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Double-secured immune protection against plant attackers

Plants have sophisticated defence mechanisms to help them fight off all kinds of pathogens. A group of researchers led by Dr. Gabriel Schaaf at the University of Tübingen’s ZMBP has now discovered that plants’ immune response is more similar to the innate immune system of humans and animals than previously thought. The biologists discovered a biosynthesis pathway in plants involving a group of signalling molecules that, until recently, has only been known from animal cells where they are essential for cell survival. In plants, these molecules known as inositol pyrophosphates are, along with the phytohormone jasmonic acid, responsible for taking action against pathogens. The scientists now want to examine the extent to which such ubiquitous molecules can be used therapeutically.

The team of researchers led by Dr. Gabriel Schaaf at the ZMBP in Tübingen is focused, amongst other things, on the complex defence strategies that plants use to fend off pathogens (from left to right: Dr. Gabriel Schaaf, Dr. Marek Dynowski, Dr. Marília K. F. de Campos, Philipp Johnen, Debabrata Laha (first author of the study), Philipp Gaugler. © University of Tübingen

Almost all plants have scores of enemies that they have to fight off day in day out in order to survive. They include caterpillars that feed on living leaves, fungi that infest and digest plant tissue as well as pathogenic bacteria. Unlike animals, plants cannot run away from their enemies. Plants therefore need different strategies from animals to ensure their survival. They have developed a whole range of biochemical defence mechanisms either to deter their enemies or adversely affect the enemies' development. Plants can distinguish whether they are being attacked by bacteria, fungi or herbivores (e.g. caterpillars), and can adapt their defence response accordingly. After damage caused by herbivores, it only takes plant cells a few minutes to produce an active form of the phytohormone jasmonic acid, which then induces the release of inhibitors and defence substances. The hormone even "notifies" neighbouring plants of the danger.

Two molecules switch on pathogen defence signalling pathway

Simplified diagram showing the activation of defence responses following an attack by insects or infestation with fungi. Under basal conditions, JAZ proteins (green) inhibit the plants’ defence mechanisms (guaranteeing optimal growth and optimal development of the plants). Insects or fungi induce the synthesis of two important molecules, namely activated jasmonic acid and inositol pyrophosphate InsP8. Both molecules need to bind to the SCFCOI1 complex in order to be able to effectively recruit the JAZ repressor. Following recruitment of the repressor, JAZ is labelled with ubiquitin (Ub, ubiquitinylation), which causes the JAZ repressor protein to degrade and results in the rapid activation of a variety of defence genes (e.g. VSP2). © American Society of Plant Biologists (modified, Gabriel Schaaf)

Dr. Gabriel Schaar and his team at the Centre for Plant Molecular Biology (ZMBP) at the University of Tübingen are studying the complex defence mechanisms of plants. Until recently, they thought that the active form of the phytohormone jasmonic acid alone was enough to switch on the defence signalling pathway. To this end, the hormone recruits so-called JAZ proteins that normally suppress the activity of defence genes. The proteins are recruited to a so-called SCFCOI1 complex that induces the degradation of these repressor proteins. The JAZ proteins lose their suppression activity, resulting in the activation of a large number of defence genes. The biologists from Tübingen have now discovered that the SCFCOI1 complex, which is responsible for the recognition of active jasmonic acid, not only binds active jasmonic acid and JAZ repressor proteins, but is also able to bind another signalling molecule, namely inositol pyrophosphate (PP-IP).

"A functioning protein complex forms best when inositol pyrophosphate is also bound. This means that two molecules dock to the complex," says Schaaf. "Plants have an extremely effective defence mechanism as they can modulate the activation of defence genes via these two proteins. Plants also have an additional safety level that stops the defence mode being triggered prematurely. If this happens, the plant loses its fight against other plants in the competition for light and nutrients," adds the researcher.

