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Dawn attack: how plants anticipate pathogen infection

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Like animals, plants are susceptible to infections from bacteria, viruses and fungi. While animals have a wide variety of immune cells and in some cases an interconnected immune system plants must rely on other methods to fight infection. A recent news bulletin from the Howard Hughes Medical Institute lists a range of exciting ways plants can defend against pathogen attack. I was able to speak to Professor Xinnian Dong about her work looking at interactions between pathogen defence and the circadian rhythm.

Pseudomonas syringae bacteria entering a plant leaf through a stoma. Image credit: James Kremer and Sheng Yang He. Reproduced with permission.

While exploring the response of the plant Arabidopsis to the downy mildew pathogen Dr. Dong’s researchers found 22 genes that, when mutated into non-functioning forms, severely compromised the plants resistance. Looking more closely at the genes they determined they could be controlled by the circadian regulator CCA-1.

The circadian rhythm is a time-cycle which switches on different plant processes at different times of the day. For plants there are many processes that only happen at certain times; petals opening, leaves moving and photosynthesis occurring. It seems that pathogen response may also be controlled as part of the circadian rhythm. By looking at the change in expression of the genes throughout the day, the researchers found that the pathogen response genes showed highest expression levels at dawn. This expression pattern was seen even when no pathogens were present.

Change in gene expression over time showing the temporal expression of resistance genes. Hours along the x-axis and relative expression up the y-axis. For full details see the reference.

In order to understand why this kind of control was necessary, the researchers studied the pathogen in closer detail. The downy mildew pathogen (Hyaloperonospora arabidopsidis) can only live on the Arabidopsis plant. The pathogen grows spores at night time, which appear on the leaves with the damp dew drops. As the dew dries in the sunshine, the spores are released. By setting up a system of defence that turns on in the early morning, the plant  is anticipating the attack from the mildew spores.

In order to test this hypothesis the researchers tried inoculating plants with the pathogen at dawn and at dusk. Those that were infected in the morning were far more resistant than those infected at night. By anticipating the time the pathogen was most likely to attack, the plant was able to boost its own defences in preparation.

Having looked at how the circadian rhythm can control plant defences, Professor Dong is now interested in how attacking pathogens can affect the circadian clock. In animals, infections can throw the internal scheduling off track, and it would be fascinating to see what affect pathogens have on the internal clocks of plants.

Reference: Wang W, Barnaby JY, Tada Y, Li H, Tör M, Caldelari D, Lee DU, Fu XD, Dong X. Timing of plant immune responses by a central circadian regulator. Nature. 2011 Feb 3;470(7332):110-4. doi: 10.1038/nature09766.

Many thanks to Professor Dong for discussing her work.

Original story from the Howard Hughes Medical Institute

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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