In a Plant's Plentiful Genes, a Chemistry Lesson

By Justin Gillis
Washington Post Staff Writer
Monday, April 8, 2002; Page A07

Last week's announcement that the rice plant probably contains more genes than a human being might
have seemed mystifying. But step back a moment to consider the harried life of a plant.

A plant has to find its nutrients wherever it happens to be. It cannot take shelter from a storm. Some
plants can deploy physical defenses, such as nettles or bark, but most cannot. If plants had a point of
view, they would see the animals of the Earth, including people, as marauding plunderers determined to
gobble them up; plants would feel naked before the onslaught.

Above all, plants lack a critical advantage that pretty much every animal on the planet takes for granted.

"For animals, if it's too hot, if it's too cold, if something is about to eat them, their general response is to leave," said Jeffrey L. Bennetzen, a biologist at Purdue University. Plants, of course, are stuck in place.

But nature has compensated plants richly for their deficits, and therein lies one explanation for why a rice
plant might have more genes than a person.

The pressure of evolution has taught plants to make tens of thousands of chemical compounds, which
they use to ward off competition from other plants, to fight infection, to respond to the environment, and
to manipulate the behavior of animals. In fact, even the lowliest weed is a chemical factory that puts
DuPont to shame.

"Plants live by chemistry," as Bennetzen put it.

The complicated evolutionary knowledge of how to make those chemicals is encoded in the plant's genes,
which are its instructions for making proteins. Biologists say that's one explanation for why plants are
turning out to have so many genes -- they are carrying what amount to chemistry instruction manuals in
their cells.

The chemical wizardry of plants is a daily fact of human life, but people don't often stop to think of it that
way. The pungent flavor of basil, the intoxicating aroma from a glass of wine, the spicy smell of a
gardenia, the cloying sweetness of a candy bar -- all are a testament to the ability of plants to make a
wide array of chemical compounds.

Plants do not learn from other plants in the way many animals learn behavior from their parents, so the
expertise to make all these chemicals must be carried in the genes.

Two scientific papers published on Friday reported the first detailed analysis of the full genetic
complement, or genome, of the rice plant. Genes are only a small portion of the total genetic material, and
scientists are still trying to fish all of them out of the mix. But they estimate that rice has somewhere
between 32,000 and 55,000 genes.

Humans are estimated to have 30,000 to 40,000 genes, so if rice finally comes out in the midrange of
estimates, it will be the first organism shown to have more genes than a person.

It probably won't be the last. Plant geneticists say some species carry enormous amounts of genetic
material, sometimes hundreds of times as much as a person. Even if much of that turns out to be "junk
DNA," the mysterious DNA scattered throughout a genome, it seems inevitable that many organisms will
be found to have more genes than people.

What all those genes are doing in rice is something the scientists are still puzzling out. But the largest single
group of genes identified so far encode instructions for making enzymes, the proteins that perform
chemical reactions in plants -- strongly implying that many of the genes in a rice plant are devoted to
producing compounds that help the plant cope with its environment.

Until the rice papers, mankind had escaped the indignity of being upstaged on gene count by a "lower"
form of life. But there have been detailed scientific reports on only five organisms to date, and some of
those came uncomfortably close.

The tiny worm Caenorhabditis elegans, for instance, has about 19,000 genes, at least half as many as a
human being -- this for a microscopic nematode that's essentially a hollowed-out tube with a rudimentary
brain.

Comparing the worm to another research organism -- Drosophila melanogaster, the fruit fly -- points
up another reason why counting genes is starting to seem less than revealing. The fly appears to be a far
more complicated organism than the worm, with a complex body plan, a bigger brain and some fairly
elaborate behavioral patterns. Yet the fly turned out to have fewer genes than the worm -- only about
13,000.

It has been known for years that animals generate biological complexity that doesn't depend strictly on
gene count. In at least some animals, unlike in plants, a single gene can get sliced and diced to make
variations on a particular protein. Like a baker using one cookie cutter but then decorating each cookie in
a different way, an animal cell seems to be able to use one gene to generate many proteins.

The new research is driving home the importance of those mechanisms, suggesting that at most, biological
complexity may be coupled to gene count only in plants. Scientists say the old idea of a straightforward
correlation between the number of genes and an organism's complexity is on its deathbed.

"I think it's absolutely collapsed," said Donald Kennedy, editor in chief of Science, the journal that
published the rice papers. "You don't tell an awful lot about an organism's complexity from the number of genes that it has."

The new research could give scientists far more insight into the chemistry of plants. Many thousands of
plant compounds are known already, and some of them have played storied roles in human history. In
1897, for instance, a German chemist named Felix Hoffmann tweaked a chemical from the bark of a
willow tree and came up with the compound known as aspirin, launching the modern pharmaceutical
industry.

Plants didn't originally devise their chemicals to fill medicine chests, of course. Typically, the compounds
are evolution's way of giving the plant a means to manipulate animals. A lot of the chemicals are aimed at
repelling animals that would otherwise eat the plant, while others are designed to attract bees or other
creatures that help a plant reproduce by spreading pollen.

Plants often employ both strategies at once. Benjamin Burr, a biologist at Brookhaven National
Laboratory, cites the example of apricots and peaches. These plants make delectable fruit that inspires
animals to chomp away. Evolution wants the animal to tote the fruit -- and the seed inside -- some
distance from the parent plant, but it does not want the animal to eat the seed. So the pits of these fruits
contain a compound that breaks down, in an animal's body, into a familiar poison.

"If you eat apricot or peach pits, you can poison yourself with cyanide," Burr said.

Scientists know of many examples of this kind of chemistry, but they assume far more are waiting to be
discovered. They expect the gene maps now being unveiled to give them powerful new insights into the
chemical wizardry of plants -- perhaps turning up new drugs or industrial compounds in the process.

                    © 2002 The Washington Post Company