Metarhizium anisopliae increases virulence against Opisina arenosella

The formation of appressoria increased the pathogenicity of Metarhikum anisopliae against larvae of Opisina arenosella. The study showed a positive correlation between the rate of formation of this infective structure and insect mortality. With greater appressorium formation, Metarhizium anisopliae killed more larvae and reduced lethal time. LC-MS analysis also identified changes in the cuticle metabolism of Opisina arenosella after the formation of the appressorium. This conclusion comes from a study by Chinese researchers.

The appressorium acts as a specialized infection control structure. It allows the Metarhikum anisopliae break the cuticle of Opisina arenosella through mechanical pressure and enzymatic action. In the experiment, the researchers used sulforaphane to reduce the formation of appressoria in Metarhikum anisopliaeThe concentrations tested included 0,00 mg/mL, 0,01 mg/mL, 0,02 mg/mL, 0,05 mg/mL, and 0,08 mg/mL. Increasing the concentration reduced conidia germination and appressorium formation.

In the treatment without sulforaphane, the appressorium formation rate reached 66,60% after 72 hours. This value exceeded the other treatments. Under the same conditions, the cumulative corrected mortality of larvae of Opisina arenosella the mortality rate reached 82,76% in seven days. The LT50 was 4,82 days. In the treatment with 0,08 mg/mL of sulforaphane, the cumulative corrected mortality reached 25,29% in seven days. The LT50 increased to 8,74 days.

The scientists also evaluated the toxicity of the YEMDT medium with sulforaphane. The treatment showed no significant difference compared to the control. This result indicates an absence of direct toxicity of the mixture on the larvae of Opisina arenosella. Thus, the reduction in mortality was related to the lower formation of appressoria by Metarhikum anisopliae.

Conidia germination

The infection dynamics showed the beginning of conidia germination 10 hours after inoculation. The germination rate reached 90,40% at 38 hours and then stabilized. Appressorium formation began at 20 hours. Between 22 and 30 hours, the process accelerated. At 48 hours, the rate reached 65,40% and also stabilized. Based on these data, the authors defined 18 hours as the point before appressorium formation and 48 hours as the point after.

Metabolomics revealed alterations in the cuticle of larvae of Opisina arenosella before and after the formation of the infective structure. Before appressorium formation, the study detected 410 differential cuticular metabolites compared to the control. Of this total, 349 were reduced and 61 were increased. The authors associated these changes mainly with pathways linked to caffeine metabolism and porphyrin metabolism.

After appressorium formation, the number of differential metabolites dropped to 151. Among them, 91 increased and 60 decreased. Comparison between the two time points showed 102 compounds exclusive to the phase after appressorium formation. These compounds included benzenes and substituted derivatives, amino acids and derivatives, as well as heterocyclic compounds.

Among the compounds that increased after appressorium formation, the study cites L-sorbitol, sparfloxacin, N-acetyl-D-glucosamine, L-aspartic acid, and 2,6-dihydroxybenzoic acid. Benzenes and substituted derivatives accounted for 24,7% of the increased compounds. Among the reduced compounds, the authors identified lincomycin, D-gulono-1,4-lactone, luteolin, manghaslin, and 4,5-dihydroxyterephthalic acid. Fatty acids accounted for 20,7% of the reduced compounds.

Metabolic defense pathways

KEGG enrichment analysis indicated activation of metabolic defense pathways in Opisina arenosella. Before appressorium formation, 152 pathways were recorded. Of these, 25 showed significant differences and nine showed highly significant differences. The most prominent pathways included biosynthesis of tropane, piperidine, and pyridine alkaloids, biosynthesis of phenylpropanoids, and biosynthesis of flavonoids.

After appressorium formation, 101 metabolic pathways were recorded. Fifteen showed significant differences. The main pathways affected involved tyrosine metabolism, histidine metabolism, and flavonoid degradation. According to the authors, these processes indicate a response of Opisina arenosella through immune defense, antifungal defense, and toxin degradation.

The researchers concluded that the cuticle of Opisina arenosella does not act merely as a physical barrier. It responds to infection by Metarhikum anisopliae through metabolic reprogramming. The interaction between Metarhikum anisopliae and the body wall of Opisina arenosella involves mechanical pressure, enzymatic hydrolysis, chemical signals, and metabolic changes.

More information at doi.org/10.3390/insects17050476