Collaboration delivers exciting rice blast discovery

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A study led by UC Berkeley has found that the rice blast fungus gains entry to plant cells in a way that leaves it vulnerable to simple chemical blockers. In collaboration with the Nick Talbot group from The Sainsbury Laboratory, they made the fascinating discovery that the rice blast fungus secretes an enzyme that punches holes in the tough outer layer of rice leaves.

Each year, blast disease, caused by the fungal pathogen Magnaporthe oryzae, attacks and kills plants that represent between 10% and 35% of the global rice crop, depending on weather conditions.

University of California, Berkeley, biochemists led by Michael Marletta and postdoctoral fellow Alejandra Martinez-D’Altoin collaborated with Nick Talbot and his team to describe the structure of an M. oryzae enzyme and how it works to help the fungus invade plants.

Because this enzyme is secreted onto the surface of the rice leaf, a simple spray could be effective in destroying the enzyme’s ability to digest the wall of the plant. The scientists are now screening chemicals to find ones that block the enzyme.

Symptoms of rice blast disease. The fungus responsible essentially kills every rice plant it infects. (Photo credit: Nick Talbot)

Nick Talbot's research group has made significant contributions to our understanding of how fungi infect plants and cause disease with a focus on the rice blast fungus, Magnaporthe oryzae.

This is why Nick and his colleague, Xia Yan, at The Sainsbury Laboratory were only too happy to collaborate with Michael and his UC Berkeley team in this exciting new study and put their expertise in plant genetics and infections to good use.

Nick Talbot's group has a special interest in the infection mechanism of the rice blast fungus.

(A) Life cycle of Magnaporthe oryzae. (B) Scanning electron micrograph with false colouring, of a dome-shaped appressorium (grey) on the rice leaf surface (green). (Graphic credit: Eseola et al., Fungal Genetics and Biology)

A little over ten years ago, Michael Marletta and his UC Berkeley colleagues discovered a family of enzymes called polysaccharide monooxygenases (PMO) in another, more widespread fungus, Neurospora.

Michael was intrigued by a small subset of these 16,000 varieties that seemed to do more than provide nutrition for fungi. MoPMO9A, in particular, had an amino acid segment that binds to chitin, a polysaccharide that forms the outer coat of fungi, but is not found in rice.

“We were curious: ‘Hey, why does this enzyme have a chitin-binding domain if it’s supposed to be working on cellulose?’” says Michael. “And that’s when we thought, ‘Well, maybe it’s secreted, but it sticks to the fungus. That way, when the fungus is sitting on the plant, it can have between it and the leaf the catalytic domain to punch the hole into the leaf.’”

Studies subsequently proved that to be the case. Magnaporthe concentrates MoPMO9A in a pressurized infection cell, called the appressorium, from which it is secreted onto the plant, with one portion of the enzyme binding to the outside of the fungus. The other end of the enzyme has a copper atom embedded in its center. When the fungus slaps the loose end of the enzyme onto the rice leaf, the copper atom catalyzes a reaction with oxygen to break cellulose fibers, helping the fungus breach the leaf surface and invade the entire leaf.

“It isn’t just rice that small molecule inhibitors could be used against. They could be widely used against a variety of different crop pathogens,” Michael adds. “I think the future for this, in terms of drug development for plant pathogens, is pretty exciting, which is why we are going to pursue both the fundamental science of it, like we always do, and try to put together pieces to spin it out as a company.”

Microscope images of Magnaporthe as it produces an appressorium – a pressurized adhesion structure – filled with the enzyme MoPMO9A, labeled with green fluorescent protein. When the enzyme is secreted by the fungus, it binds both to the fungus and the plant and catalyzes a reaction that breaches the leaf wall. (Image credit: Nick Talbot)

Michael Marletta and Nick Talbot are now testing other pathogens that produce PMOs to see if they use the same trick to enter and infect leaves. If so, it opens avenues to attack them with a spray-on fungicide as well.

Nick says, “Given the importance of the PMO to plant infection, it may be a valuable target for developing new chemistries that could be applied at much lower doses than existing fungicides and with less potential environmental impact. It might also be a target for completely chemical-free approaches, too, such as gene silencing.”

He adds that he is particularly excited about these new findings because of the urgent need for more sustainable control strategies for rice blast disease, particularly in South Asia and sub-Saharan Africa.