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BERKELEY ? Researchers at the University of California, Berkeley, have
identified a critical gene for plants that start their lives as seeds buried
in soil. They say the burial of seeds was an adaptation that likely helped
plants spread from humid, wet climates to drier, hostile environments,
September 2004 by Sarah Yang .

In a study published in the Sept. 24 issue of the journal Science, the
researchers found that a gene called phytochrome-interacting factor 1, or
PIF1, affects the production of protochlorophyll, a precursor of the
chlorophyll used by plants to convert the sun's energy into food during
photosynthesis.

While a seed germinates under soil, in the dark, it is producing a
controlled amount of protochlorophyll in preparation for its debut above
ground. Much like a baby takes his or her first breath of air after emerging
from the womb, seedlings must quickly convert protochlorophyll into
chlorophyll once they are exposed to light for the first time.

"It's a delicate balancing act," said Peter Quail, professor of plant and
microbial biology at UC Berkeley's College of Natural Resources and
principal investigator of the study. "The young plant needs some
protochlorophyll to get the ball rolling in photosynthesis. But if the plant
accumulates too much of the compound, it leads to photo-oxidative stress,
which is seen as bleaching on the leaves. The overproduction of
protochlorophyll is like a ticking time bomb that is set off by the sun."

Quail is also research director of the Plant Gene Expression Center, a joint
research center of the Agricultural Research Service of the U.S. Department
of Agriculture and the University of California.

The researchers targeted the PIF1 gene because it binds to phytochrome, a
protein that is triggered by light and that controls a plant's growth and
development. The researchers disabled the PIF1 gene in the species
Arabidopsis thaliana, a mustard plant, and compared the mutant seedlings
with a control group of normal plants.

They grew the seedlings in the dark to mimic conditions beneath the soil,
bringing groups out into the light at different time points throughout a
six-day period. In nature, seeds are typically buried under 2 to 10
millimeters of soil, taking anywhere from two to seven days to germinate and
break through the soil surface.

"We found that mutated plants had twice the levels of protochlorophyll than
normal, wild-type plants, suggesting that phytochrome acts as a negative
regulator for protochlorophyll," said lead author Enamul Huq, who conducted
the study while he was a post-doctoral researcher at UC Berkeley's
Department of Plant and Microbial Biology. "We also saw that the longer the
seedlings were grown in the dark, the more likely they would die when they
were exposed to light."

The mutated seedlings failed to switch off production of protochlorophyll
throughout the germination period, so the longer the seedlings stayed in the
dark, the more toxic the levels became.

Huq, now an assistant professor of molecular cell and developmental biology
at the University of Texas at Austin, pointed out that it is an "unbound"
form of protochlorophyll that is toxic. Normal plants, he said, produce
enough of an enzyme, called protochlorophyllide oxidoreductase, to bind with
typical levels of protochlorophyll. But not enough of the enzyme is produced
to handle the overabundance of unbound protochlorophyll churned out by the
mutant seedlings.

The researchers say the ability of plants to precisely regulate production
of protochlorophyll was probably an evolutionary development designed to
ensure seed survival among higher plants.

Primitive plants, such as mosses and some species of fern, thrive in moist,
humid environments where their spores can stay safely above the soil
surface. But all higher plants - from grasses to trees to agricultural crops
such as wheat and corn - must have the ability to transition from the
darkness of an underground environment to life above ground.

"The development of seed burial in plants provided a long-term survival
benefit through protection from predators and hostile surface conditions,"
said Quail. "The true test of our hypothesis would be to verify whether
primitive plants have the PIF1 gene, and whether the gene is functional."

The finding may also have implications for agricultural biotechnology,
allowing researchers to manipulate the gene to improve the efficiency with
which plants carry on photosynthesis.

Other co-authors of the study are Bassem Al-Sady and Matthew Hudson of UC
Berkeley's Department of Plant and Microbial Biology, and Chanhong Kim and
Klaus Apel of the Swiss Federal Institute of Technology's Institute of Plant
Sciences in Zurich, Switzerland.

The study was supported by grants from the Department of Energy, the
National Institutes of Health, the USDA and Syngenta.

[www.berkeley.edu]

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