70 and Hsp90 genes, and provides a preliminary basis for exploring the genetic structure of the genus Isaria using the sequences of Hsp70 and Hsp90 as molecular markers.

Preservation affects the vegetative growth and fruiting body production of Cordyceps militaris

Cordyceps militaris is a model species of Cordyceps fungi, and has been traditionally used as an edible and medicinal fungus due to its richness of bioactive and pharmacological metabolites. The fruiting bodies of this fungus are widely used as healthy food and nutrition supply. In industrial production, fruiting bodies are cultivated on artificial media, but their yield and quality are usually affected by the quality of fungal strains. In this study, the effect of colony growth rate of the fungal strains, fungal age and repeated subculturing on the fungal biomass accumulation was investigated. The results indicated that the fungal biomass was positively correlated with the colony growth rate and not affected by fungal age and the repeated subculturing. The preservation conditions for stock cultures, including choice of cultures, lyophilization, temperature and protective agents were optimized based on the mycelial formation and conidia production in artificial inoculum. The development of fruiting bodies from the fungal strains stored under the optimized preservation conditions were further analyzed to determine the ideal time period of preservation. Results indicated that storing the fungus at 4 °C could maintain the fungal vitality and fruiting body producing capacity for at least 12 months. This study established practical criteria of fungal inoculum for artificial cultivation of fruiting body and provided a simple and efficient preservation method for C. militaris. The results may shed light on preservation for other Cordyceps species and other edible fungi.