Fungi need to eat, same as you and me – they can’t make food out of air and sunlight like a plant can. But they have no mouths, so they can’t bite and swallow their food like your average animal; instead, a fungus grows into its food source, releasing digestive enzymes to break down food and absorbing the results. The kinds of fungus you see on dead logs or making fairy rings in your yard are decomposers – they grow through (and eat) plant and animal tissues that are already dead. But there are plenty of species of fungus that chow down on living plant or animal tissues instead. These parasitic fungi face more of a challenge. Dead plants and animals can’t object to fungal digestion; living ones can and do, releasing a variety of cellular or chemical defenses to fight off an invading fungus. But a host only reacts if it detects the parasite after it moves in. A parasitic fungus has a lot to gain if it can keep its host from finding out that it’s arrived.
An interesting strategy for avoiding detection is found in a plant parasite known by the charming name ‘corn smut,’ described in this month’s issue of PLoS Biology. Corn smut fungus (Ustilago maydis) is a parasite of (unsurprisingly, given the name) corn (Zea mays). It infects living corn stalks, growing long, filament-like cells called hyphae through the tiny spaces between the corn plant’s cells. There’s plenty to eat in those narrow alleys – sugars move through them as they leave the photosynthetic cells where they’re made and head for other parts of the plant. The predominant form of sugar moving through these spaces is sucrose -- a two-part sugar built of a molecule of glucose and a molecule of fructose. Your average fungus would dump digestive enzymes into the space between the cells, break the sucrose into glucose and fructose, then absorb the simpler molecules. But because there isn’t supposed to be a lot of simple sugar in that extracellular space, the glucose spike becomes a signal that tells the plant something is wrong and triggers defensive systems to attack the fungus. The corn smut fungus avoids this state of affairs with a unique transport protein that lets it move sucrose directly into its cells.
The protein, called Srt1, sits in corn smut’s cell membranes and shuttles sucrose from the outside of its cells to the inside of its cells. That’s it. But what an effect it has! Srt1 lets corn smut digest sugar inside its body, hiding the glucose signal that would tell its host it has a parasite. Unmolested, corn smut uses the sugar it harvests to grow rapidly, forming hyphae-filled tumors all over the plant (most impressively inside corn kernels, where smut turns the seeds into giant, gray fungal breeding grounds). Mutant forms of corn smut lacking Srt1 can still colonize corn plants, but they don’t form tumors, suggesting that when the plant’s defense systems can identify the fungus they can keep it from growing out of control. Even more interesting, the gene for Srt1 is only expressed when the smut is inside plant tissue: otherwise normal corn smut hyphae didn't bother to build sucrose-transporting proteins when they were growing on agar plates in the lab. The signal that tells the fungus it's in a plant and starts it on its stealthy sugar diet is still unknown.Wahl, R., Wippel, K., Goos, S., Kämper, J., & Sauer, N. (2010). A Novel High-Affinity Sucrose Transporter Is Required for Virulence of the Plant Pathogen Ustilago maydis PLoS Biology, 8 (2) DOI: 10.1371/journal.pbio.1000303
Photo: © 2008 MAVI RODRIGUEZ GARCIA