Fungal peptide turns defense into vulnerability

Researchers from the United States and Germany have discovered that a gene present in some commercial barley varieties, responsible for activating defense against pathogens, can, paradoxically, make them more vulnerable to Bipolaris sorokiniana.

The necrotrophic fungus B. sorokiniana causes dark spots on barley leaves leading to cell death and a significant drop in productivity.

Since the 1990s, a highly virulent strain of the fungus has ravaged fields in North Dakota, the main barley-growing region in the United States. Although some varieties have been shown to be resistant, the genetic basis of susceptibility has remained a mystery until now.

The role of the Scs6 gene

A team led by Shaobin Zhong of North Dakota State University identified the Scs6 gene, part of the MLA family of immune receptors.

This gene, essential in the defense against biotrophic pathogens, was shown to be vulnerable when activated by a peptide produced by B. sorokiniana. Instead of fighting the pathogen, activation of Scs6 results in cell death, facilitating infection.

The team used chemical mutagenesis to create variations of the gene in barley plants. They found that the presence of Scs6 conferred high susceptibility to the fungus. Furthermore, inserting the gene into previously resistant plants resulted in their vulnerability.

The fungus' strategy

The fungal peptide is part of a non-ribosomal molecule that activates the Scs6 receptor, inducing cell death. This process was confirmed by experiments in both barley and tobacco, where the introduction of the gene generated similar responses.

Additional studies revealed that the Scs6 gene is present in approximately 16% of commercial barley varieties and in several wild populations.

Implications for genetic improvement

The discovery reinforces the need for caution in developing disease-resistant varieties.

"Evolution has allowed some pathogens to exploit plants' immune receptors, transforming defense weapons into tools to increase susceptibility," highlights Florian Kümmel, co-author of the study.

For agricultural engineers, the challenge lies in balancing resistance to different types of pathogens. The creation of synthetic receptors that are immune to manipulation by necrotrophs emerges as a possible solution.

More information can be found at doi.org/10.1111/nph.20289