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How plants detect infections

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Published online 12 July 2010 | Translated by Sophie Haesen, ACCESS!

The smallest fragments of bacteria suffice for plants to trigger a defense reaction to the microbes. Researchers of the Universities of Basel and Wurzburg describe the molecular details of this reaction in the renowned periodicals «The Plant Journal» and «Journal of Biological Chemistry».

In the wild, the life of plants is constantly in jeopardy: they are threatened by unfavorable environmental conditions such as: persistent drought or by harmful substances in the atmosphere, soil and water. In addition, aggressive fungi, bacteria and viruses are lurking all the time. If these pathogens had their way, the flora would not be as lusciously green and resplendent. Plants are therefore capable of holding their little enemies at bay. How do they do this, not being able to run away or reach for the medicine chest?

Research teams from the Universities of Basel and Wurzburg answer this question. Dr. Thomas Boller from Basel and the Wurzburg team, led by the biophysicist Dr. Rainer Hedrich, have published their results in the renowned periodicals «The Plant Journal» and «Journal of Biological Chemistry». The researchers show how plants restrain potential pathogens by means of their natural immune system. They studied this phenomenon using Pseudomonas bacteria, which are known to cause rot and other damage in plants, and the classic plant model species, Arabidopsis thaliana (mouse-ear cress).

Receptor detects bacteria fragments
The bacteria Pseudomonas was not chosen at random, as the team around Thomas Boller had achieved a decisive breakthrough before. The Basel researchers identified a receptor (FLS2) in the plasma membrane of plant cells. It detects fragments of flagella, the bacterial organs of locomotion, even in minuscule amounts. “At a conference, we agreed to combine our expertise in biophysics (Wurzburg) and biochemistry (Basel),” Rainer Hedrich explained. The researchers’ objective is to clarify the early processes that plants use to detect pathogens.

Electrical excitation by bacterial fragments
From Basel, the Wurzburg researchers received a peptide chain of 22 amino acids (Flg22) from the flagella component flagellin, as well as receptor mutants of Arabidopsis. With this material they observed that the bacterial peptide excites the plant cells electrically. About a minute after having administered the bacterial fragment to the plants, they noticed an increase in calcium concentration as well as a membrane depolarization lasting for ten minutes. “Depending on calcium, the flagellin receptor activates an anion channel in the membrane,” says Dirk Becker.

Plant distributes antibacterial substances
At the same time, the researchers discovered that the excitation of the membrane is transmitted to the nucleus and stimulates the immune system: the plant activates defense genes, assembles antimicrobial substances and enzymes and showers the intruding bacteria with them. To prevent the microbes from spreading, whole cell groups around the source of infection commit sacrificial death as a last resort. Brown spots and minuscule “scars” remain, witnessing the successful defense against the pathogen.

Hundreds of early warning systems against intruders
But what if the plant overlooks the flagellin that is produced by many bacteria? “No problem,” says Thomas Boller, “The plant identifies intruders simultaneously via several receptors – this way, it takes a typical fingerprint of the respective pathogens.” The natural immune system of plants consists of hundreds of such early warning systems. Among others, this includes those of the PEPR1/2 type that detect endogenous peptides from the interior of the cell. As soon as microbes harm a plant cell, these peptides reach the surface receptors of surrounding cells and signal the danger.

Anion channel transmits danger signals
From their results, the German-Swiss research alliance concludes that the diverse danger signals detected by these receptors are translated into an electrical signal via the same anion channel. Hedrich added, “Right now, we are busy tracking the gene for this central anion channel. We have found two gene families encoding anion channels. Now we need to nail the prime suspect.”

Early signaling through the Arabidopsis pattern recognition receptors FLS2 and EFR involves Ca2+-associated opening of plasma membrane anion channels, Elena Jeworutzki, M. Rob G. Roelfsema, Uta Anschütz, Elzbieta Krol, J. Theo M. Elzenga, Georg Felix , Thomas Boller , Rainer Hedrich, and Dirk Becker, Plant Journal 2010, Volume 62 Issue 3, Pages 367 – 378 (Published Online), DOI: 10.1111/j.1365-313X.2010.04155.x (PDF / Abstract)

Perception of the Arabidopsis Danger Signal Peptide 1 Involves the Pattern Recognition Receptor AtPEPR1 and Its Close Homologue AtPEPR2, Elzbieta Krol, Tobias Mentzel, Delphine Chinchilla, Thomas Boller, Georg Felix, Birgit Kemmerling, Sandra Postel, Michael Arents, Elena Jeworutzki, Khaled A. S. Al-Rasheid, Dirk Becker, and Rainer Hedrich, J. Biol. Chem. 2010 285: 13471-13479. First Published on March 3, 2010. DOI:10.1074/jbc.M109.097394 (PDF / Abstract)

Source: University of Basel

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