Rice University biologists found that plants with altered circadian clocks were unable to defend themselves against leaf-eating cabbage looper caterpillars. Credit: Tommy LaVergne/Rice University |
In
a study of the molecular underpinnings of plants’ pest resistance, Rice
University biologists have shown that plants both anticipate daytime
raids by hungry insects and make sophisticated preparations to fend them
off.
“When
you walk past plants, they don’t look like they’re doing anything,”
said Janet Braam, an investigator on the new study, which appears this
week in the Proceedings of the National Academy of Sciences. “It’s
intriguing to see all of this activity down at the genetic level. It’s
like watching a besieged fortress go on full alert.”
Braam,
professor and chair of Rice’s Department of Biochemistry and Cell
Biology, said scientists have long known that plants have an internal
clock that allows them to measure time regardless of light conditions.
For example, some plants that track the sun with their leaves during the
day are known to “reset” their leaves at night and move them back
toward the east in anticipation of sunrise.
In
recent years, scientists have begun to apply powerful genetic tools to
the study of plant circadian rhythms. Researchers have found that as
many as one-third of the genes in Arabidopsis thaliana—a widely studied
species in plant biology—are activated by the circadian cycle. Rice
biochemist Michael Covington found that some of these
circadian-regulated genes were also connected to wounding responses.
“We
wondered whether some of these circadian-regulated genes might allow
plants to anticipate attacks from insects, in much the same way that
they anticipate the sunrise,” said Covington, now at the University of
California, Davis.
Danielle
Goodspeed, a graduate student in biochemistry and cell biology,
designed a clever experiment to answer the question. She used 12-hour
light cycles to entrain the circadian clocks of both Arabidopsis plants
and cabbage loopers, a type of caterpillar that eats Arabidopsis. Half
of the plants were placed with caterpillars on a regular day-night
cycle, and the other half were placed with “out-of-phase” caterpillars
whose internal clocks were set to daytime mode during the hours that the
plants were in nighttime mode.
“We
found that the plants whose clocks were in phase with the insects were
relatively resistant, whereas the plants whose clocks were out of phase
were decimated by the insects feeding on them,” Goodspeed said.
Wassim
Chehab, a Rice faculty fellow in biochemistry and cell biology, helped
Goodspeed design a follow-up experiment to understand how plants used
their internal clocks to resist insect attacks. Chehab and Goodspeed
examined the accumulation of the hormone jasmonate, which plants use to
regulate the production of metabolites that interfere with insect
digestion.
They
found that Arabidopsis uses its circadian clock to increase jasmonate
production during the day, when insects like cabbage loopers feed the
most. They also found that the plants used their internal clocks to
regulate the production of other chemical defenses, including those that
protect against bacterial infections.
“Jasmonate
defenses are employed by virtually all plants, including tomatoes, rice
and corn,” Chehab said. “Understanding how plants regulate these
hormones could be important for understanding why some pests are more
damaging than others, and it could help suggest new strategies for
insect resistance.”
The
research was supported by the National Science Foundation and Rice
University. Additional co-authors include Rice undergraduate Amelia
Min-Venditti.
Arabidopsis synchronizes jasmonate-mediated defense with insect circadian behavior.
