Tenebrionid secretions and a fungal benzoquinone oxidoreductase form competing components of an arms race between a host and pathogen

Abstract

Entomopathogenic fungi and their insect hosts represent a model system for examining invertebrate-pathogen coevolutionary selection processes. Here we report the characterization of competing components of an arms race consisting of insect protective antimicrobial compounds and evolving fungal mechanisms of detoxification. The insect pathogenic fungus Beauveria bassiana has a remarkably wide host range; however, some insects are resistant to fungal infection. Among resistant insects is the tenebrionid beetle Tribolium castaneum that produces benzoquinone-containing defensive secretions. Reduced fungal germination and growth was seen in media containing T. castaneum dichloromethane extracts or synthetic benzoquinone. In response to benzoquinone exposure, the fungus expresses a 1,4-benzoquinone oxidoreductase, BbbqrA, induced >40-fold. Gene knockout mutants (ΔBbbqrA) showed increased growth inhibition, whereas B. bassiana overexpressing BbbqrA (Bb::BbbqrAO) displayed increased resistance to benzoquinone compared with wild type. Increased benzoquinone reductase activity was detected in wild-type cells exposed to benzoquinone and in the overexpression strain. Heterologous expression and purification of BbBqrA in Escherichia coli confirmed NAD(P)H-dependent benzoquinone reductase activity. The ΔBbbqrA strain showed decreased virulence toward T. castaneum, whereas overexpression of BbbqrA increased mortality versus T. castaneum. No change in virulence was seen for the ΔBbbqrA or Bb::BbbqrAO strains when tested against the greater wax moth Galleria mellonella or the beetle Sitophilus oryzae, neither of which produce significant amounts of cuticular quinones. The observation that artificial overexpression of BbbqrA results in increased virulence only toward quinone-secreting insects implies the lack of strong selection or current failure of B. bassiana to counteradapt to this particular host defense throughout evolution.

Occurrence of entomopathogenic fungi on insect pests of stored wheat and maize in Central and South Anatolia in Turkey

“…are known to be difficult to kill with entomopathogenic fungi in laboratory bioassays (Rice & Cogburn, 1999;Padin et al, 2002;Akbar et al, 2004;Wakefield et al, 2005;Wakefield, 2006;Michalaki et al, 2006Michalaki et al, , 2007. This was recently shown to be due in part to the production of cuticular defensive secretions that inhibit growth of B. bassiana and other microbes (Pedrini et al, 2015). also found differences in the number of fungal isolates with respect to the location where they had been sampled and a similar variation was noted in our study, especially in sites sampled in the city of Adana, which contributed most of the observed variation.…”

Snapshot of insect–fungus arms race

“…Entomologists examine these host-pathogen interactions not only to obtain better insights on the forces driving natural selection, but also to devise novel environmentally safe alternatives to control insect population while reducing our nation's dependence on conventional agricultural chemicals. In PNAS, the multidisciplinary and multinational team of Pedrini et al unravel a mystery in the insect-fungus arms race (2).…”

“…These quinones are very toxic (7) and keep fungus at bay. Intrigued that degradation of quinone derivatives by other fungi is very common, Pedrini et al (2) asked the question: Why doesn't B. bassiana use its biochemical machinery to degrade/ detoxify quinones and get one step ahead in this race? Surprisingly, the authors found that the fungus is working on it.…”

“…To unambiguously determine a direct role of the enzyme in the inhibitory effect of growth, Pedrini et al (2) first measured enzyme activity of extracts from the WT and two engineered strains grown in the presence and absence of benzoquinone. Clearly, enzyme activity was increased in the WT strain when grown in the presence of benzoquinone, and enzymatic activity was low in the silent strain, whereas in the overexpressing strain enzymatic activity was very high both in the presence and absence of benzoquinone.…”

A multidisciplinary approach to study virulence of the entomopathogenic fungus Beauveria bassiana towards malaria mosquitoes

“…Despite B. bassiana being able to infect a broad range of arthropods, there are reports of insects that are refractory to infection by B. bassiana. For instance, the red flour beetle Tribolium castaneum is not susceptible to infection by B. bassiana, and this is attributed to its ability to produce quinone-containing cuticular secretions that act as antimicrobial defence compounds (Pedrini et al, 2015;Yezerski et al, 2007). Nevertheless, as explained above, EPF have a lower killing capacity than insecticides, which allows mosquitoes to reproduce before dying.…”