Bio-research links plants' defenses to natural cycles

By Johanna Ohm     3/7/12 6:00pm

In the battle between plants and their insect predators, it's a caterpillar-eat-plant world. But while plants may seem to be an easy target for insects, plants have their own mechanisms of defense, similar in many ways to those of the human immune system.

Researchers in Rice's Biochemistry and Cell Biology department have recently found that the complex system of hormones and secondary metabolites which comprise plant defenses are cyclically activated to follow day-and-night cycles. According to plants' circadian rhythms, these changes in plant hormones alter their state of defense against insects.

Janet Braam, chair of the Department of Biochemistry and Cell Biology, led the research that was published in the Proceedings of the National Academy of Sciences last week. Graduate student Danielle Goodspeed designed the experiment and received input and help with the project from Rice faculty fellow Wassim Chehab and then-undergraduate Amelia Min- Venditti (Duncan '11).

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Goodspeed said she is excited with the direction her research has taken, though she never expected to be working on a project relevant to ecology and evolution.

"I didn't even want to work with plants when I first came here," Goodspeed said.

She cited a fascination with circadian rhythms with sparking her interest in the lab she now works in, which focuses primarily on plant hormones.

"The circadian clock really fascinated me because humans have a circadian clock, too, that controls our eating patterns, our sleeping patterns and a lot of things," Goodspeed said. "Plants have something very similar to us that works in a very similar fashion, but nobody's done a lot of studies with plants about the physiological relevance of this clock."

Braam credits Goodspeed with the design of the project and with bringing the idea of testing the effects of circadian rhythm to her lab.

"It was a collaborative effort but the project started with a very clever experimental design by Danielle," Braam said. "She figured out that she could easily test whether time of day affected plants' susceptibility to insect attack."

In Goodspeed's experiments, specimens of Arabidopsis thaliana, a plant species commonly used in biological studies, were grown in constant light-dark cycles and then exposed to cabbage loopers, a type of plant-eating insect, that were either "in-phase," defined as being on the same light-dark circadian rhythm or "out-of-phase," defined as being on an opposite light-dark cycle. With the "out-of-phase" insects, insects were in their daytime state when exposed to plants that were in their night phase. The results showed that, when in opposite phases, plants are more vulnerable to attack and get easily decimated by loopers.

"The plant has set up different hormones to defend against different pathogens at different times when those pathogens are going to attack," Goodspeed said. "Cabbage loopers attack during the middle of day to dusk. So jasmonate [a defensive hormone] levels, which are required for defense against loopers, are highest during the middle of the day and drop off at night."

Min-Venditti worked on the project during the 2010-11 academic year, assisting Goodspeed with her experiments.

"My job was to help with the optimization experiments, to make sure all the results were reproducible," Min-Venditti said. "It's kind of cool. We found that plants raised on a ‘normal' cycle fared significantly better than plants on an ‘off' cycle, and the difference was significant."

Min-Venditti said she was excited to hear that the research was accepted for publication and thought Goodspeed's project would be important to the plant biology community for its broader applications and unique findings.

"It's novel [research]," Min-Venditti said. "The fact that Danielle [Goodspeed] is working on circadian rhythms, genomics and ecology all at once is pretty exciting."

Goodspeed said similar research is being done with mammalian immune systems to see if defensive mechanisms become strengthened at times of the day when the body is more susceptible to attack.

"A lot of people think this is convergent evolution. [...] Almost all organisms developed this circadian clock independently but ended up with something very similar because it was so important," Goodspeed said.

Goodspeed said she is continuing her research and doing follow-up experiments which she hopes will be published later this year.

"We believe that these initial studies will lead to significant later understandings of how the plant clock works and affects plant fitness," Braam said.

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