A fungus affecting rice crops worldwide invades plant cells in a way that makes it vulnerable to simple chemical blockers, a discovery that could lead to new fungicides to reduce the significant annual losses of rice and other valuable crops.
Every year, the blast disease caused by the fungal pathogen Magnaporthe oryzae affects and kills crops that account for between 10% and 35% of the world rice crop, depending on climatic conditions.
Biochemists at the University of California, Berkeley, led by Michael Marletta, professor of chemistry and molecular and cell biology, found that the fungus secretes an enzyme that bores holes in the tough outer layer of rice paddies. Once inside, the fungus grows rapidly and inevitably kills the plant.
In an article published this week in the journal Proceedings of the National Academy of Sciences, Marletta and colleagues describe the enzyme’s structure and how it helps the fungus enter plants. Since the enzyme is secreted on the surface of the rice leaf, a simple spray could effectively destroy the enzyme’s ability to digest the plant wall. Scientists are now looking at chemicals to find those that block the enzyme.
“It’s estimated that if you could eliminate this fungus, you could feed 60 million more people around the world,” said Marletta, Choh Hao and Annie Li, professors of molecular biology of diseases at UC Berkeley. “This enzyme is a single target. Our hope here is that we look for unique chemicals and start a company to develop inhibitors of this enzyme. »
This target is part of a family of enzymes called polysaccharide monooxygenases (PMOs) that Marletta and his colleagues at UC Berkeley discovered just over 10 years ago in another more common fungus, Neurospora. Polysaccharides are sugar polymers that include starches as well as the tough fibers that make plants strong, including cellulose and lignin. The PMO enzyme breaks down cellulose into smaller pieces, making the polysaccharide vulnerable to other enzymes such as cellulases and accelerating the breakdown of plant fibers.
“There is an urgent need for more sustainable blast response strategies, particularly in South Asia and sub-Saharan Africa,” said Nicholas Talbot, colleague and co-author of Marletta, crop disease expert and managing director of The Sainsbury’s. Laboratory in Norwich UK. “Given the importance of polysaccharide monooxygenase in plant infections, it could be a valuable target for the development of new chemicals that could be applied at much lower doses than existing fungicides and with less potential environmental impact.” . also free approaches like gene silencing. »
Will Beeson and Chris Phillips, graduate students at Marletta and UC Berkeley, were originally interested in these enzymes because they break down plant cellulose much faster than other previously described enzymes and therefore had the potential to convert biomass into sugar polymers that are more easily fermented into biofuels . . Fungi use PMOs to provide a food source.
He and his colleagues at UC Berkeley later discovered evidence that certain fungal PMOs might do more than just turn cellulose into food. These PMOs were activated in the early stages of infection, meaning they are more important in the infection process than in providing food.
That’s what Marletta, Talbot, and their colleagues found. Led by postdoc Alejandra Martinez-D’Alto, UC Berkeley scientists biochemically characterized this unique PMO, called MoPMO9A, while UC Berkeley’s Talbot and postdoc Xia Yan showed that eliminating the enzyme reduced infection in rice plants.
Marletta and her colleagues at UC Berkeley found similar PMOs in fungi that attack grapes, tomatoes, lettuce and other important crops, meaning the new findings could have broad application against fungal diseases.
“It’s not just rice that small molecule inhibitors could be used against. They could be used on a large scale against a variety of different plant pathogens,” said Marletta. “I think the future is pretty exciting in terms of drug development for plant pathogens, so we will both do the basic research like we always do and try to collect coins to run it as a business. »
Biofuels pave the way for attack by fungal pathogens
Marletta specializes in identifying and studying new and unusual enzymes in human cells. But 10 years ago, just as people were getting excited about biofuels as a way to combat climate change, he received a grant from UC Berkeley’s Energy Biosciences Institute to research enzymes in other life forms that digest plant cellulose faster than enzymes known at the time. . The goal was to convert tough cellulose fibers into short-chain polysaccharides that yeast could ferment into fuel.
“I said to two of my freshman students, Chris Phillips and Will Beeson, ‘You know, there must be organisms that eat cellulose quickly,'” Marletta said. “These are the ones we want to find because we know the enzymes that eat it slowly, and they’re not particularly useful in a biotechnological sense because they’re slow. »
Phillips and Beeson managed to find fast-acting enzymes in a common fungus, Neurospora, which is among the first fungi to attack dead trees after a fire and quickly digest wood for nutrients. They isolated the enzyme responsible, the first known PMO, and described how it works. Since then, Marletta students have identified 16,000 species of PMO, most in fungi but a few in wood-boring bacteria. So far, these have been successful in accelerating the production of biofuels as part of a cocktail of other enzymes, but they do not make biofuels competitive with other fuels.
But Marletta was intrigued by a small subset of those 16,000 varieties that seemed to do more than feed the fungi. In particular, MoPMO9A had an amino acid segment that binds to chitin, a polysaccharide that forms the outer layer of fungi but is not found in rice. And although all PMOs are secreted, MoPMO9A was secreted during the infection cycle of the fungus.
Studies then showed that Magnaporthe concentrated MoPMO9A in a pressurized infection cell called the appressorium, from which it was secreted onto the plant, part of the enzyme that binds to the outside of the fungus. The other end of the enzyme has a copper atom embedded in its center. When the fungus encounters the free end of the enzyme on the rice leaf, the copper atom catalyzes a reaction with oxygen to break down the cellulose fibers, helping the fungus to break through the leaf surface and invade the entire leaf.
“We were curious, ‘Hey, why does this enzyme have a chitin-binding domain when it’s supposed to act on cellulose?’ to Marletta. “And then we thought, ‘Well, maybe it’s secreted, but it’s attached to the fungus. That way, when the fungus sits on the plant, it can have the catalytic domain between itself and the leaf to punch the hole in the leaf.” »
This turned out. Marletta and Talbot are currently testing other pathogens that produce PMOs to see if they use the same trick to penetrate and infect leaves. If so – Marletta is convinced – it also opens up the possibility of attacking them with a fungicide spray.
“The only place you find such PMOs is in plant pathogens, which need to get to their host. So they’re almost certainly going to work the same way,” Marletta said. “I think the scope of work to develop inhibitors of this particular PMO goes well beyond rice, although that in itself is quite important. We will be able to use them in other important cultures. »
The paper’s other co-authors are Alejandra Martinez-D’Alto, Tyler Detomasi, Richard Sayler, and William Thomas of UC Berkeley. Marletta is a member of the Berkeley Branch of the California Institute for Quantitative Biosciences (QB3). Research was funded by the National Science Foundation (CHE-1904540, MCB-1818283) and the National Institutes of Health (F32-GM143897).