Wang also watched expression patterns of these defense genes in the absence of any pathogens and found them cycling daily. "It suggests that the plants are programmed to anticipate infection according to a circadian schedule," Dong said.
To test this hypothesis, Wang exposed plants to the mildew spores at "dawn" or "dusk" in a greenhouse with artificial 12-hour days. He found infection rates were much higher at dusk when spores were not expected in nature.
To identify the clock components that control this immune defense, Wang began a series of experiments on so-called "clock mutants," which are Arabidopsis plants that lack portions of the circadian time-keeping system. He found that the mutant missing the central clock component CCA1 suffered much higher infection rates than normal plants. Conversely, a plant line expressing CCA1 all the time had heightened resistance.
Over the last decade, plant researchers have identified several systems that work on a circadian clock, including starch metabolism, photosynthesis and frost-resistance, said Robert McClung, a Dartmouth University biologist who was not involved in this research, but is writing an accompanying commentary on the paper for Nature. "This sort of completes the suite of environmental insults that the clock manages," he said.
Although this finding is specific so far to Arabadopsis and its exclusive pathogen Hyaloperonospora, it's likely to be a system that will be found in other plants and pathogens, McClung said.
While it makes logical sense that clock mechanisms would be involved with the plant's immune system, this is still "a remarkable discovery," said Philip Benfey, the Paul Kramer professor of biology at Duke. "This required an experimental tour de force combined with inspired insight to make the connection between gene express
|Contact: Karl Leif Bates|