The researchers from Tübingen became aware of the function of the signalling molecule when they studied special thale cress (Arabidopsis thaliana) collections in which individual genes were switched off. They found that the plants without active PP-IP looked perfectly normal, but had major problems with insect pathogens. Using a combination of biochemical, structural and molecular methods, Schaaf and his team were able to elucidate and publish the signalling compounds' mechanism of action.1

Inositol pyrophosphates are also interesting for medical applications

The discovery of the second molecule has attracted attention beyond the plant science world. Physicians are also interested in the PP-IPs because the molecules have been shown to be important for humans as well. "The function of these molecules in the immune response of animal cells is still largely unexplored," says Schaaf. "However, numerous parallels between plants and animals exist, for example, innate immunity, which is the ability to react quickly to foreign substances." Before the biologists from Tübingen discovered the signalling substances in the thale cress, inositol pyrophosphates were mainly known in animal cells, as well as in amoebae and yeasts, where they are involved in important functions such as programmed cell death, chemotaxis, telomere ageing processes, intracellular membrane transport and insulin signalling. However, the exact mechanism of action of most signalling molecules is still unclear.

Structural formula displaying inositol pyrophosphate biosynthesis in plants and animals.
Simplified diagram showing the biosynthesis of inositol pyrophosphate in plants, animals, humans and yeast. InsP7 and InsP8 carry a diphosphate group and therefore belong to inositol pyrophosphates (PP-IPs). 'P' is the symbol for phosphate groups. The Arabidopsis thaliana enzymes VIH1 and VIH2 are InsP8 synthetases that induce the biosynthesis of InsP8. The synthesis of InsP7 from InsP6 has not yet been clarified but the participation of VIH1 in this reaction cannot currently be excluded. © Gabriel Schaaf

Potential therapeutic application of PP-IPs

Artist's representation of an Arabidopsis thaliana leaf infested with a cabbage butterfly caterpillar (Pieris rapae). Caterpillar attack induces an immune response in the plant. The schematic shows the jasmonic acid receptor, an F-box protein complex, under high magnification. The recruitment and degradation of the so-called jasmonate ZIM domain (JAZ) repressor (dark red) only happens through simultaneous binding of inositol (pyro)phosphate (magenta red) and active jasmonic acid (or a jasmonic acid analogue, yellow), which then sparks defence reactions. © Gabriel Schaaf and Hans van Pelt

In cooperation with chemist Prof. Dr. Henning Jessen from the University of Zurich (now at the University of Freiburg) and molecular biologist Prof. Dr. Adolfo Sajardi (Medical Research Council London), the plant researchers from Tübingen now want to find out whether PP-IPs are also suitable as therapeutic agents. The molecules are currently being modified in the labs in Zurich and Freiburg. "The PP-IPs have numerous phosphate groups, which makes them strongly negative, and therefore unable to pass through plasma membranes," says Schaaf. "Jessen's team of researchers had to resort to trickery. They produced molecules whose charge was masked so that they could be transported without difficulty across the membrane into the cell, where they were cleaved by cellular enzymes. Active PP-IP was then released. We have demonstrated this for different types of cells and documented our findings in a paper published in the journal Angewandte Chemie International."2 The pyrophosphate bonds make these molecules rather instable and energy-rich, and they are therefore able to exert strong signalling effects in the cell. They have to be degraded quite quickly so that metabolic processes can be carried out correctly. The scientists are currently studying in cell cultures whether the signalling molecules might be of pharmaceutical interest.

Targeted pest control is another application that might become feasible thanks to the detailed insights into plant immunity. Using green biotechnology methods, it might also be possible to create crops that will upregulate the biosynthesis of these natural protective molecules upon pest infestation. "We have been able to show in the model plant Arabidopsis thaliana that such plants are far more resistant to plant-feeding insects and fungal infections. This would enable farmers to substantially reduce the use of insecticides and fungicides," says Schaaf.

1Original publication:
Laha D, Johnen P, Azevedo C, Dynowski M, Weiß M, Capolicchio S, Mao H, Iveng T, Steenbergen M, Freyer M, Gaugler P, de Campos MKF, Zhen N, Feussner I, Jessen HJ, Van Wees SC, Saiardi A, and Schaaf G (2015): VIH2 Regulates the Synthesis of Inositol Pyrophosphate InsP8 and Jasmonate-Dependent Defenses in Arabidopsis. Plant Cell, 27, 1082-97

2Original publication:
Pavlovic I, Thakor DT, Bigler L, Wilson MSC, Laha D, Schaaf G, Saiardi A, Jessen HJ (2015): Pro-Metabolites of 5-diphospֺho-myo-inositol pentakisphosphate. Angew Chem Int Ed 54, 9622-6

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