Towards better fates for wheat crops: Gene-level insights into a deadly pathogen
After decades, researchers finally have some gene-level insights into an age-old evolutionary arms race between wheat and the devastating fungal pathogen stem rust disease. The results of two new studies could allow for DNA testing to identify whether a rust pathogen in a particular wheat crop can overcome a resistance gene that is being introduced in high-yielding wheat varieties in many countries. In two separate efforts, scientists studying stem rust strains from different parts of the globe narrowed in on genes for products that trigger wheat's built-in defenses – the Sr50 resistance gene. Globally, wheat provides about 20% of the food calories consumed by humans every day. Puccinia graminis f. sp. tritici, or stem rust, is a fungus that threatens wheat crops worldwide, and is undeterred by many of the disease-resistance genes that otherwise protect wheat crops. In many cases, plants can fend off pathogens when their immune systems detect distinct peptides excreted by invaders. However, to date, it has been very difficult to identify the peptides released by stem rust because the fungus harbors a genome riddled with repetitive sequences and complexity. Here, in one study, Andres Salcedo et al. identified versions of a particularly nasty strain of the fungus that could infect resistant wheat strains, which drew their attention to the AvrSr35 gene in stem rust. The fungal gene encodes a peptide that increases in abundance over the course of plant infection, the authors report. Through a series of experiments, they show that mutated forms of the peptide are able to evade detection by the plant's immune system, demonstrating how this particularly infectious form of stem rust is winning the evolutionary arms race over its host. In a separate study, Jiapeng Chen et al. identify another fungal peptide, AvrSr50, which binds to the plant's receptor, Sr50, for the rust-resistance gene. They confirmed this relationship by expressing Sr50 in other types of plants and exposing the plants to fungal strains expressing AvrSr50, and did further work to determine when AvrSr50 is released throughout the course of fungal infection. The findings of both studies could help in quickly identifying whether or not a given wheat crop needs to be sprayed with expensive fungicide to protect against rust – which would otherwise devastate the crop in a matter of weeks. These findings are highlighted in a related Perspective by Matthew J. Moscou and H. Peter van Esse.
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