A Tale of Two Fungi: Wheat Rust and Its Parasite

Reference article: Zhang, H., He, M., Fan, X., Dai, L., Zhang, S., Hu, Z. and Wang, N., 2022. Isolation, Identification and Hyperparasitism of a Novel Cladosporium cladosporioides Isolate Hyperparasitic to Puccinia striiformis f. sp. tritici, the Wheat Stripe Rust Pathogen. Biology11(6), p.892. https://doi.org/10.3390/biology11060892

Feature image caption: Wheat stripe rust is a fungal pathogen that causes orange bumps to appear on wheat leaves. These bumps are actually filled with large numbers of spores that allow the fungus to spread. Wind and rain are the main dispersal agents of the spores. The fungus is also growing inside the plant tissue, depleting the leaves of its valuable nutrients. Photo credit: A. Yaqub/CIMMYT. License: CC BY-NC-SA-2.0

A World Without Wheat

Imagine a world without wheat. It’s difficult to do because wheat forms the backbone of our food supply. However, wheat production is currently threatened by damaging fungi. The primary enemies of wheat are a group of pathogenic fungi called rusts. Rusts are complex fungi, as sometimes they can alternate between two hosts. Additionally, they form multiple (up to five!) different spore types. The spore types vary based on the current host, season, and whether the fungus is reproducing sexually or asexually. Spores are produced en masse and are easily spread by wind currents, animals, or even people. Before spores are even produced, the fungus grows inside the plant tissue, freely using and depleting the plant’s nutrients. Wheat rust, specifically stripe rust (Puccinia striiformis), can reduce the yield and quality of wheat grain. This pathogen has resulted in astronomical economic loss and crop damage. During severe outbreaks of the pathogen, crop yield can be reduced by as much as 60%!

Fighting Fungi with Fungi

For obvious reasons, significant efforts are underway to control this pathogen. Fungicides are on the frontline of the battle, but repeated use of chemicals breeds fungicide resistance in the pathogen. Likewise, using solely resistant cultivars (varieties that are the result of selective breeding) will allow those fungal mutants that can overcome the plant’s resistance to grow and still cause disease. Biological control is an additional option that is being explored. Such measures involve using another living organism to disactivate or destroy the pathogenic fungus. Recently published research sought to use fellow members of the fungal kingdom to accomplish this. By employing a parasite of the rust, a parasite itself, so-called ‘hyperparasitism’ may be used in agriculture.

Stripe rust is a fungal pathogen belonging to the genus Puccinia. Rust infection can reduce wheat yield by depleting the plants’ nutrients that have been dedicated for the developing seed. Photo credit: Thomas Lumpkin/CIMMYT. License: CC BY-NC-SA-2.0

Zhang et al. noticed something strange on some wheat that was afflicted by the stripe rust. They saw that the yellow spore pustules, called uredinia, were turning a brownish color. They hypothesized that a hyperparasitic fungus was attacking the rust. In order to study this new potential biological control agent, they first isolated it into pure culture. Isolation of a fungus occurs in the laboratory using solid growth media. Once in pure culture, they described the new isolate using both molecular and physical characteristics. Molecular characterization involves extracting the DNA from the fungus and sequencing it. Once the sequence of the DNA is obtained, the scientists compared the data to other similar sequences linked to scientific names. They discovered that they had a novel isolate of Cladosporium cladosporioides. Interestingly, this isolate was more genetically similar to a C. cladosporioides isolate found on dragonfly larvae than another isolate of the same species that also came from wheat stripe rust.

The scientists conducted a controlled trial where they applied
C. cladosporioides onto rust that had already infected wheat leaves. As time progressed towards 9 days post-inoculation with C. cladosporioides, the yellow wheat rust uredinia became more and more brown (B-F). Photo from: Zhang et al., 2022. Isolation, Identification and Hyperparasitism of a Novel Cladosporium cladosporioides Isolate Hyperparasitic to Puccinia striiformis f. sp. tritici, the Wheat Stripe Rust Pathogen. Biology, 11(6), p. 892.

Deactivation and Decline of the Rust

The scientists used a sophisticated microscope, called a scanning electron microscope, to visualize the interactions between the two fungi up close. They observed C. cladosporioides spores (referred to as conidia) land on the rust spore (urediospore). In approximately 120 hours, the conidia had germinated and had completely overgrown the urediospore!

These images from the scientists’ scanning electron microscopy show the stages of C. cladosporioides infection. First conidia land on the rust urediospore (A). They then germinate and form a growth structure called a germ tube (B). After it grows around the rust urediospore, C. cladosporioides begins producing reproductive structures or conidiophores (C). These produce many more spores. Finally, the parasite completely envelops the rust spore, rendering it useless (D). Photo from: Zhang et al., 2022. Isolation, Identification and Hyperparasitism of a Novel Cladosporium cladosporioides Isolate Hyperparasitic to Puccinia striiformis f. sp. tritici, the Wheat Stripe Rust Pathogen. Biology, 11(6), p. 892.

Although the microscope confirmed hyperparasitism, the scientists conducted further studies to demonstrate the harmful effects of C. cladosporioides on wheat stripe rust. The scientists tracked the change in the rust pathogen over time, following inoculation with C. cladosporioides. First, they counted the number of uredinia and demonstrated that as time progressed, the number of yellow uredinia per leaf area decreased as C. cladosporioides took over. They also discovered that the rust spores experienced a significant decrease in germination rate after parasitism occurred. In fact, spore germination decreased 80% five days after inoculation with C. cladosporioides. Finally, the ratio of fungal DNA to wheat DNA decreased as well over time.

A Parasite of a Parasite- Hope for the Future?

Through a combination of microscopic and macroscopic characterization, Zhang et al. discovered a virulent parasite that has a harmful effect on wheat rust. The ability of C. cladosporioides to diminish spore germination is crucial; without viable spores, the pathogen cannot spread as effectively. As with all potential biological control agents, further research is warranted before marketing can be considered. Regardless, this study sheds a bright light on a fungus that could bring hope to wheat growers and all of us who depend on it.

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Maria Marlin

I am a plant pathologist living in Oregon and working for Oregon State University extension. I study soilborne and foliar pathogens that attack ornamental crops, but the vast majority of my time is spent conducting outreach! I train nursery workers in scouting and detecting signs and symptoms of plant disease. I love to write and share my love of science with others! In my free time, I love to horseback ride and adventure through the magical Pacific Northwest that I am so fortunate to call home. Whether it is chasing mountain summits, exploring the rugged coast, or basking in the silence of the mossy, misty, and moody forests, I am my happiest and most inspired when surrounded by nature.

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