Plants never cease to reveal their unique and astonishing methods of protecting themselves from pathogens. Here on Bioscription we’ve previously discussed the topic of the pathogen-associated molecular pattern (PAMP) system and how it allows plants to uptake and conserve important foreign molecules for future immune system response. It is, in many ways, similar to the animal immune system and how our memory T cells provide us a swift disease response if we’re exposed to the same pathogen again, hence the importance of initial exposure through vaccines. 

But the extent to which PAMPs can be used goes far beyond just a simple immune process and instead allows those same organisms to directly assault and combat the pathogens it recognizes. The pattern-triggered immunity (PTI) is vast in its potential output and today we are going to discuss how that can be applied to a more scent-based defense. 

Nematodes And Their Pheromones

Nematodes are among the more annoying of plant pathogens and they are largely ubiquitous throughout the soil environment. They parasitize most commercial crops in use and are responsible for billions of dollars in damage to farmers’ livelihoods every year. So better understanding how to protect against them is a major area of research in the field of plant pathology and learning how plants manage it themselves could be helpful toward that goal. 

A collaboration between Cornell University researchers and the Boyce Thompson Institute began looking into this subject with a focus on the molecular group known as ascarosides. These ascarylose sugar molecules with a fatty acid related side chain are, as currently known, unique to nematodes and are important in their ability to communicate through chemical signals and are involved in other parts of their biological development. It has only been in the last decade and especially in the past few years that experiments have shown that these molecules can also be detected by plants and fungi and have long since been incorporated into the PAMP system. 

When the eighteenth known ascaroside (ascr#18) was applied to plant tissue, an extreme defense signaling pathway was activated. The actual molecules and signals in this pathway are not yet explained and it is not known how this recognition is done, as every PAMP response is based around using the relevant molecule to chemically modify the plant’s own systems. Based on the strong response, it is likely that ascr#18 is uptaken by plants and metabolized. The research team were interested in finding out just what sort of biochemical editing the plants do to the ascarosides after incorporating them and how they play a role in the PAMP response. 

Hijacking The Hijackers

What they found is that, first, only a very small concentration of ascaroside molecules are needed to trigger the response, similar to microbial molecules the plants respond to, such as flagellin. However, unlike those, ascarosides are comparatively tiny molecules and are very highly conserved only in nematodes, so how are they processed into a response so quickly and at a similar minuscule amount?

The researchers found that plants relatively quickly begin breaking down the side chains of the ascarosides that they then excrete into the surrounding soil rhizosphere. These shortened pieces act as repellents that drive away the nematodes and prevent an ongoing invasion. It is believed by the research team that ascr#18 and ascr#9 may act as a method for the nematodes as a population to find host plants to colonize and that a large amount of the molecules in the environment signals an overpopulation within the host plant already. This results in the nematodes leaving to search for a more worthwhile host. 

Intriguingly, this would imply that during a successful infestation by nematodes, they may themselves secrete these pheromones from the plant by hijacking its beta-oxidation system (which created the shortened side chains in the aforementioned PAMP response in the first place) in order to ensure that not too many of their brethren infest the same plant. Since overpopulation could also kill off the nematodes themselves by taking away their source of food. Thus, random mutations by natural selection appear to have given plants the capability to produce these altered ascarosides themselves by uptaking them from the soil environment before the nematodes are able to infect the plants’ tissues.

A World Of Communication

The scientists note that this is the first example discovered of a PAMP molecules being edited by an enzyme within a plant in order to set off the pattern-triggered immunity process. They also showcased that knocking out the relevant oxidation enzyme genes in the model plant Arabidopsis thaliana prevents the plant from being able to mount a successful defense using ascarosides, as they are unable to edit the molecules. It is possible that the development of the peroxisomal oxidation pathway, which is so critical now for the production of auxin hormones and jasmonic acid to regulate plant growth, may have been prompted far in the past thanks to the stresses and methods created by nematode pathogens. 

This knowledge also implies that enzymatic editing may be used in the immunity system for other PAMP molecules, including those used for defense against microbial invaders. This has been suggested, but not confirmed, for several other molecules and now this study can act as a boost to research searching for such mechanisms. In fact, the revelation that plants can so directly “hear” the biochemical communications going on in the surrounding soil microbiome may point to there being many other chemical signals they may be active in modifying. And not just as defense, but also for their symbiotic relationships. Only time will tell whether this hypothesis is proven correct, but we are certainly in an exciting time for plant pathological experimentation and study.

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Photo CCs: Radopholus similis from Wikimedia Commons

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