Pathogen Profile #16

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inbox!

It’s rare to come across a plant pathogen that sells for more than the crop it’s infecting. I was surprised to learn that it’s the case for today’s pathogen which infects a staple crop. Scroll down to learn about this week’s pathogen. If you’re a Robigalia subscriber, you also have access to my online database and today’s title song!

Ustilago maydis, a fungal pathogen that infects Zea mays (corn/maize), causes a disease known as corn smut, transforming kernels into large, tumour-like galls. While considered a pest in agriculture due to significant yield losses, U. maydis is also a delicacy in Mexican cuisine, known as huitlacoche or "Mexican truffle."

Huitlacoche has been a staple in Mexican cuisine for centuries, celebrated for its rich, earthy flavour reminiscent of mushrooms and truffles, with a subtle sweetness from the corn. Its cultural significance dates back to pre-Columbian times, where the Aztecs considered it a gift from the gods.

Today, huitlacoche is gaining global popularity, appearing in innovative dishes worldwide, and valued for its culinary, cultural, and nutritional richness, being a good source of protein, fibre, vitamins, and minerals. Recent studies have shown that extracts and compounds isolated from huitlacoche possess antioxidant, antimicrobial, anti-inflammatory, antimutagenic, antiplatelet, and dopaminergic properties, highlighting its potential health benefits beyond its culinary applications.

Ustilago maydis has a complex life cycle that involves both sexual and asexual reproduction. It starts as a single-celled spore that grows into a thread-like mycelium. When two compatible mycelia meet, they join to form a new mycelium with two nuclei. This new mycelium can infect corn plants. Once inside the plant, the fungus causes the formation of large, tumour-like galls. Inside these galls, the two nuclei fuse together and divide, producing new single-celled spores. When the galls mature and break open, these spores are released and can infect other corn plants, starting the cycle all over again.

Ustilago maydis thrives in hot, dry weather conditions, like those during droughts and enters the plants through sites of physical damage. High nitrogen levels in the soil, often due to heavy manure application, can further increase disease prevalence. The fungus spreads via spores released from galls on infected plants, which then infect other corn plants, continuing the cycle. These teliospores can be dispersed by wind, water, or mechanical means like contact with humans, animals, or equipment.

Ustilago maydis is a double-edged sword when it comes to farming and food. It can be a major headache for farmers. Conversely, as huitlacoche, it can fetch a high price as a gourmet ingredient. If you’re interested in diving into the science of Ustilago maydis, I’ve summarised a few articles below. Robigalia subscribers can scroll down to access my online database, which includes full summaries of these and all other featured Robigalia articles.

Ustilago maydis papers

Novel Secreted Effectors Conserved Among Smut Fungi Contribute to the Virulence of Ustilago maydis

• The study uncovers the crucial role of novel secreted effector proteins conserved among smut fungi in promoting virulence. By analysing 53 core effector groups, researchers found 20 contribute significantly to the pathogen's ability to infect maize, highlighting potential targets for developing disease-resistant crops.

Progress in pathogenesis research of Ustilago maydis, and the metabolites involved along with their biosynthesis

• Ustilago maydis employs a variety of effector proteins and secondary metabolites to successfully infect maize. The article reviews the current understanding of U. maydis pathogenesis, focusing on key effectors, their biosynthesis and regulation, and the roles of secondary metabolites like ustilagic acids and mannosylerythritol lipids in facilitating infection.

Ustilago maydis PR-1-like protein has evolved two distinct domains for dual virulence activities

• The study on reveals the PR-1-like protein, UmPR-1La, has evolved dual functional domains that contribute to fungal virulence. One domain binds plant-derived phenolics, prompting protective hyphal structures, while the other is cleaved by a plant cysteine protease to release peptides that suppress plant immunity.

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Until next week,

Alyssa