Science 20 November 2009:
Ninth International Plant Molecular Biology Congress, 25-30 October 2009, St. Louis, Missouri:
Chloroplasts seem to rely on the polymerization of protein filaments to make their way across a cell, researchers reported at the 9th International Plant Molecular Biology Congress, and they can move quickly—or slowly—depending on the circumstances.
For more than a century, researchers have known that chloroplasts move. But it’s taken 25 years for Masamitsu Wada to figure out how these photosynthesizing organelles make their way across a cell. His labeling and imaging experiments have revealed that they slide from one place to another, never turning, twisting, or rolling. They seem to rely on the polymerization of protein filaments to pull them along, he reported at the meeting, and they can move quickly—or slowly—depending on the circumstances. The work represents “an outstanding combination of genetics and cutting-edge cell biology,” says Eberhard Schäfer, a plant physiologist at the University of Freiburg, Germany.
Chloroplasts have evolved to make the most out of the available light. Photoreceptors on the plant cell surface relay light-intensity information to chloroplasts, which move toward weak light and quickly scatter in strong light that might damage them. “This response is vital to plant survival,” says Christian Fankhauser of the University of Lausanne, Switzerland.
Wada, a plant physiologist at Kyushu University in Japan, typically studies chloroplast movements in fern gametophytes, the tiny sexual phase of the plant, which at first grow as a single layer of highly photoreactive cells. He uses a microscope to direct a microbeam of light to specific parts of a cell and records the result with time-lapse videos.
When he shined strong light at one end of a chloroplast, it took less than a minute for it to respond: It slid away from the light at about 1.5 micrometers per minute. When he gave another pulse of light on one side of the advancing chloroplast with another pulse of bright light, the chloroplast stopped and retreated, changing direction without reorienting itself in any way, he reported. “They don’t have any head or tail,” says Wada. “They can move in any direction.”
To get a better handle on what propels the chloroplast, Wada and his colleagues turned to a mutant of the model plant Arabidopsis, whose chloroplasts don’t move. They homed in on the defective gene and figured out that it codes for a protein that sits in the chloroplast membrane and has the ability to latch on to actin protein fibers. Actin is often involved in the movement of organelles. Using genetically modified Arabidopsis plants that produced actin-binding proteins linked to green fluorescent protein, Wada and his colleagues made actin filaments visible and traced their whereabouts.
Before the researchers turned on their strong light beam, the chloroplast was surrounded by actin filaments. But in the light, the filaments disappeared—likely depolymerizing—within 30 seconds. A minute later, they began to reappear but only on what became the leading edge of the moving chloroplast, Wada reported at the meeting and in the 4 August issue of the Proceedings of the National Academy of Sciences. “I think actin might pull the cell along,” says Wada.
Next, Wada wants to pin down the signal that travels from the photoreceptor and causes the actin filaments to dissolve and reform. “The speed of signal transduction is very slow, so it must not simply be a diffusible, small molecule,” he says.