Researchers from the United States and Germany have identified a peptide that makes barley, grown in the U.S.’s key barley-producing region, more vulnerable to leaf blotch disease.
The fungus responsible for the disease uses this peptide to activate an immune receptor in the plant, causing affected cells to die. Over time, more immune receptor genes have been incorporated into new, disease-resistant barley varieties. However, the leaf blotch pathogen appears to exploit this and turns the plant’s defense against itself. This discovery highlights the importance of careful breeding to prevent inadvertently increasing susceptibility to other pathogens while targeting specific diseases.
Barley is one of the world’s most important grain crops, with 155 million tonnes produced annually, the majority of which are used for animal feed. Like other crops, barley is vulnerable to pathogenic microorganisms that can cause significant yield losses.
One of the most damaging diseases is spot blotch, caused by a necrotrophic fungus that kills infected plant cells. This disease is characterized by dark, chocolate-colored blotches that lead to leaf drying. In the 1990s, a new, highly destructive strain of spot blotch spread to North Dakota, a major barley-growing region in the U.S. Interestingly, some barley plants were highly susceptible to this new strain, while others were resistant.
To identify the gene(s) responsible for this susceptibility, Shaobin Zhong’s team at North Dakota State University in Fargo treated barley seeds with a substance that induces random mutations. By analyzing plants that were no longer susceptible to spot blotch, the researchers discovered that all the resistant plants shared mutations in a gene called Scs6. When Scs6 was introduced into resistant barley lines, they became highly susceptible to the disease.
Mutations Make Susceptible
Thirty years after the new spot blotch strain first caused devastation in North Dakota, the gene responsible for making some barley plants highly susceptible was finally identified. However, one mystery remained: when the sequence of Scs6 was decoded, it was found to belong to the MLA family of immune receptors, which are typically known to provide immunity against biotrophic pathogens—those that require living plant hosts. What could explain this contradiction?
To investigate further, Zhong’s team collaborated with Paul Schulze-Lefert’s group in Germany. Based at the Max Planck Institute for Plant Breeding Research in Cologne, Schulze-Lefert’s team has extensive expertise in studying immune receptors. Graduate student Florian Kümmel conducted experiments to understand why an immune receptor, typically associated with immunity, was paradoxically making plants more susceptible to spot blotch infection.
Peptide Causes Cell Death
Zhong’s team had previously discovered that a unique molecule, a non-ribosomal peptide, is responsible for the virulence of barley spot blotch. The researchers hypothesized that the pathogen uses this unusual molecule to target and activate the Scs6 immune receptor, triggering a cell death response typically aimed at defending against biotrophic pathogens. When wash-out from infected barley leaves was injected into healthy barley leaves carrying the Scs6 gene, a cell death response occurred. Notably, when Kümmel conducted the same experiment on leaves of another plant species that had been induced to express Scs6 but no other barley proteins, he also observed cell death. This suggests that the cell death triggered by Scs6 results from a direct interaction between the receptor and the non-ribosomal peptide.
Barley domestication and breeding for resistance has led to the enrichment of many immune receptor genes such as Scs6 in commercially grown barley elite varieties. Could this be a doubled edge sword? As co-first author Kümmel points out: “Evolution has seemingly led some pathogens to hijack host immune receptors, turning the plants weapons against themselves to make them more susceptible to disease. In breeding for resistance to one type of disease, we need to ensure that we are not inadvertently making our crops more susceptible to other pathogens.”
