Warding off microbes – the green way

Scientists from Wageningen UR have unravelled the mechanism of how a plant defends itself against pathogens, finally solving a mystery that has troubled botanists for years

Plants are constantly challenged by pathogens such as fungi, viruses and bacteria. Just like humans, plants almost always succeed in warding off pathogens by using special receptors that detect the invading pathogen and subsequently trigger the plant to activate an immune response.

These so-called immune receptors are either present at the plant cell surface or inside the plant cell. Receptors located inside the cell recognise pathogens that enter the cell interior, such as viruses and various types of fungi, whereas cell surface receptors protect against extracellular pathogens, such as bacteria and specialised fungi. Extracellular receptors usually also have a domain that protrudes through the cell membrane into the cell interior. This so-called kinase domain is able to activate interacting proteins through phosphorylation thereby warning the cell of the intruder and stimulating it to take action. This results in the activation of a signalling cascade that triggers plant defence responses, a process that generally culminates in the hypersensitive response. This response involves ‘programmed cell death’ of the plant cells surrounding the microbe, ensuring that the intruder can no longer absorb nutrients from living cells.

Although much is known about the defence system of plants, there are still various mysteries to be solved. For quite some time, for instance, plant scientists know about the existence and functioning of so-called RLK-receptors. These are receptors that are located at the surface of plant cells and have a domain both on the inside and the outside of the cell. At the outside a typical receptor-like structure is present which consists of leucine-rich repeats (LRRs), whereas in the cell a kinase domain is present. Whenever they receive a signal from the outside - from a fungus, for example - the kinase at the inside of the cell activates a signal to mount a defence response against the invading fungus.

In addition to RLK-receptors there are also RLP-receptors. These receptor-like proteins are also located at the plant cell surface and carry extracellular LRRs, but they do not have a kinase domain on the inside of the cell to pass on signals. The first RLP-receptor was identified in tomato plants (Solanum lycopersicum) about 20 years ago by researchers from The Sainsbury Laboratory in Norwich, UK. This is the Cf-9 resistance protein, which is an RLP that provides resistance of tomato to the extracellular fungal pathogen Cladosporium fulvum. Cf-9 specifically mediates recognition of the avirulence-9 (Avr9) protein of the fungus and triggers the plant cells to mount the hypersensitive response, resulting in plant resistance and avirulence of C. fulvum. Ever since the identification of Cf-9, and subsequent additional RLPs, scientists have been mystified as to how these receptors that lack a kinase domain, manage to warn the plant to enable it to protect itself against pathogens.

[caption id="attachment_34745" align="alignright" width="200" caption="Receptor-like protein (RLP)-receptors recruit the receptor-like kinase (RLK)-receptor SOBIR1 to activate defence against invading pathogens. When SOBIR1 is absent, a defence response is not mounted upon perception of a fungal signal molecule by the RLP-receptor (plant cell on the left) and the plant will be diseased. When SOBIR1 is present, the protein will be recruited by the RLP-receptor upon perception of a fungal signal molecule and defence will be activated through the kinase domain of SOBIR1 (dark green structure; plant cell on the right) and the plant will be resistant to the pathogen and stay healthy. Diagram made by: Thomas Liebrand, Matthieu Joosten and Guozhi Bi"][/caption]

We now know that all plant species contain such RLPs. For example, tomato contains around 180 different RLPs and the function of only some is known. Scientists developed the hypothesis that RLP-receptors involved in defence against attacking microbes possibly work together with RLK-receptors to pass on signals, but such an RLK-receptor remained to be identified. The scientists purified an RLP-receptor complex containing a Cf protein from leaves of tomato plants by using the latest biochemical techniques available. Subsequent analysis of the exact composition of the complex by mass-spectrometry revealed that a number of RLP-receptors indeed do recruit an RLK-receptor. This RLK, which can be regarded as a kind of co-receptor for RLPs, is referred to as SOBIR1 and has been related to plant immunity before. It appeared that RLPs are already present in a complex with SOBIR1 before pathogen perception. Possibly, SOBIR1 is being activated upon signal perception by the RLP when the pathogen attacks. The scientists describe that switching off SOBIR1 causes the RLP-receptors to be non-functional. In that case the plants do not recognise the fungal Avr protein anymore and become susceptible to the pathogen. In this way they showed that RLP-receptors cannot warn the cell without cooperating with the RLK SOBIR1. The results of their research have recently been published in the scientific journal PNAS.

All plant species use RLP-receptors to protect themselves against pathogens and they all appear to contain a gene closely related to SOBIR1. For example, the model plant Arabidopsis thaliana, which is a small weed that is preferentially used by plant scientists in their studies, also contains a SOBIR1 protein that is recruited by its RLPs. Therefore, this RLK-receptor is likely to be an essential and universal link in the defence system of plants.

To be able to infect plants, pathogenic microbes suppress the activation of defence responses. It is conceivable that during co-evolution between plant and pathogen, the RLK SOBIR1 has become a preferential target for pathogens to manipulate, thereby inhibiting its defence signalling activity. The discovery of SOBIR1 being a putative central regulator of plant defence responses therefore provides many opportunities for further studies on this type of defence system. Once more is known about the essential links in plant defence systems, it will be easier to breed plants that are more resistant to pathogenic microbes, which in turn would lead to a reduced use of pesticides. The Wageningen UR scientists will now continue to study what exactly occurs in the plant cells once the SOBIR1 kinase sends out warning signals. Possibly the perception of a signal from the pathogen, such as an avirulence protein, results in a change of the phosphorylation status (the “activation status”) of the kinase domain of SOBIR1, causing proteins required for defence signalling to be recruited. When it is known how the phosphorylation status determines the activation of plant immunity, SOBIR1 might be modified to more efficiently trigger resistance against a broad range of pathogens. Furthermore, the kinase domain of SOBIR1 might be manipulated by pathogens in such a way that its defence signalling is suppressed. When it is exactly known what this manipulation consists of, possibly a variant of SOBIR1 can be generated that cannot be manipulated by pathogens. This approach could also lead to broad-spectrum pathogen resistance.

The research was performed by several scientists from the Laboratory of Phytopathology, with the PhD student Thomas Liebrand as the lead researcher and the group leader Dr Matthieu Joosten as a last author. Studies were done together with colleagues from Plant Research International (PRI), the Centre for BioSystems Genomics (CBSG) and the Sainsbury Laboratory in the UK. It was financed by the Centre for BioSystems Genomics (CBSG), the Netherlands Organisation for Scientific Research (NWO) and the Gatsby Charitable Foundation.

Author: Matthieu Joosten, Associate Professor, Department of Phytopathology

Contact:

Matthieu.Joosten@wur.nl / +31 317483411

http://www.pnas.org/content/early/2013/05/22/1220015110.abstract.html?etoc