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 whe
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Contact: Sarah Yang
scy@pa.urel.berkeley.edu
510-643-7741
University of California - Berkeley
23-Sep-2004