Plants on a Plane
By Kate Fink
Samuel L. Jackson has nothing on Dr. Marshall Porterfield of Purdue University. Though Jackson battled snakes on a plane in last summer’s blockbuster movie, Porterfield has put in flying time with plant spores….in zero gravity.
A miniscule new biochip that houses sixteen cells and probes them with 64 electrodes has allowed Porterfield and colleagues to study fern spores’ responses to gravity by monitoring ions entering and leaving the cells. The chip may eventually have medical applications in drug development and screening.
A biochip functions as an extremely small-scale laboratory. The chip was developed by Porterfield, an associate professor in the Department of Agricultural and Biological Engineering, and colleagues and is featured in a recent publication in Sensors and Actuators B. Approximately the size of your big toenail, it contains sixteen small pores shaped like pyramids, each housing one cell. Four electrodes—above, below, and on either side—poke into each cell’s chamber but stop short of penetrating the cell itself. A chemical coating on the electrodes lets researchers specifically detect calcium ions coming and going from the cell. For other applications, using different electrodes could allow for detection of different ions.
Calcium currents in cells have garnered fame for guiding development in organisms from frog eggs to algae, but this biochip is the first device to measure the current in single cells being stimulated in different ways, said Stan Roux, professor at the University of Texas at Austin and an author on the paper. The researchers used this chip to examine calcium currents in fern spores. These seed-like cells start to grow when nestled in the ground or floating in water. Spores take cues from gravity to ensure that root-like rhizoids burrow downward while the leafy greens head for the sunshine. The cells generate a current of positively charged calcium ions: the ions entered at the bottom of a spore and exited the top, streaming through the cell and giving the cell polarity, like a magnet with an opposite charge at each end.
These spores may translate the pull of gravity into the chemical signal of a calcium current simply by throwing their weight around. Each cell is over 100 microns in diameter in normal gravity, hefty by plant standards. The pull of gravity makes them slump, bulging at the bottom, Roux said, perhaps stretching the cell’s membrane. This physical stretching can open calcium channels. When calcium floods in, the cell sends little packets of growth materials towards this concentrated pool of ions, and the cell prepares to grow a rhizoid at that site.
Why, then, place these plant cells in petite pyramids on a plane? The researchers wanted to test how the fern spores reacted to too much or too little gravity, attempting to answer the questions of how fast these cells respond to changes in gravity and whether those changes alter the calcium current flowing through the cells. Since the strength of the gravitational field on earth wasn’t budging, they intrepidly turned to NASA’s Zero-G aircraft, commonly known as the Vomit Comet. Older methods of testing for ions involved patch clamping—a delicate, laborious procedure of using one electrode on one cell at a time. With the biochip set-up, “we just kept an eye on it during the flight. We just got to focus in on enjoying the flight. You go from weighing twice your body weight to almost being weightless once every minute,” Porterfield said. The researchers are now analyzing data from the flight, and the group expects to publish their results within the next year.
There are other devices that use electrodes to detect specific ions. Dr. Olivier Guenat, associate professor at the Polytechnic School of Montreal, and colleagues developed electrodes that detect potassium or ammonium ions, which clues researchers in to activities such as death or differentiation in liver cells. He notes that most electrode-measuring systems take stock of these ion levels just outside a cell, without penetrating it. This can make it difficult to interpret results if the ion soup surrounding a cell already contains many of the ions researchers are trying to detect. This background noise could make small changes in ion concentration nearly imperceptible, said Guenat.
All agree that broad applications await these devices, though. Bio-artificial liver and toxicology applications may come from Guenat’s work. Roux looks forward to using the biochip to test bone cells for gravity-regulated calcium currents similar to those seen in the fern spores, which could have direct applications for NASA astronauts and their health. Porterfield says the biochip may also have particular importance in drug screening because about 15% of medications in development aim to modify ion channel activity.
Plants on a plane may not be a blockbuster movie, but they could someday lead to a blockbuster drug.