A pollen grain extending a pollen tube.
Credit: University of Montreal
A real-time look at plant sex in an environment simulating microgravity reveals that agriculture in space might face challenges.
The study also illuminates how gravity works on intercellular transport, a crucial process for mating plants and communicating human brain cells alike.
Sex in space
Though not as titillating as humans getting busy, plant sex is a great way to examine how cells transport materials inside their walls. When a pollen grain lands on a stigma, the female part of a flowering plant, it grows a pollen tube that acts as a tunnel for sperm cells to travel down to reach the egg. The pollen tube is the fastest-growing cell in the plant kingdom. [50 Sultry Facts About Sex]
Fast growth is key for studying the way cells move in real-time. Using any other plant cell, you'd have to wait weeks to see a response to gravity, said study researcher Anja Geitmann, a biologist at the University of Montreal. In pollen tubes, a response takes mere seconds.
Pollen tubes are also good models in which to examine how intercellular transport works, because they don't sense gravity. Any response pollen tubes have is due only to the physical effects of the gravitational force, not the cell sensing gravity and changing its behavior accordingly.
Some plant cells do sense gravity; tiny structures called statoliths in root cells ensure that plant roots grow down, for example. But growth of pollen tubes follows the chemical signal from a female plant, so they don't need gravitational information. In that way, they work like any cell with a nucleus, including animal cells.
Beyond 1 g
No pollen tubes were blasted into space in the making of this study. Instead, Geitmann and her co-researchers availed themselves of the tools of the European Space Agency (ESA). They used a spinning centrifuge 26 feet (8 meters) in diameter to expose growing pollen tubes to forces of gravity up to 20 times normal Earth gravity (known as 1 g). They also put pollen tubes in the ESA's Random Positioning Machine, which turns specimens in all directions at a particular speed, essentially canceling out the effects of gravity from each side. This creates conditions that simulate the microgravity of space.
"It's not true zero gravity," Geitmann told LiveScience. "There is continuously 1 g on the sample, but it simply changes direction."
The researchers used microscopy to watch their samples in real-time. The results revealed that while the pollen tube may not sense which way is up, gravity affects it nonetheless. The diameters of the tubes grown in simulated microgravity were 8 percent smaller than a tube grown in 1 g. At five times Earth's gravity, the tubes were 8 percent wider, and at 20 times Earth's gravity, they were 38 percent wider.
The surface expansion rate of the tubes also dropped 39 percent in the simulated microgravity.
Because forming a pollen tube is essentially a tiny cellular construction project, cells transport little bubbles, or vesicles, of material to build out the cell walls in the direction the tube is growing. The researchers found that the distribution of two of these materials, cellulose and callose, was disrupted in hyper- and microgravity.
"The intercellular trafficking, which occurs in very precisely defined paths in these cells, was affected," Geitmann said. She and her colleagues reported their findings today (March 13) in the journal PLOS ONE.
Animal reproduction isn't similar enough to plant reproduction to draw any conclusions about the result of human sex in space from this study, Geitmann said. Concerns about human reproduction in space include the effects of radiation exposure on a developing fetus as well as unknowns about microgravity, according to a 1996 paper in the journal Acta Obstetricia et Gynecologica Scandinavica. [Animal Sex Quiz: Test Your Smarts]
But don't shrug off microgravity plant sex just yet. Intercellular transport is important in a variety of human cells, particularly lengthy neurons, Geitmann said. Researchers studying fish brains reported in the journal Advanced Space Research in 2002 that synaptic formation was influenced by microgravity. Anecdotal reports and small studies of astronauts also suggest that cognitive performance declines in space, but individuals varied widely, according to a 2012 report by NASA.
Causes for that decline could range from sleep deprivation and stress to radiation, NASA found, but no one has looked at whether intercellular transport in neurons might play a role, Geitmann said.
"Many neuronal diseases, such as Huntington's or Parkinson's or Alzheimer's, are related to trafficking," she said.
Humans also need to understand plant sex in space should our species ever need to feed itself on long-duration missions or colonies on other planets.
"If we ever want to do agriculture in space, so to say — it's a long-term vision! — then we have to take this into account," Geitmann wrote in an email. "In order to actually do long-term plant cultivation, we have to look for species that can actually reproduce under zero gravity conditions."