A fungus that afflicts rice crops worldwide is entering plant cells in a way that makes it vulnerable to simple chemical blockers, a discovery that might lead to new fungicides to reduce substantial annual losses of rice and other valuable cereals.
Each year, blast disease, caused by the fungal pathogen Magnaporthe oryzaeattacks and kills plants that represent between 10% and 35% of the world’s rice harvest, depending on climatic conditions.
University of California, Berkeley, biochemists led by Michael Marletta, a professor of chemistry and molecular and cellular biology, found that the fungus secretes an enzyme that drills holes in the tough outer layer of rice leaves. 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 structure of the enzyme and how it works to help the fungus invade plants. Since the enzyme is secreted on the surface of the rice leaf, a simple spray might be effective in destroying 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 might eliminate this fungus, you might feed 60 million more people around the world,” said Marletta, Choh Hao and Annie Li Professor of Molecular Biology of Diseases at UC Berkeley. “This enzyme is a single target. Our hope here is that we will screen for unique chemicals and create 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 starch 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 susceptible to other enzymes, such as cellulases, and accelerating the breakdown of plant fibers.
“There is an urgent need for more sustainable blast control strategies, particularly in South Asia and sub-Saharan Africa,” said Nicholas Talbot, colleague and co-author of Marletta, plant disease expert and executive director. from The Sainsbury’s. Laboratory in Norwich UK. “Given the importance of polysaccharide monooxygenase for plant infection, it may be a valuable target for developing new chemistries that might be applied at much lower doses than existing fungicides and with less potential environmental impact. . free approaches too, such as gene silencing. »
Will Beeson and Chris Phillips, PhD students at Marletta and UC Berkeley, were originally interested in these enzymes because they degrade plant cellulose much faster than other previously described enzymes and therefore had the potential to transform biomass into sugar polymers that can be more easily fermented into biofuels. . Fungi use PMOs to provide a food source.
He and his colleagues at UC Berkeley later discovered clues that certain fungal PMOs might do more than just turn cellulose into food. These PMOs were activated in the early stages of infection, implying that they are important in the infection process rather than providing food.
That’s what Marletta, Talbot and their colleagues discovered. Led by postdoctoral fellow Alejandra Martinez-D’Alto, UC Berkeley scientists biochemically characterized this unique PMO, called MoPMO9A, while Talbot and UC Berkeley postdoctoral fellow Xia Yan showed that eliminating the enzyme reduced infection in rice plants.
Marletta and her UC Berkeley colleagues found similar PMOs in fungi that attack grapes, tomatoes, lettuce and other major crops, meaning the new findings may have broad application once morest fungal diseases. Plant.
“It’s not just rice once morest which small molecule inhibitors might be used. They might be widely used once morest a variety of different crop pathogens,” Marletta said. “I think the future for this, in terms of drug development for plant pathogens, is quite exciting, so we’re going to pursue both basic science, as 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, when people got excited regarding biofuels as a way to fight climate change, he received a grant from UC Berkeley’s Energy Biosciences Institute to research enzymes in other forms. of life that digest plant cellulose faster than the enzymes known at the time. . The goal was to turn tough cellulose fibers into short-chain polysaccharides that yeast might ferment into fuel.
“I said to two of my freshman graduate students, Chris Phillips and Will Beeson, ‘You know, there must be organisms that eat cellulose fast,'” 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 biotech sense because they’re slow. »
Phillips and Beeson succeeded in finding fast-acting enzymes in a common mushroom, Neurospora, which is among the first fungi to attack dead trees following a fire and does a quick job of digesting 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 varieties of PMO, most in fungi, but some in wood-boring bacteria. To date, these have been successful in accelerating the production of biofuels as part of a cocktail of other enzymes, although they have not made 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 mushrooms. MoPMO9A, in particular, 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.
Subsequent studies have shown that Magnaporthe concentrates MoPMO9A in a pressurized infection cell, called an appressorium, from which it is secreted onto the plant, part of the enzyme binding to the exterior of the fungus. The other end of the enzyme has a copper atom embedded in its center. When the fungus strikes the free end of the enzyme on the rice leaf, the copper atom catalyzes a reaction with oxygen to break the cellulose fibers, helping the fungus to break through the leaf surface and invade the whole sheet.
“We were curious: ‘Hey, why does this enzyme have a chitin-binding domain if it’s supposed to work on cellulose?’ according to Marletta. “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 in the leaf.’ »
This turned out to be the case. 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 they do – it also opens the way to attack them with a fungicide spray.
“The only place you find PMOs like this is in plant pathogens that need to get to their host. So they’re almost certainly going to work the same way,” Marletta said. “I think the scope of the 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 crops. »
The other co-authors of the paper 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). The research was funded by the National Science Foundation (CHE-1904540, MCB-1818283) and the National Institutes of Health (F32-GM143897).