Young plants prepare to see the light.

Researchers have found what may be the key steps that allow newborn seedlings to burst out of the soil without a spot of ray-catching chlorophyll but still start using light.

While underground, seedlings rely on lunches their mothers packed, nutrients in the seed. Although conifer sprouts under soil prepare for daylight by making chlorophyll, flowering plant sprouts cannot synthesize it in the dark.

As soon as they emerge, these pale sprouts can nevertheless harness light to power their shift to adult metabolism, thanks to a novel complex of compounds, report Christiane Reinbothe of Bayreuth University in Germany, and her colleagues. They and other researchers had identified some bits of the system. Now, coauthor Steffen Reinbothe says, "we put them together to find a way out of the dark." The results appear in the Jan. 7 NATURE.

Studying barley seedlings grown in darkness, the researchers focused on two enzymes called PORA and PORB, which catalyze a step in the transformation of precursor pigments into chlorophyll. Researchers had not known how the activity of these two forms of the enzyme protochlorophyllide oxidoreductase differs, Reinbothe was surprised to find that PORA and PORB bind different chlorophyll precursor pigments having extremely similar structures. "It was very unexpected," he says.

Reinbothe's group proposes that in a seedling, the two enzymes and the pigments form part of a complex that waits for the first blast of light. Mixing ingredients in the laboratory, the researchers did find such a complex. When they flashed light that only the pigment bound by PORA could absorb, they saw reactions in the PORB-pigment pair. They suggest that the pigment bound by PORA acts as an antenna, soaking up rays and passing along energy to power PORB's step in making chlorophyll.

The work "took a number of leaps over the current dogma," says Robert Willows of Macquarie University in Sydney, Australia. He says the most dramatic change is the new role for protochlorophyllide b, the pigment bound by PORA. Most physiologists assumed it didn't show up so early in development, Willows says. However, he speculates that "the main reason no one's found it is that no one's looked very hard."

Timothy W. McNellis of the University of California, Berkeley was intrigued by the way the complex protects seedlings. Without chlorophyll, "it's as if an albino person goes out in bright sunlight without sunblock," he says. Reinbothe, however, found that if protochlorophyllide b receives more energy than the complex can use, it fluoresces, emitting the excess harmlessly.