A study that could lead to new fungicides to lessen the significant annual losses of rice and other vital cereals reveals how a fungus that destroys rice crops all over the world enters plant cells and is susceptible to basic chemical blockers.
Depending on the season, the fungus Magnaporthe oryzae causes blast disease attacks and kills between 10% and 35% of the world’s rice harvest each year.
Michael Marletta, a professor of chemistry and molecular and cell biology at the University of California, Berkeley, and his team of biochemists have found that the fungus secretes an enzyme that pierces the strong outer layer of rice leaves. After entering, the fungus quickly multiplies and inexorably kills the plant.
Marletta and his coworkers detail the structure of the enzyme and how it functions to aid the fungus in invading plants in a research that was just published in the Proceedings of the National Academy of Sciences publication. A straightforward spray could be helpful in stopping the enzyme’s capacity to breakdown the plant wall because the enzyme is secreted onto the surface of the rice leaf. The search for compounds that inhibit the enzyme is currently underway by the scientists.
According to estimates, eliminating this fungus would increase global food production by 60 million people, according to Marletta, the Choh Hao and Annie Li Chair in the Molecular Biology of Diseases at UC Berkeley. “Targeting this enzyme is different. In this case, we’re hoping to search for certain special compounds and launch a business to create inhibitors for this enzyme.”
This target is a member of the polysaccharide monooxygenases (PMO) family of enzymes, which Marletta and his UC Berkeley colleagues first identified in Neurospora, a more common fungus, a little more than ten years ago. Starch and the hard fibers that give plants their sturdiness, such as cellulose and lignin, are both components of polysaccharides, which are sugar polymers. PMO enzyme accelerates the breakdown of plant fibers by dissolving cellulose into tiny particles that are more accessible to cellulases and other enzymes.
In particular in South Asia and sub-Saharan Africa, more effective control methods for rice blast disease are urgently needed, according to Nicholas Talbot, Marletta’s co-author and fellow plant disease expert who also serves as executive director of The Sainsbury Laboratory in Norwich, United Kingdom. “The polysaccharide monooxygenase may be a useful target for creating novel chemicals that might be used at much lower dosages than current fungicides and with less possible environmental damage. This is because of the enzyme’s significance to plant infection. It might also be a target for methods that use no chemicals at all, including gene silencing.”
These enzymes initially piqued Marletta’s interest, as well as that of UC Berkeley PhD candidates Will Beeson and Chris Phillips. They degrade plant cellulose much more quickly than other enzymes that have been previously described, and as a result, they have the potential to convert biomass into sugar polymers that can be fermented more easily into biofuels. PMOs serve as a source of food for fungi.
Later, he and his UC Berkeley colleagues discovered clues that some fungal PMOs may be capable of more than just converting cellulose into food. These PMOs were activated during the initial phases of infection, suggesting that they serve a purpose other than feeding the organism.
That is what Marletta, Talbot, and the rest of the team discovered. The UC Berkeley researchers, under the direction of postdoctoral fellow Alejandra Martinez-D’Alto, biochemically identified this particular PMO, known as MoPMO9A, and Talbot and postdoctoral fellow Xia Yan demonstrated that infection in rice plants was reduced when the enzyme was knocked off.
Similar PMOs have been discovered by Marletta and his UC Berkeley colleagues in fungi that attack grapes, tomatoes, lettuce, and other important crops, suggesting that the new research may have broad use against plant fungal infections.
“Small molecule inhibitors aren’t just effective against rice. They could be widely employed against numerous crop diseases, “added Marletta. We will investigate both the underlying science of it, like we always do, and try to put pieces together to spin it out as a company because we believe the future for this in terms of drug discovery for plant pathogens is pretty interesting.
Biofuels lead way to attacking fungal pathogen
Marletta has developed a specialty in locating and researching novel and rare enzymes in human cells. But ten years ago, when interest in biofuels as a means of combating climate change grew, he received funding from UC Berkeley’s Energy Biosciences Institute to look for enzymes in other living forms that degrade plant cellulose more quickly than the enzymes that were already known. The idea was to convert cellulose fibers, which are difficult to work with, into short-chain polysaccharides that yeast could then ferment into fuel.
There must be species out there that devour cellulose quickly, I told two of my first-year graduate students, Will Beeson and Chris Phillips, Marletta said. “Those are the ones we want to identify,” the scientist said. “We know the enzymes that consume it slowly, and because they’re sluggish, they’re not very valuable in a biotechnological perspective.”
A common fungus called Neurospora, which is one of the first fungi to attack dead trees after a fire and does a swift job of digesting wood for nutrients, was successfully identified by Phillips and Beeson as having fast-acting enzymes. They identified the offending enzyme, the first known PMO, and explained how it functioned.
Since then, Marletta’s students have discovered 16,000 different types of PMO, the majority in fungi but also in bacteria that consume wood. As a combination with other enzymes, these have so far had modest success in accelerating the generation of biofuels, but they haven’t made biofuels competitive with other fuels.
However, a small portion of these 16,000 variations caught Marletta’s attention since they appeared to do more than just offer food for fungi. In particular, MoPMO9A has an amino acid sequence that interacts with chitin, a polymer that makes up the outer layer of fungi but is absent from rice. All PMOs are secreted, but MoPMO9A was only secreted during the fungus’s infectious cycle.
Following research, it was shown that Magnaporthe concentrates MoPMO9A in the appressorium, a pressured infection cell from which it is discharged onto the plant, with one portion of the enzyme attaching to the fungus’s exterior. A copper atom is located in the center of the enzyme’s opposite end. The copper atom catalyzes a reaction with oxygen to break cellulose fibers when the fungus slaps the enzyme’s free end onto the rice leaf. This reaction helps the fungus penetrate the leaf surface and infiltrate the entire leaf.
Marletta added, “We were wondering why this enzyme has a chitin-binding domain if it’s supposed to be operating on cellulose. We then reasoned that perhaps it is secreted but still adheres to the fungus. In this manner, the fungus will have a catalytic domain between it and the leaf while it is sitting on the plant and ready to puncture the leaf.
That turned out to be true. To find out if other viruses that generate PMOs employ the same ruse to enter and infect leaves, Marletta and Talbot are currently studying them. If this is the case—and Marletta is certain that it is—it creates opportunities to also attack them with a fungal spray.
“PMOs of this type are exclusively present in plant diseases that must enter their host. They will therefore likely function in the same manner “explained Marletta. “Even while rice is really important in and of itself, I believe the scope of study to find inhibitors to this specific PMO will go much beyond that. They will be useful in other significant agricultural plants.”
