Flower or Fungus? That is the Question
By R. Gary Raham:
Pollinators of yellow-eyed grass (Xyris spp.) growing on the savannas of Guyana, South America, may question their senses as they swoop in for a landing looking for a sweet treat. Bees, for example, will usually find the attractive yellow grass flowers that are on their shopping list. Sometimes, however, they will land on the pseudoflower produced by the fungus Fusarium xyrophilum and inadvertently help the fungus transport spores to its next grass host. F. xyrophilum not only looks like a flower, but smells like one, too. In addition, the yellow-orange pigments of the pseudoflower reflect UV light just like the natural flower—an important cue for bees. This elaborate fungal deception is rare. In fact, Kerry O’Donnell, who is a microbiologist at the U.S. Dept. of Agriculture and co-author of the original paper about the fungus, said “This is the only example that we know of, anywhere on planet Earth, where the false flower is all fungal.”
Other species of Fusarium practice less elaborate forms of deception. One species modifies the leaves of blueberry plants to smell like flowers and even produce nectar. Another species of fungal rust, Puccinia monoica, forms pseudoflowers on Drummond’s Rockcress (in the mustard family) that release scents and sugars, but don’t look like the plant’s flowers. The yellow-orange pigments that F. xyrophilum produces to make petal-like structures probably served the fungus originally as a chemical deterrent to fungus-munching soil arthropods, according to a March 8, 2021 article in Current Biology. F. xyrophilum also produces plant hormones like auxin, cytokinins, and gibberellin analogs that allow it to stifle the yellow-eyed grass’s own reproduction.
Deceit is a common strategy, of course, in life’s tool box. Biologists call such deceptions mimicry, from the Greek word for mime. Viceroy butterflies look like glycoside-filled Monarch butterflies to convince birds that eating them could be nauseating or fatal. Some preying mantises look like flowers hoping that an insect will come close enough that they can snatch and eat them. Crab spiders use floral colors and markings planning for a similar result. I once encountered a geometrid moth caterpillar ineffectively trying to look like a twig while perched on a log. And certain orchids mess with the love life of wasps by emitting pheromones and even resembling a wasp femme fatale so that male wasps looking for a good time become pollen transporters instead.
I also discovered, while perusing the pages of The Lives of Fungi by Britt A. Bunyard (Princeton University Press, 2022) that a species of Dracula orchid in Columbia turns the tables on fungi by mimicking a mushroom. Dracula orchids possess blood-red flowers and long, rather sinister-looking sepal spurs sporting an excellent imitation of a mushroom—both visually and aromatically. Because these orchids live on water-soaked ledges where few other flowering plants germinate, they co-opt mushroom-feeding flies to accomplish pollination. Flies come for filet of fungus, but leave with a load of pollen to complete the sex life of the Dracula orchid instead.
Species of Fusarium fungi have a venerable history that stretches back at least 91 million years. The first flowers to evolve may already have had to contend with Fusarium partners that were either pathogens, incidental hangers-on, or symbiotic in some way. In fact, Fusarium species may have evolved gibberellin plant hormones that effectively mess with the chemistry controlling stem elongation, germination, and floral development in various flowering plants. A close relative of Fusarium causes bakanae or “foolish seedling” disease in rice. Rice stems grow too quickly with a loss of seed production. One Fusarium species prolongs flowering in its host plant so that hummingbirds are more likely transport it’s spores (along with the plant’s pollen, of course) to extend their intimate relationship into the future.
As a paleobiologist I’m always fascinated to discover the elaborate networking that has evolved between the various domains of life—microscopic and macroscopic. Sometimes one species gets more out of the arrangement than another (parasitism), sometimes both partners benefit (mutualism), and sometimes their association results in a unique composite organism, like the obligatory symbiosis of algae and fungi in lichens. And some of these cross-kingdom liaisons, as I’ve described, involve clever deceptions. Fusarium may provide additional surprises in the future.
So, the next time you go outside to smell the roses—or the yellow-eyed grass— “Is it a flower or a fungus?” might be the right question to ask.
R. Gary Raham writes and illustrates both science fact and science fiction. His book, Confessions of a Time Traveler (Penstemon Publications, 2015) contains examples of both, including award-winning contributions to Colorado Gardener magazine. Check out his website at www.rgaryraham.com.