Researchers identify the hierarchical regulatory network controlling infection by the devastating blast pathogen

Fungal plant diseases such as the blast pathogen Magnaporthe oryzae can devastate rice and wheat crops. Rice blast is one of the most serious threats to rice production worldwide, while wheat blast outbreaks are showing up in continents where the disease was never a threat before. Developing effective and robust management strategies for blast diseases is becoming increasingly urgent as we deal with the exacerbating effects of climate change on agriculture and a rising need for secure food supplies.

Gaining a fundamental understanding of how M. oryzae infects its host plants is crucial for developing targeted disease intervention strategies.

Researchers from Nick Talbot’s group at The Sainsbury Laboratory are studying how M. oryzae penetrates the leaf surface via a specialized infection cell called the appressorium. By generating enormous osmotic pressure, the rigid penetration hypha of the appressorium can pierce a host plant’s cuticle. Preventing formation of appressoria would therefore be a good way to incapacitate the fungus, before it can cause disease.

To do this, we need to answer the question: What are the master regulators of appressorium development?

In a study recently published in Nature Microbiology, researchers set out to answer this question by using a comparative transcriptomics approach which involved extensive RNA-sequencing. The team discovered that 6000 genes, representing 40% of the M. oryzae genome, changed their expression during appressorium development in response to contact with hydrophobic glass surfaces which trigger the infection process by mimicking the surface characteristics of rice leaves.

By comparing RNA-seq data for M. oryzae mutants lacking a mitogen activated protein (MAP) kinase called Pmk1 and a transcription factor Mst12, which work in a hierarchy to control appressorium development, the main players that regulate these genes were identified. This led to the discovery that Pmk1 acts through a novel master regulator called Hox7, which is necessary for an M. oryzae spore to successfully infect a host.

Hox7 controls the expression of 4000 genes, whereas 2000 genes depended on Mst12.  These transcription factors therefore form a two-tier transcriptional model downstream of the MAP kinase Pmk1, which acts at the top of the hierarchy to phosphorylate Hox7, followed by Mst12. These transcription factors regulate different aspects of the infection process. Hox7 controls appressorium development through inhibiting the formation of hyphae, while promoting development of infection cells, whereas Mst12 predominantly controls maturation of the appressorium and how this facilitates plant tissue colonization events such as the secretion of effector proteins to disable the host immune system.

The study presents a novel model for transcription regulation in which Pmk1 is a global regulator of a network of transcription factors during appressorium development. Through a small number of Pmk1-dependent master regulators, such as Hox7 and Mst12, the expression of a very large number of downstream genes is effectively controlled.

In their future work, the study’s authors aim to characterize the whole network of transcription factors that they found. By identifying signalling modules during the infection process, more targets are exposed for future disease interventions. A fundamental understanding of how plants and pathogens interact will increase the likelihood of effectively managing blast disease in the future.

Dr Míriam Osés-Ruiz, lead author of the study, says “We hope to be able to further dissect how the hierarchical regulation of transcription factor networks work at the molecular level and determine what processes they specifically control. In this way, we can start thinking about control strategies that interrupt these networks to prevent the disease”

Prof Nick Talbot, group leader, says “We have known for many years that appressorium development by the blast fungus depends on the action of a kinase called Pmk1, but we never knew how. This new work shows us that Pmk1 is the central hub in a cascade of regulators that all need to act in concert. Only then can the blast fungus infect plants. We have discovered how the top of this hierarchy of regulators functions, so that leaves us able now to work out the whole mechanism. This will pave the way for completely new interventions to control this devastating disease.”

Dr Frank Menke, Proteomics team leader, says “Through the combination of (chemical) genetics, RNA seq and targeted phosphoproteomics we were able to delineate a regulatory framework controlling appressorium development and identify Hox7 as a direct Pmk1 target. Further work characterizing additional targets of Pmk1 is ongoing and is expected to show how this framework functions in molecular detail.”

Collaboration was at the heart of this study’s success. Through the proteomics work led by Frank Menke the two-step phosphorylation of Hox7 and Mst12 by Pmk1 was established. With Mathias Nielsen, a PhD student from Caroline Dean’s laboratory at the John Innes Centre, and Dr. Jitender Cheema, the optimization and analysis of the ChIP-seq protocol needed to identify the direct targets of transcription factors was made possible.

You can find the article “Appressorium-mediated plant infection by Magnaporthe oryzae is regulated by a Pmk1-dependent hierarchical transcriptional network” here.