EMBARGOED FOR RELEASE:
THURSDAY, JUNE 26, 1997, 5 P.M. ET
Contact: Larry Bernard
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ITHACA, N.Y. -- An "information superhighway" inside plant cells allows
chloroplasts -- the minuscule green bodies that carry out photosynthesis
inside cells -- to communicate directly with each other, Cornell University
scientists have found.
That means chloroplasts are not independent green globules that float
around unattached inside the cells, but they may communicate by exchanging
proteins or other material to facilitate coordination of cellular
activities.
"We were surprised to find chloroplasts that were attached to each other by
long, slender tubules," said Maureen R. Hanson, Cornell professor of plant
molecular biology and of genetics and development who led the work. "We
don't know why chloroplasts communicate, but they are clearly exchanging
proteins and perhaps other types of molecules."
Hanson and co-authors Rainer Kˆhler and Jun Cao, postdoctoral research
associates in her lab; Watt W. Webb, Cornell professor of applied and
engineering physics; and Warren Zipfel, research associate in Webb's lab,
reported their findings in the journal Science (27 June 1997), in work
sponsored by the U.S. Department of Energy.
Chloroplasts trap light energy and use it to produce the organic molecules
plants use to grow and reproduce. It had been thought they were
independent within plant cells, but the Cornell researchers found these
slender tubes attaching some of the chloroplasts in some cells. This
occurs not just in genetically engineered plants, Hanson said, but in all
plants.
The researchers made the discovery quite by accident. Hanson and
colleagues wanted to view chloroplasts more easily under a microscope, so
they genetically engineered plants that contained a fluorescent protein
gene in the nucleus, from jellyfish. This made the chloroplasts glow green
when viewed under the microscope.
Examining plant leaves under the microscope, the scientists easily could
see the brightly glowing chloroplasts. "But Kˆhler was excited when he
also saw glowing tubules emanating from some of the fluorescent
chloroplasts," Hanson said.
Hanson and colleagues wondered what function the chloroplast tubules might
have, so they designed a new experiment. Using a sophisticated two-photon
laser microscope developed at Cornell as part of the Developmental Resource
for Biophysical Imaging and Opto-electronics, they tested whether proteins
could move from one chloroplast to another. Zipfel, research associate in
Webb's laboratory, used the laser to zap just one chloroplast that was
connected via a tubule to a second chloroplast. The laser bleached the
first chloroplast so it stopped glowing, but after a few seconds, it became
bright again.
"Some of the fluorescent protein clearly was being transferred into it from
the unbleached chloroplast through the tubule," Hanson said. "The
chloroplast that donated the fluorescent protein became less bright because
it lost some fluorescent protein through the tubule."
Why do the chloroplasts communicate and, more important, what are they
saying? Scientists think chloroplasts are remnants of photosynthetic
bacteria that long ago became parts of plant cells. Like bacteria,
chloroplasts contain genes and divide to reproduce, and bacteria are known
to communicate with each other and exchange genes through thin tubules
called pili.
"Perhaps the chloroplast tubules are similar in structure or function to
connections between bacterial cells," Hanson said. "At this point, we just
don't know."
After first finding these slender tubules, the Cornell researchers searched
the old scientific literature for mention of any such structures. They
realized that Sam Wildman and his colleagues at the University of
California at Los Angeles had described, in a 1962 paper, something
protruding from chloroplasts.
"But because his light microscope technique was 35 years ahead of its time,
other researchers could not find the tubules. So scientists first doubted
and then forgot about his discovery," Hanson explained. "Now we know the
connections are there, and they are being used to communicate."
Even today, the tubules are almost impossible to see unless they are
labeled with fluorescent protein by genetic engineering, a technology that
had not been invented in the 1960s when Wildman did his studies. Now 84,
he is professor emeritus at UCLA and was "extremely pleased" when he
learned that his earlier description of structures emanating from
chloroplasts finally had been corroborated, Hanson said.
The technology that made these findings possible, a two-photon laser
microscope, was developed by Webb, the professor of applied and engineering
physics. The microscope uses pulsed lasers and fluorescent markers to
detect and image cellular activity with sensitivity to detect and recognize
tens of individual molecules in extremely small volumes. These advanced
microscopes can reveal fundamental biological processes in living cells --
plant biology, metabolism, wound healing, behavior of malignant cells and
nerve communication -- opening a new world for investigators of biological
systems.
Webb has been developing user-friendly instrumentation and methods for
biophysical investigations for the last six years with pre- and
postdoctoral students. Cornell holds the patent on the technology, which
is available for licensing. Webb also is director of Cornell's
Developmental Resource for Biophysical Imaging and Opto-electronics, funded
by the National Institutes of Health and the National Science Foundation,
which made the images of this research.
Hanson said the discovery reported today is just the first of many that
will be possible using advanced imaging techniques. "We are uncovering the
secret lives of plant cells. Now we want to find out how chloroplasts
interact with other components of the cell," she said. "As microscopes and
recombinant DNA methods get better, they provide new tools to improve our
understanding of plant biology."
-30-
Photos and diagrams are available on the web at
news.cornell.edu/science/June97/plantcells/lb.html
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