Cycloheximide-Producing Streptomyces Associated with Xyleborinus saxesenii and Xyleborus affinis Fungus-Farming Ambrosia Beetles

Symbiotic microbes help a myriad of insects acquire nutrients. Recent work suggests that insects also frequently associate with actinobacterial symbionts that produce molecules to help defend against parasites and predators. Here we explore a potential association between Actinobacteria and two species of fungus-farming ambrosia beetles, Xyleborinus saxesenii and Xyleborus affinis. We isolated and identified actinobacterial and fungal symbionts from laboratory reared nests, and characterized small molecules produced by the putative actinobacterial symbionts. One 16S rRNA phylotype of Streptomyces (XylebKG-1) was abundantly and consistently isolated from the nests and adults of X. saxesenii and X. affinis nests. In addition to Raffaelea sulphurea, the symbiont that X. saxesenii cultivates, we also repeatedly isolated a strain of Nectria sp. that is an antagonist of this mutualism. Inhibition bioassays between S. griseus XylebKG-1 and the fungal symbionts from X. saxesenii revealed strong inhibitory activity of the actinobacterium towards the fungal antagonist Nectria sp. but not the fungal mutualist R. sulphurea. Bioassay guided HPLC fractionation of S. griseus XylebKG-1 culture extracts, followed by NMR and mass spectrometry identified cycloheximide as the compound responsible for the observed growth inhibition. A biosynthetic gene cluster putatively encoding cycloheximide was also identified in S. griseus XylebKG-1. The consistent isolation of a single 16S phylotype of Streptomyces from two species of ambrosia beetles, and our finding that a representative isolate of this phylotype produces cycloheximide, which inhibits a parasite of the system but not the cultivated fungus, suggests that these actinobacteria may play defensive roles within these systems.

Effectiveness of the chemical stabilizers of Talaromyces flavus in biological control of tomato and greenhouse cucumber vascular wilt disease

Fungal antagonist,Talaromyces flavus, is one of the most important biological agents of soil-borne fungal diseases including Verticillium and Fusarium wilt. In this study, to increase the effectiveness ofT. flavusisolates obtained from greenhouse cucumbers and field grown tomatoes five chemical stabilizers were evaluated. Based on the results of previous studies, the most effective substrate for the growth, sporulation and stability ofT. flavusisolates related to the above-mentioned plants was a mix of rice bran and peatmoss. Different chemical stabilizers were mixed with the above-mentioned substrate containing spore suspensions of variousT. flavusisolates. For each plant, a completely randomized experiment was conducted under greenhouse conditions with seven treatments and three replications. The results of this study indicated that treatments containing sodium nitrate and D-cycloserine were more effective than those containing other stabilizers. The overall results of this study suggest that the use of some chemical stabilizers may enhance the biocontrol potential of fungal antagonists in controlling different plant diseases including Verticillium and Fusarium wilt.