My parents have a garden. Every year, most of the plants in that garden remain perfectly healthy and even those that suffer from pests and diseases seldom host more than one or two at a time. That does seem odd: plants live in an environment filled with thousands of different species of fungi, bacteria, viruses, nematodes, insects and so on that prey on plants. In other words, every plant is resistant to most pests and pathogens. Resistance is the rule, disease the exception.
The reason for that impressive level of resistance is that plants have an immune system. While it doesn’t work in quite the same way as the human one, but conceptually it does the same three basic things: it recognizes invaders, fights them and learns from the experience. And just like the human immune system, the plant immune system tends to win most of the battles it has to fight. Only a handful of pests and diseases ever manage to seriously damage any given plant.
Humans learnt long ago that they could use vaccination to train their immune system to win even more fights than it normally could. Fundamentally, a vaccine is a simple thing: you take a dead or weakened pathogen, or even just a little fragment thereof, and expose the immune system to it. It then memorizes the experience and responds much more quickly and intensely when it encounters the real pathogen later on.
Could we do the same for plants? It turns out we can. One of the most interesting approaches is called priming. Priming is conceptually like a human vaccine: you expose the plant to something that triggers a benign immune response and the plant memorizes that response. When a real pathogen attacks, the immune response of a primed plant is faster and more intense than that of an unprimed plant. The ‘something’ that primes the plant can be a beneficial micro-organism, a small molecule or a protein.
There’s one big difference between priming and vaccination though. Vaccination gives nearly complete immunity to a specific pathogen, while priming gives partial resistance against a wide range of pathogens. While that lack of specificity might seem like a weakness of the plant immune system, it’s actually an advantage in the context of priming. You can prime a plant pre-emptively at the start of the season and then enjoy the benefits against a huge range of pests and diseases that might later occur.
Of course, a general immune response will not suffice to completely fight off an attacker. However, it might well be enough to prevent a pest or disease from crossing the damage threshold at which economic losses occurs, or failing that it will at least reduce the number of pesticide applications needed to achieve a good level of control. A major additional advantage of priming is that the plant immune response involves a wide range of molecules, barriers and signals, which makes it unlikely that pests or pathogens could develop resistance against priming. By contrast, most pesticides depend on a single molecular target, which means that the odds of resistance emergence are much higher.
The environmental benefits of priming are significant. Priming agents are designed to help plants, not to kill pests or diseases. As a rule, they have few or no side effects on non-target organisms – unlike conventional pesticides, which cannot always be used without harming beneficials. The pre-emptive nature of priming and its inherent respect for beneficial organisms means that it’s a perfect fit in the wider Integrated Pest Management (IPM) toolbox.
A crop protection product that pre-emptively reduces the likelihood that a given disease will cross the damage threshold while respecting biodiversity sounds great. Quite a few priming agents have been described in the lab. So why are relatively few priming-based products used in the field?
Priming is hard.
Finding good priming agents is hard. They don’t show up in the high-throughput assays companies routinely perform and we don’t understand priming well enough to rationally design priming agents. Finding them is still mostly a matter of luck.
Producing, formulating, storing and using priming agents can be hard. Many priming agents are living micro-organisms or fragile macromolecules isolated from them. Getting them to the farmer is a challenge by itself; making sure that they can work consistently is even harder. Dozens of environmental factors, from temperature to the soil microbiome, modulate plant immunity and hence the efficacy of priming.
At the moment, there’s no perfect priming agent that ‘just works’. For all we know, there never might be one. However, we are getting a little bit closer every day.
As part of my research into plant-pathogen interactions, I discovered a class of small molecules capable of priming a wide range of crops. These molecules are stable and easily formulated, which increases the likelihood that they will work consistently in the real world. They’re also easy to produce, non-phytotoxic and they have shown an impressive breadth of activity in lab tests.
Without going into too much detail, these molecules work by temporarily blocking the phenylpropanoid pathway. This pathway, which is present in all land plants, turns an amino acid, phenylalanine, into a huge range of phenolic molecules. These phenolic molecules have a wide range of functions: some give flowers and fruits their bright colours, some are used as building blocks for wood, some act as natural sunscreen for plants and yet others help fight plant pests and diseases.
If you block the phenylpropanoid pathway permanently, you kill the plant. But if you block it briefly with a short-lived molecule, you prime the plant. Many pests and diseases try to suppress the phenylpropanoid pathway as part of their infection process, so a plant sees any tampering with that pathway as an alarm signal. A brief immune response is triggered, memorized and the plant is primed. Moreover, the plant responds to a blocked phenylpropanoid pathway by making more copies of the enzymes of this pathway. Those enzymes stick around after the priming and help the phenylpropanoid pathway to do its job more effectively when a real pathogen attacks.
At the moment, we’re testing these molecules in collaboration with an industrial partner in a wide range of plant-pathogen systems and at different scales, from in vitro experiments to field trials. It’s still far too early to draw firm conclusions, but initial results against nematodes and fungi appear promising. If our molecules live up to their potential, they will provide farmers with an affordable, convenient and sustainable tool to help control pests and diseases across a range of farming systems.
Simultaneously, we are using a range of scientific techniques, including metabolomics, transcriptomics, microscopy and biochemical assays, to learn more about how our molecules, and priming agents in general, work at a fundamental level. The insights from that research might help develop other priming agents and could even find use in molecular breeding or gene editing for disease resistance. In short, I’m working on a project at the interface between applied crop protection and fundamental plant biology – a dream come true!
So in conclusion: our phenylpropanoid pathway inhibitors just might help the crops of tomorrow protect themselves.
Willem Desmedt, Belgium