Many of our valuable cereal crops are from the same grass family, Poaceae, which includes barley, wheat, rice, and maize. Understanding the molecular mechanisms behind this lineage’s survival over millennia could better equip us in ensuring that these plants continue to flourish and feed us in the years to come.
For as long as grasses have evolved into the thriving varieties we know today, the diseases that infect them have been evolving alongside them. The Pucciniales are an order of fungal pathogens that cause rust diseases on a variety of plants. This includes stripe rust, Puccinia striiformis, which is present in all major wheat growing areas and is responsible for an average global yield loss of approximately 2% per year (loss of 2.2 billion pounds/2.8 billion US dollars per year).
There is a close link between any given pathogen and host such that some pathogen species evolve forms that are highly specialized to specific host species. These are called formae speciales and Puccinia striiformis has evolved several specialised forms including wheat stripe rust that infects wheat and barley stripe rust that infects barley.
Puccinia striiformis is an adaptable pathogen. However, while wheat stripe rust has been endemic in Australia for more than 60 years, it has not adapted to infect barley despite both crops being grown in the same regions. To date, the source of this resistance has remained unclear.
Barley has non-adapted resistance to wheat stripe rust, despite not experiencing any selective pressure from the pathogen. The Matthew Moscou group at The Sainsbury Laboratory has been investigating the mechanisms behind this kind of disease resistance and identified three resistance (R) gene loci designated Rps6, Rps7, and Rps8, as contributing to this non-adapted resistance of barley to wheat stripe rust.
To better understand the role of these R genes in barley, scientists in the Moscou group fine-mapped Rps8 to a region on chromosome 4H, which encompasses a presence/absence variation across diverse barley accessions.
Using a forward genetic screen, natural variation, and transgenic complementation, they found that Rps8-mediated resistance to wheat stripe rust is conferred by a genetic module: a receptor kinase (Pur1) and a Poales-specific Exo70 (Exo70FX12).
Exo70 is one of eight proteins making up the exocyst complex which is crucial for exocytosis. Exocytosis is needed for cell secretion and membrane extension, which are critical requirements for cell function and tissue development.
This complex is highly conserved across kingdoms and is present in fungi, plants and animals. Up until recently, it has been assumed that the exocyst complex plays a similar role in other organisms as it does in the model organism, yeast. However, new functions for the complex are starting to emerge from research in plants.
Exo70 is hugely expanded and more diverse in plants in comparison to the other kingdoms, which indicates that it may have been under heavy selection pressure. Phylogenetic analyses of the Exo70FX clade within the Poales lineage (which includes all grasses) suggests that it is experiencing a different evolutionary trajectory to other Exo70 families.
To date, only one other member of the Exo70FX clade has been characterized and appears to be involved in powdery mildew resistance. Given that the only two characterised Exo70FX members are both involved in defense, it seems likely that the clade is involved in plant immunity.
The authors believe that the rapid expansion and diversity of the Exo70FX clade is due to the component losing its original function in regulating cell secretion and gaining a new overall role in immunity. That would explain the sudden diversification under pathogen pressure and getting caught in an evolutionary arms race with adaptable pathogens.
This is an exciting discovery in plant immunity and cereal evolution, with implications for plant breeding. With this information we know that to transfer wheat stripe rust immunity traits to another variety, both Pur1 receptor kinase and ExoFX12 would need to be moved together. Many potential investigative routes have opened up to take this research forward, and the Moscou group’s next steps will be to find out what exactly the Exo70FX gene family is doing and how it plays a role in receptor kinase function.
Lead author, Samuel Holden says "This paper is the result of an interdisciplinary collaboration between scientists from several different groups and areas of expertise. I’m very excited to see what new avenues of research it opens up in plant immunity and beyond."
You can read the article "A lineage-specific Exo70 is required for receptor kinase-mediated immunity in barley" at science.org