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A team of scientists led by University of Connecticut plant biologist
Roberto Gaxiola has discovered an overlooked genetic key to generating
plants that are more productive, more drought resistant and can grow in
soils low in nutrients. Their work is the first to successfully test in
cells a 30-year-old hypothesis that explains the movement of a primary
growth and development hormone through plants and is expected to prompt
biology textbooks to be rewritten, October 2005.

The researchers from UConn, Purdue University and Pennsylvania State
University determined that one of three proton pumps found within plant
cells, previously believed to have an extremely limited function, plays a
critical role in plant root and shoot system growth and development by
controlling cell division, expansion and hormone transport. Over-expressing
the single gene that encodes this particular proton pump significantly
enhances the transportation of the primary plant growth hormone, auxin, and
results in plants with stronger, more extensive root systems and as much as
60 percent more foliage, the researchers report in the Oct. 7 issue of the
prestigious journal Science.

"This discovery has the potential to revolutionize agriculture worldwide,"
said Gaxiola, an assistant professor-in-residence in UConn's plant science
department. "This over-expression regulates the development of one of the
most important parts of the plant, the roots. A plant with larger roots is a
healthier and more productive plant, because, with a larger root system, the
plant is able to get water and nutrients from larger soil areas.

"Biology textbooks tell you there are three pumps inside a plant's cell but
one is less important. Our research shows that is not the case," Gaxiola
said. "As it turns out, that tiny pump is required to shuttle the master
pump, the plant's major engine, to the plasma membrane. That, in turn,
allows the master pump to facilitate the transport of more of the growth
hormone, auxin, through the plant's plasma membrane and through the plant's
root and shoot systems, resulting in enhanced cell division and growth."

All plants contain three proton pumps ? a master pump, known as the P-type
H+-ATPase, that facilitates transport of nutrients in and out of plant
cells, and two other pumps that work within plant cells. Biologists have
shown that only the P-type H+-ATPase pumps protons into the space outside
the cell to create changes that drive the transport of small molecules in
and out of cells. Until now, they believed the AVP1 H+-PPase that Gaxiola's
group over-expressed merely controlled pH levels within plant vacuoles, or
large storage areas inside plant cells, and served primarily as a back-up
pump to a larger vacuolar pump known as V-ATPase. Scientists believed that
the larger vacuolar pump was the only one to help shuttle the master pump to
and from the plant cell's plasma membrane.

In collaboration with scientists at the Massachusetts Institute of
Technology and Harvard University, Gaxiola previously had created plants in
which the AVP1 gene was over-expressed using the research plant Arabidopsis
thaliana. As Gaxiola predicted, these plants were salt- and
drought-resistant and sequestered more salt ions in their vacuoles.
Surprisingly the plants also had abnormally large root and shoot systems.

Simon Gilroy, a Pennsylvania State University cell biologist, provided
another piece to the puzzle when he discovered that the pH, which indicates
proton concentration, was unchanged inside the cells. But the extra-cellular
pH was lower, meaning it was more acidic and had a higher proton
concentration.

The next clue came from plant cell biologist Angus Murphy and his colleagues
at Purdue University.

"When Simon reported the acidity and the proton gradient was increased
between the inside and outside of plant cells in Roberto's over expression
lines, we saw an opportunity to test the model that had been used to explain
the transport of the plant hormone auxin for the last 30 years," Murphy
said. "This model predicts that an increased proton gradient should result
in a faster rate of auxin transport. This theory never had been tested
directly tested in plants where the proton gradient had been manipulated by
molecular genetic techniques. When we determined that the rate of transport
was increased, but the overall auxin content was not, the auxin transport
model was validated."

They determined AVP1's critical role by comparing the transgenic plants to
both ordinary Arabidopsis plants and mutant versions of the plant that were
devoid of AVP1. They discovered that the AVP1 mutants didn't develop
functional root systems and their shoots were tiny and deformed.

Gaxiola specializes in manipulating plant proton pumps for crop improvement
and relied on Murphy and Purdue colleague Wendy Peer, for expertise in auxin
transport in plants, and Gilroy for expertise in plant cell biology with an
emphasis on roots.

Additional authors are UConn doctoral students Jisheng Li, Haibing Yang,
Soledad Undurraga and Mariya Khodakovskaya; Purdue doctoral students Joshua
Blakeslee, Anindita Bandyopadhyay, Boosaree Tiapiwantakun, Elizabeth
Richards; Penn State doctoral student Gregory Richter; and University of
South Carolina Biology Professor Beth Krizek.

Gaxiola said that early experiments to duplicate the Arabidopsis results in
other crops, such as tomatoes, rice, cotton and poplar trees, indicate the
team's discovery could have implications for increasing the world's food
production and aiding global reforestation efforts. He predicts that within
the next five years there will be a "boom" of crops genetically engineered
using his team's approach. The research team's findings are likely to be
particularly significant for farmers in developing countries, including
Gaxiola's native Mexico, because many live in arid regions and lack
irrigation systems and money for the amount of expensive fertilizers needed
to feed plants with less expansive root systems.

[www.eurekalert.org]

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