In Michael Knoblauch’s lab, the gap between fundamental research and practical applications is a narrow one.
Knoblauch studies the inner workings of phloem (FLOAM), the channels that transport water and nutrients throughout a plant. Research doesn’t get much more basic than that—yet one of his recent discoveries is leading him straight to the patent office.
He’s found that structures in the phloem of some plants have great potential as high-tech, microscopic valves, sensors, and motors.
Knoblauch named the structures “forisomes,” which means “gate-bodies.” He found that they keep the phloem from leaking after it’s been injured.
Phloem is comprised of parallel tubes, or sieve elements, each of which is made of long, narrow cells laid end-to-end. The end of each cell, the sieve plate, is pocked with holes that allow fluid to move into the neighboring cell.
In healthy sieve cells, forisomes resemble toothpicks. They were first observed through the light microscope more than 100 years ago, but nobody knew what they did. Knoblauch and his colleagues in Germany noticed that the toothpicks only appeared in samples that were bathed in EDTA, a chemical that binds calcium. That was significant; structures inside plant cells usually don’t encounter free calcium, since most plant cells actively pump calcium out of their cytoplasm. But these cells had been broken open during preparation for microscopy. Could it be that the toothpicks occurred in intact cells or in EDTA, but disappeared when exposed to calcium?
Knoblauch tested that proposition. When he replaced the EDTA with a calcium-containing solution, the toothpicks instantly disappeared—or seemed to. Closer examination revealed that they had converted to a shorter, plumper form that couldn’t be seen with normal light microscopy. He added EDTA again, and presto! the toothpicks reappeared. He went on to show that pricking an intact sieve cell with a microneedle—mimicking an attack from a sap-sucking insect—instantly triggers the shift. Whenever a sieve element is damaged and calcium enters a cell, the slender forisome changes into a gloppy plug that stops flow through the sieve plate. When the cell heals the break in its membrane and pumps out the remaining calcium, the forisome returns to its toothpick form. The sieve plate is unblocked, and flow resumes.
Further test showed they also change shape in response to barium, strontium, a change in pH, or an electrical impulse.
Knoblauch says their gap-plugging ability makes forisomes prime candidates for use as valves in microfluidic systems such as “labs on a chip,” in which diagnostic tests are run on tiny glass or polymer chips. “It’s minimization of the equipment, for example, to detect individual cancer cells in the blood,” he says. “So you don’t need a half liter of blood, you just need a drop. You put it on the lab chip, and in the chip [all the tests] are performed.” The technology has already become a billion-dollar industry, but making secure valves is still a problem. The most effective in use today involve pressing rubber into the fluid channel, he says, “but all of these valves leak. And the forisome doesn’t leak.”
Forisomes could also be used as micromotors. During their shape changes, they push as well as pull, and they generate the same amount of force in both movements.
“It’s not as strong as muscle, but it’s not too far away from it,” says Knoblauch. “The force which is generated would be sufficient to lift [tiny] cargoes.”
Perhaps most amazing of all, forisomes accomplish their shape changes without consuming ATP, the usual energy source in living systems. They also appear to last forever. They outlasted Knoblauch, at any rate. He and a colleague once tested their endurance by sitting down with a microscope, a forisome, and electrodes to spark the change. After 4,200 cycles of expansion and contraction, Knoblauch stopped the experiment. He had other things to do, but the forisome was still going.
Knoblauch’s lab is now working to identify the proteins forisomes are made of, in hopes of developing a way to produce them for industrial use. Even if forisomes turn out to be unsuitable for high-tech purposes, he says, once we understand how their structure enables them to do what they do, they could serve as a model for engineers to design devices that work in a similar way.
He already holds one patent, for a microinjection system, and is enjoying this new foray into bioengineering. Still, his primary interest remains figuring out how phloem works. Sieve elements contain many more proteins and structures that can be seen in electron micrographs.
“And nobody has an idea what they are for,” he says. “They are some of the most abundant structures in sieve elements—and if it’s there, it has a function—I think a very important function”—perhaps one we can harness or be inspired by.