Azoles Trigger Two Self-Destruction Pathways in Fungi

Scientists have shown that the most common class of antifungals used worldwide induces microorganisms to self-destruct. Research from the University of Exeter may enhance strategies for preserving human life and food security.

Up to 25% of the world's crops are lost due to fungal infections. These infections can be deadly for people with compromised immune systems and pose a risk to humans as well.

Azole fungicides are the most effective “weapon” against fungal plant diseases. At over £3 billion annually, these chemical compounds make up about a fifth of the global market for agricultural fungicides. The fact that antifungal azoles are frequently used to treat harmful fungi that can kill humans makes them even more crucial to the efforts to control fungal growth.

Azoles target the pathogen cell's ergosterol-producing enzymes, resulting in compounds that resemble cholesterol. Ergosterol is an essential part of cellular biomembranes. Azoles deplete ergosterol, killing the pathogen cell. Despite the significance of azoles, scientists do not fully understand the true cause of pathogen death.

Scientists from the University of Exeter have discovered the biological mechanism by which azoles destroy pathogenic fungi. The research was published in the journal Nature Communications.

With funding from the BBSRC, the group of scientists, led by Professor Gero Steinberg, used molecular genetics and live-cell imaging techniques to investigate why the crop pathogenic fungus Zymoseptoria tritic (Z. tritici) dies when its ergosterol synthesis is inhibited.

This fungus causes wheat to develop septoria leaf blotch, a significant disease in temperate areas estimated to cost the UK more than £250 million annually in lost harvest and fungicide spraying expenses.

The Exeter group treated the living Z. tritici cells with agricultural azoles, watched them, and examined their response. They disproved the generally held belief that azoles destroy pathogen cells by rupturing the outermost cell membrane.

Rather, they discovered that azole-induced ergosterol decrease increases the activity of cellular mitochondria, the “powerhouse” of the cell, which is necessary to provide the cellular “fuel” that powers the pathogen cell's metabolic operations.

While creating more “fuel” does not necessarily mean harm, the process produces more poisonous byproducts. These byproducts cause the pathogen cell to undergo apoptosis, a kind of “suicide.”

Furthermore, a second “self-destruct” pathway brought on by lower ergosterol levels enables the cell to “self-eat” its own nuclei and other essential organelles; this process is called macroautophagy. The authors demonstrate that the fatal action of azoles is derived from both cell death mechanisms. They conclude that by starting self-destruction, azoles force the fungal infection to commit “suicide.”

The scientists discovered that the rice-blast fungus Magnaporthe oryzae uses the same mechanism for azole-induced pathogen cell death. The fungus-caused disease can kill up to 30% of rice, a staple food crop for more than 3.5 billion people worldwide.

The group also investigated terbinafine, sulfonate, and fluconazole, three more therapeutically significant anti-fungal medications that interfere with ergosterol production.

All elicited identical reactions in the pathogen cell, indicating that ergosterol biosynthesis inhibitors generally cause cell death.

Our findings rewrite common understanding of how azoles kill fungal pathogens. We show that azoles trigger cellular “suicide” programs, which result in the pathogen self-destructing. This cellular reaction occurs after two days of treatment, suggesting that cells reach a “point of no return” after some time of exposure to azoles.”

Gero Steinberg, Professor, Lead Author and Director, Bioimaging Centre, University of Exeter

Steinberg said, “Unfortunately, this gives the pathogen time to develop resistance against azoles, which explains why azole resistance is advancing in fungal pathogens, meaning they are more likely to fail to kill the disease in crops and humans.”

Our work sheds light on the activity of our most widely used chemical control agents in crop and human pathogens across the world. We hope that our results prove to be useful to optimize control strategies that could save lives and secure food security for the future.”

Gero Steinberg, Professor, Lead Author and Director, Bioimaging Centre, University of Exeter