RESUMO
Metarhizium spp. is used as a biocontrol agent but is limited because of low tolerance to abiotic stress. Metarhizium robertsii is an excellent study model of fungal pathogenesis in insects, and its tolerance to different stress conditions has been extensively investigated. Priming is the time-limited pre-exposure of an organism to specific stress conditions that increases adaptive response to subsequent exposures. Congo red is a water-soluble azo dye extensively used in stress assays in fungi. It induces morphological changes and weakens the cell wall at sublethal concentrations. Therefore, this chemical agent has been proposed as a stressor to induce priming against other stress conditions in entomopathogenic fungi. This study aimed to evaluate the capacity of Congo red to induce priming in M. robertsii. Conidia were grown on potato dextrose agar with or without Congo red.The tolerance of conidia produced from mycelia grown in these three conditions was evaluated against stress conditions, including osmotic, oxidative, heat, and UV-B radiation. Conidia produced on medium supplemented with Congo red were significantly more tolerant to UV-B radiation but not to the other stress conditions assayed. Our results suggest that Congo red confers trans-priming to UV-B radiation but not for heat, oxidative, or osmotic stress.
Assuntos
Metarhizium , Metarhizium/fisiologia , Vermelho Congo , Raios Ultravioleta , Esporos Fúngicos/fisiologiaRESUMO
Soybean, corn, and cotton crops are afflicted by several noctuid pests and the development of bioinsecticides could help control these pests. The fungus Metarhizium rileyi has the greatest potential because its epizootics decimate caterpillar populations in the absence of insecticide applications. However, insect-pathogenic fungi when used for insect control in agriculture have low survival mainly due to the deleterious effects of ultraviolet radiation and heat from solar radiation. In this study, fourteen isolates of M. rileyi were studied and compared with isolates ARSEF 324 and ARSEF 2575 of Metarhizium acridum and Metarhizium robertsii, respectively, whose sensitivity to UV-B radiation had previously been studied. Conidia were exposed at room temperature (ca. 26 °C) to 847.90 mWm-2 of Quaite-weighted UV-B using two fluorescent lamps. The plates containing the conidial suspensions were irradiated for 1, 2, and 3 h, providing doses of 3.05, 6.10, and 9.16 kJ m2, respectively. A wide variability in conidial UV-B tolerance was found among the fourteen isolates of M. rileyi. Isolate CNPSo-Mr 150 was the most tolerant isolate (germination above 80% after 2 h exposure), which was comparable to ARSEF 324 (germination above 90% after 2 h exposure), the most tolerant Metarhizium species. The least tolerant isolates were CNPSo-Mr 141, CNPSo-Mr 142, CNPSo-Mr 156, and CNPSo-Mr 597. Nine M. rileyi isolates exhibited similar tolerance to UV-B radiation as ARSEF 2575 (germination above 50% after 2 h exposure). In conclusion, the majority of M. rileyi isolates studied can endure 1 or 2 h of UV-B radiation exposure. However, after 3 h of exposure, the germination of all studied isolates reduced below 40%, except for CNPSo-Mr 150 and ARSEF 324.
Assuntos
Metarhizium , Animais , Raios Ultravioleta , Esporos Fúngicos , InsetosRESUMO
The fungal species Trichoderma is frequently found in soil antagonizing plant-pathogenic fungi as well as parasitizing plant-pathogenic nematodes. Metarhizium species are insect-pathogenic fungi that are used throughout the world to control agricultural insect pests. Here, we determine whether the antagonism (A) of Trichoderma atroviride to Metarhizium robertsii during growth and spore formation can impact the stress biology of M. robertsii conidia. Cultures of M. robertsii were either produced without exposure to T. atroviride (control) or in the presence of T. atroviride. M. robertsii was grown in dual culture with T. atroviride on potato dextrose agar (PDA) using the following treatments: 1) Trichoderma inoculated at the same time with Metarhizium (A0); 2) Trichoderma inoculated two days after the inoculation of Metarhizium (A2); 3) Trichoderma inoculated four days after Metarhizium (A4); 4) Trichoderma inoculated 6 d after Metarhizium (A6); 5) M. robertsii grown alone on PDA medium (control); and 6) M. robertsii grown alone on minimal medium (Czapek-Dox medium without sucrose) (MM). Germination of M. robertsii conidia from all six treatments was then assessed under osmotic, oxidative, UV-B, and thermal stress. M. robertsii conidia produced on MM were the most tolerant to all stress conditions. For all stress conditions, conidia from treatments A0 and A2 were not viable. For osmotic stress, conidia produced in treatment A4 were the most tolerant, followed by conidia from treatment A6, which were both more tolerant than the control. For oxidative stress, conidia produced in both A4 and A6 treatments were similarly tolerant and more tolerant than conidia produced in the control. For thermal stress, conidia produced in treatments A4, A6, and control (PDA) were similarly heat-tolerant. For UV-B stress, conidia produced in treatments A4 and A6 were equally tolerant and more tolerant than conidia produced in the control. The germination speed of conidia produced in all treatments, A0, A2, A4, and A6 was also tested. Conidia produced on MM germinated faster than the other treatments. Conidia produced in the A4 treatment were the second fastest, followed by conidia produced in treatment A6. Both A4 and A6 conidia germinated faster than conidia produced in the control treatment. Conidia produced in the treatments A0 and A2 did not germinate in 24 h. In summary, moderate levels of biotic stress from a fungal competitor or low-nutrient conditions can enhance the stress tolerance of M. robertsii conidia.