Preparing for infection

Plants can prepare for future infections by turning on the expression of defense genes during the times of day when they are most likely to encounter pathogens, according to a study published this week in Nature.

Arabidopsis thaliana
Image: Roepers, Wikimedia

Plants achieve this timing thanks to the synchronization of those resistance genes with their own circadian clock, the daily rhythms that regulate many aspects of plant physiology, the authors report."This is a really elegant piece of work," says molecular biologist at the University of Cape Town in South Africa, linkurl:Laura Roden,;http://www.mcb.uct.ac.za/Staff/Roden/lroden.htm who did not participate in the research. "They discovered this circadian component of disease resistance."With no specialized immune system to speak of, plants rely on a large set of genes that code for receptors and other proteins that specifically recognize molecules, or effectors, from a wide range of pathogens. Known as disease resistance (R) genes, they are responsible for jump starting a series of physiological processes that thwart the progression of infection.Thanks to the R gene known as RPP4, the model plant __Arabidopsis thaliana__ is naturally resistant to the fungus-like parasite, __Hyaloperonospora arabidopsis__ (Hpa) that causes downy mildew disease, which stunts plant growth and causes yellow spotting and localized necrosis in leaves.Lead scientist and molecular biologist from Duke University, linkurl:Xinnian Dong,;http://www.biology.duke.edu/donglab/research.html was initially interested in finding the other genes that work with __RPP4__ to enable resistance against the parasite. Using expression data from wildtype and __rpp4__ mutants, Dong and her colleagues identified 22 genes that worked downstream of __RPP4__ to initiate defense mechanisms such as the programmed cell death of infected cells, the accumulation of antimicrobial compounds, and the scarring of the infection site, which helps block the pathogen from moving on to neighboring cells.But looking more closely at the promoter regions of the 22 downstream genes revealed an unexpected surprise. It turned out that the majority contained sequences that bind to the major circadian clock transcription factor known as CCA1, as well as other clock elements. This meant that these defense genes were not only regulated by __RPP4__ in response to downy mildew infection, but they were also intimately tied to the plant's circadian cycle.

Downy mildew (dark blue) infecting
Arabidopsis thaliana leaf cells (light blue)
Emmanuel Boutet, Wikimedia

When the team went back to monitor the expression of the genes in __Arabidopsis__ over the course of 48 hours after infection, they found that indeed, the expression of these genes ebbed and flowed, peaking at dawn and then shutting off during the day."People had suspected that this was the case," Dong said. "But there were no examples of it. Here we have specific molecules that are involved in immune response but are regulated by the circadian clock."The team got further evidence of this when they found that __Arabidopsis__ plants containing mutant versions of CCA1 were more vulnerable to downy mildew.Interestingly, even in the control samples (plants that had been sprayed with water instead of the parasite), the genes were expressed in the same oscillatory fashion. "The defense genes have a rhythmic expression even without infection," Dong said, suggesting that the plant was anticipating infection risk. Sure enough, the genes' most active period at dawn corresponds to the time of day when the downy mildew releases its spores into the air. During the rest of the day, on the other hand, when the genes are essentially inactive, Hpa infection is highly unlikely. It wasn't a surprise then, that when the researchers exposed the plants to the pathogen at dusk, the plants were much more susceptible to downy mildew disease. "When we studied defense gene expression, we would normally take a couple of time points and never really think that these genes would oscillate between day and night," Dong said. If other plants also show cyclical defense strategies against their common parasites, it could have serious implications for understanding and dealing with devastating crop diseases such as the potato blight, she added. W. Wang, et al., "Timing of plant immune responses by a central circadian regulator," Nature, 470:110-4, 2011.
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