Serendipity in the wrestle between Trichoderma and Metarhizium

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.