The ribosome-inactivating protein restrictocin deters insect feeding on Aspergillus restrictus

Abstract

The fungus-feeding beetle, Carlpophilus freemani, consumed equal quantities of young mycelia, fewer phialides bearing mature spores and much fewer phialides bearing developing spores of Aspergillus restrictus compared to those of Aspewillus nidulans when tested in diet choice assays. The degree to which specific fungal structures were consumed was inversely related to the localization of high levels of restrictocin, a ribosome-inactivating protein, to those structures. Pure restrictocin added to the insect diet at 1000 p.p.m. killed 385 O/ O of C. fbemani larvae and 62.5 O/ O of SpoCroptera frugiperda larvae in 48 h, but did not affect C. freemsni adults or Helicowerpa zea larvae over the same interval. In diet choice assays, 1000 p.p.m. of restrictocin deterred feeding by adult C. freemani and Sitophilus zeamais compared to control diets. Thus, restrictocin production and localization may have a natural defensive role against insect feeding at times critical to spore formation by A. restrictus, and may have potential as an insect control agent.

Fungal extracellular ribotoxins as insecticidal agents

“…The discovery of the insecticidal protein HtA and the demonstration that it was another member of the ribotoxins family opened the door to speculate about insecticidal activity being their so long searched function. A function that had been already suggested by some other authors (Brandhorst et al, 1996(Brandhorst et al, , 2001) but that was far from being proven in full. Furthermore, HtA is the most singular of all ribotoxins known, given its small size and its much lower degree of sequence identity with the other members of the family.…”

“…Could we then extrapolate HtA insecticidal properties to the whole ribotoxins family? In that case, it would contribute to answer the long time question about the unknown function of this family of toxins (Brandhorst et al, 1996(Brandhorst et al, , 2000Herrero-Galán et al, 2008;Viegas et al, 2009). …”

“…In fact, the assignment of an insecticidal function to fungal ribotoxins has been suggested before (Brandhorst et al, 1996(Brandhorst et al, , 2000. These authors studied the behavior of the beetle Carpophilus freemani against two different Aspergilli: The ribotoxin producer A. restrictus, and A. nidulans, which genome does not contain a gene for this type of proteins (Brandhorst et al, 1996).…”

“…These authors studied the behavior of the beetle Carpophilus freemani against two different Aspergilli: The ribotoxin producer A. restrictus, and A. nidulans, which genome does not contain a gene for this type of proteins (Brandhorst et al, 1996). These results showed a significant decrease in feeding on the first of these two fungi when conidia were starting to develop, while no change in the insect behavior was observed against maturing A. nidulans.…”

Characterization of a new toxin from the entomopathogenic fungus Metarhizium anisopliae: the ribotoxin anisoplin

“…The toxic character of anisoplin suggests some relevant function during insect infection but further research needs to be done. Other ribotoxins like restrictocin have a deterrent effect on insects feeding as a protection mechanism during conidia maturation of Aspergillus (Brandhorst and Kenealy, 1992;Brandhorst et al, 1996). Considering the importance of Metarhizium in a broad spectrum of biotechnological applications, including malaria vector control, the characterization of its molecules involved in pathogenesis is a useful tool (Schrank and Vainstein, 2010;de Bekker et al, 2013;Staats et al, 2014).…”