Researchers in the field of bioballistics – how plants, fungi and animals accelerate seeds, spores or even parts of their body to high speed – have found an amazing variety of techniques to overcome friction with the air, the main limitation for small spores and seeds.
"Understanding how Sclerotinia is discharging its spores and getting them onto the plants will eventually lead us to new ways of looking at plant architecture," said co-author Helene Dillard, a plant pathologist who heads Cornell University's Cooperative Extension and is associate dean of the College of Agriculture and Life Sciences. "When plant breeders are developing new varieties of crops – such as beans, cabbage or sunflowers – they can keep in mind how Sclerotinia gets the spores to reach their targets, which is usually the flowers."
Scientists have recognized for more than 100 years that many spore-producing fungi – the ascomycetes – release their spores in plumes that carry them long distances. More than 50 years ago, scientists noted that these spore plumes create a wind of their own, but the physics of the plumes was not understood, Roper said. In addition, little work has been done on how seeds or spores cooperate to improve dispersal to new environments.
With training in the mathematics and physics of fluid flow, Roper and Seminara decided to investigate in collaboration with Pringle, a Harvard mycologist.
For the current PNAS paper, the researchers used high-speed video to clock the speed of spores ejected by Sclerotinia, finding that they are expelled at a speed of about 8.4 meters per second (19 miles per hour). However, because the spores are so small – 10 microns long – air drag brings them to a stop in a mere 3 millimeters. When thousands of spores are ejected at the same time, however, some can travel more than 100 millimeters, or 4 inches.
These high-speed video images enabled Roper and Seminara to model spore plume movement precisely with standard equations of hydrodynamics. They showed that the thousands of spores ejected at the same time quickly eliminate all drag and allow the spores to travel about a centimeter, by which time the wind generated by the spores captures and whisks them to a speed of 60 centimeters per second. Their upward motion is stopped only by gravity, Roper said.
The added range from "hydrodynamic cooperation" allows fungi on the ground to shoot their spores into flowers or plant wounds, whence they can quickly spread throughout the plant and kill it.
Often called white mold, Sclerotinia rot, or wilt, the fungus attacks more than 400 species of plants, Dillard said, including beans, sunflowers, soybeans, canola and peanuts, and can wipe out entire fields. In spring and summer, the fungus produces cups (apothecia) about one-half centimeter across that spew spores into the air to infect plants. The fungus produces overwintering seed-like bodies called sclerotia on the infected plant tissues.
"It grows across a cabbage head and produces these small sclerotia that look like mouse droppings," Dillard says. "The sclerotia fall on the ground, and are then in position to initiate the infection process the following year."
The researchers were also curious how fungi manage to eject their spores simultaneously. To investigate this, they grew another mold, a coprophilic fungus from the genus Ascobolus, on horse dung and focused their high-speed video camera on the two-millimeter, cup-shaped fruiting body containing tens of thousands of spore sacs (asci), each containing eight spores. They found that, while the spore sac that ejects first seems to be random, after the first one or two go off, a wave of ejection travels outward as successive rings of spore sacs rupture in sequence. Because this happens in one-tenth of a second, the ejection seems simultaneous.
"What looks like a plume is actually a series of sheets going off," Roper said.
By tweaking their mathematical model to take account of this, Roper and Seminara discovered that cooperative ejection in sheets is a highly effective method for shooting spores long distances. The scientists continue to investigate how spore ejection is initiated, and whether and how spores can cheat to make sure that they get ejected farther than their companions.
In a few tenths of a second, Sclerotinia expels hundreds of thousands of spores in a plume that can rise 20 cm, much higher than any single spore by itself. At right, the spores ejected first (blue) do not travel as far or as fast, but they create a wind that carries spores released milliseconds later (yellow) much higher, and the final spores (red) even higher and faster. Filmed at 3,000 frames per second.
(Photo Credit: Mahesh Bandi, Agnese Seminara/Harvard University and Marcus Roper/UC Berkeley)
The spore cups, or apothecia, of the fungus Sclerotinia sclerotiorum are about a half-centimeter across and produce thousands of spores throughout the spring and summer. The fungus is often hidden in fields, where they infect crops ranging from peanuts and cabbage to sunflowers.
(Photo Credit: Helene Dillard/Cornell University)