By Robert Sanders
A fungus that plagues rice crops worldwide gains entry to plant cells in a way that leaves it vulnerable to simple chemical blockers, a discovery that could lead to new fungicides to reduce the substantial annual losses of rice and other valuable cereals.
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, professor of chemistry and of molecular and cell biology, have discovered that the fungus secretes an enzyme that punches holes in the tough outer layer of rice leaves. Once inside, the fungus rapidly grows and inevitably kills the plant.
In a paper published this week in the journal Proceedings of the National Academy of Sciences, Marletta and his colleagues describe the structure of the enzyme and how it works to help the fungus invade plants. Because the 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.
"The estimates are that if you could knock out this fungus, you could feed 60 million more people in the world," said Marletta, the Choh Hao and Annie Li Chair in the Molecular Biology of Diseases at UC Berkeley. "This enzyme is a unique target. Our hope here is that we'll screen to find some unique chemicals and spin out a company to develop inhibitors for this enzyme."
This target is one of a family of enzymes called polysaccharide monooxygenases (PMO) that Marletta and his UC Berkeley colleagues discovered a little over 10 years ago in another more widespread fungus, Neurospora. Polysaccharides are sugar polymers that include starch as well as the tough fibers that make plants sturdy, including cellulose and lignin. The PMO enzyme breaks cellulose into smaller pieces, making the polysaccharide susceptible to other enzymes, such as cellulases, and speeding up the breakdown of plant fibers.
"There is an urgent need for more sustainable control strategies for rice blast disease, particularly in South Asia and sub-Saharan Africa," said Nicholas Talbot, who is Marletta's colleague and co-author, a plant disease expert and executive director of The Sainsbury Laboratory in Norwich in the United Kingdom. "Given the importance of the polysaccharide monooxygenase 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."
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