Don’t mess with Mum
Plants also do a good job of defending themselves. Their
defenses range from crude and always deployed—think thorns—to
downright sneaky. How else to describe the fate that befalls
certain insect larvae, as told by biochemist Clarence “Bud”
Ryan?
“When they chew on the plant, their saliva mixes with the
wounded plant tissue, and it gives off volatile [chemicals],” says
Ryan. “Predator wasps pick up the smell. Then they come and inject
their eggs into the larvae.” When the baby wasps hatch, they gnaw
on the host larvae from the inside. The larvae, understandably,
stop chewing on the plant.
Ryan pioneered the study of another line of defense, which has
come to be known as the systemic wound response. A protein chemist
by trade, he started in the early 1970s trying to understand how
plant protease inhibitors work. Those are chemicals that block the
digestive enzymes (proteases) in the gut of any animal that takes a
bite out of the plant. Raw potatoes contain protease inhibitors,
which is why munching on them can cause severe stomachache.
Ryan found the inhibitors were present in the leaves of potato
plants as well—but that some potato plants were loaded, while
others had none.
“And I got the idea that maybe they’re there because an insect
was attacking the plant,” he recalls. “So I went out and borrowed
some Colorado potato beetles from a friend of mine, and let them
chew on potato plants in the greenhouse.” A day later, the
previously inhibitor-free plants were full of protease
inhibitors.
Ryan then repeated the experiment, confining a beetle to a
single leaf with a shield of aluminum foil. The next day, leaves on
the opposite, uninjured side of the plant contained just as much
inhibitor as the bugged leaf. News of an attack at one point on the
plant had spread—and caused a response—throughout the entire
plant.
“That was a huge discovery, because nobody had ever seen
anything like that,” he says. The breakthrough opened a whole new
field of research, and secured Ryan’s election to the National
Academy of Sciences in 1986. In 1991, his research team isolated
the signal chemical, a small peptide they dubbed systemin. It was
the first signal peptide ever found in plants.
Plant protease inhibitors do more than just give bugs an upset
tummy: they send a message from the gut to the brain that ruins a
bug’s appetite.
“It’s a satiety thing,” Ryan says. “It’s telling the insect,
'You’re full, you shouldn’t eat so much.' At the same time, the
insect is starving to death.
“It’s wicked!"
And it happens to work the same way on human hunger pangs. Ryan
and research associate Greg Pearce devised a large-scale method for
producing the inhibitor, which is now being sold by Kemin
Industries as a human weight-loss aid called
Satise®.
Recent explorations in his lab have revealed the presence of a
different signal peptide that activates the plant’s defenses
against microbial pathogens. In the innate immune response, as it
is called, an initial attack by a pathogen prompts nearby cells to
make the new peptide, which then travels through the plant’s
vascular system and causes cells throughout the plant to make
substances that fight the pathogen—and which also stimulate the
production of more peptide. This “amplification response,” as Ryan
calls it, allows a plant to respond in a big way to a small attack.
If the initial attack is followed by more, the plant will be
prepared.
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