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.
In a study published in the Sept. 24 issue of the journal Science, the researchers describe how 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.
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