For more than three decades, scientists wondered how specific resistance genes could rapidly spread among diverse populations of crop pathogens like the causal agents of rice blast and wheat stem rust. Now, a team of international scientists led by researchers at the University of California, Davis, have determined how these avirulence (AVR) genes jump between fungal species by hitchhiking on mobile elements, answering a longstanding question that has puzzled plant pathologists for decades.

The study was published in the journal Nature Communications.

"Our team discovered that some AVR genes are part of a discrete group of mobile elements that can rapidly move around genomes, not only within species, but also between different fungal species," said study lead author Menglong Chen, a postdoctoral scholar in the Department of Plant Pathology at UC Davis. "We named these mobile elements Rapidly Moving Elements Associated with Virulence (RMEAVs)."

Plant pathogens are microorganisms, including fungi, bacteria and oomycetes, that can cause diseases in plants. Many important plant pathogens contain genes that encode AVR proteins. AVR genes play an important role in triggering a plant's immune response and causing disease. AVR proteins are recognized by corresponding disease resistance proteins in the host plant, leading to a defense reaction that can limit or prevent infection. As such, AVR genes have been used for decades in plant breeding efforts to develop resistant varieties.

"For resistance breeding to be successful, the resistance genes used need to be durable, meaning that they can provide effective resistance over a long period of time," said co-author Brent Threlfall, an assistant project scientist in the Department of Plant Pathology at UC Davis. "However, the rapid and frequent emergence of new pathogen strains that are able to overcome these resistances has frustrated these efforts."

The pathogen strains that emerge, often referred to as "virulent" strains, contain new or mutated versions of AVR genes. The ability of these AVR genes to change means they can evade detection by the plant's immune system and cause disease on previously resistant plants.

The researchers used a variety of approaches, including comparative genomics, bioinformatics, molecular biology and functional studies to understand the mysterious spread of AVR genes among different fungal species. They focused on two of the most destructive fungal pathogens of cereals worldwide, Magnaporthe oryzae, the causal agent of rice blast disease, and Zymoseptoria tritici, the causal agent of Septoria tritici blotch in wheat.

The scientists were able to show that the RMEAVs are autonomous replicons that replicate and spread using the host fungus' replication and transcription machinery. They also found that RMEAVs can act as selfish elements, meaning they provide no direct benefit to the fungus but can still successfully spread through a population.

"Understanding the mechanisms behind the rapid spread of AVR genes among different fungal species is critical for developing strategies to improve the durability of resistance genes in crops," said study senior author Jian-Min Yuan, a professor in the Department of Plant Pathology at UC Davis. "Our findings provide new clues about how some pathogens can quickly adapt and overcome plant resistance, helping us stay one step ahead in the battle against devastating plant diseases."