Induced Resistance and Plant Immunity: Everything You Need to Know

The endless crusade to protect plants from pests and disease is a struggle as old as agriculture itself, but the idea of using induced resistance as a pathway to improving plant immunity is a fairly recent discovery.

Induced resistance (IR) is a phenomenon where biological or chemical agents trigger, or induce, a state of resistance in plants. This response can be triggered by beneficial microbes in the rhizosphere, the introduction of certain biological or synthetic compounds, or following stimulation by a pathogen, insect, or wound.

The practice of implementing IR for disease management only began in recent decades. But some research suggests it has potential as a holistic, long-lasting, and environmentally friendly solution for disease control that may reduce our dependence on costly and environmentally damaging chemical pesticides.

The Different Types of Induced Resistance

Depending on the mode of induction, IR is typically classified as either induced systemic resistance (ISR) or systemic acquired resistance (SAR).

Unlike chemical or biological pesticides, ISR and SAR work by activating a plant’s inherent abilities to defend against pests and diseases. Rather than a localized response to attacks, these two modes of resistance induce a defensive response throughout the entire plant—they are systemic, meaning that even plant parts not exposed to pathogens develop resistance.

The two terms were sometimes used interchangeably as research in IR evolved through the late 20th century. Initially, the different biochemical and genetic pathways that distinguish ISR and SAR were still unknown. This has created lasting confusion about what exactly differentiates these two types of IR. Current research now defines them as:

Induced Systemic Resistance (ISR): Resistance that is triggered in plants by certain beneficial microbes or chemical agents. When a plant comes into contact with these, it activates its immune system and produces phytochemicals that help to fight off potential infections.

Systemic Acquired Resistance (SAR): Resistance that develops in plants after exposure to a necrotizing (plant tissue-killing) pathogen or after treatment with synthetic or natural compounds that simulate exposure. Following exposure, the plant accumulates salicylic acid. This immune response can also protect the plant from future infections by the same or similar pathogens.

The biochemistry behind IR is complex, but in short, the primary difference between SAR and ISR is that SAR is triggered by pathogenic microbes or synthetic compounds, and plants acquire future resistance to that pathogen. ISR is triggered by beneficial microbes or synthetic compounds, inducing a plant immunity response. Both mechanisms help plants to defend themselves against pathogens, but ISR can provide protection even before an infection occurs.

How ISR and SAR Work

ISR and SAR are initiated through distinct molecular and chemical modes of action. Here is an overview of how they both work in practice.

ISR

ISR uses jasmonic acid (JA) and ethylene (ET) to send chemical signals that trigger defence responses throughout the plant. Beneficial microbes are often what initiate this physiological reaction, called “priming”. Plants show stronger and faster defense responses, thus preparing them for potential pathogenic invasions.

Certain strains of Bacillus and Pseudomonas bacteria have been shown to elicit an ISR response in many greenhouse and field crops, including tomato, bell pepper, watermelon, sugar beet, tobacco, and cucumber.

Inoculations featuring Bacillus and Pseudomonas can significantly reduce the prevalence and severity of many diseases, including leaf-spotting fungal and bacterial pathogens, crown-rotting fungal pathogens, root-knot nematodes, damping-off, blue mold, and late blight diseases.

SAR

SAR is often triggered by exposure to pathogenic microbes, setting off the accumulation of salicylic acid (SA) and pathogenesis-related proteins to help inhibit or kill off infectious bacteria or fungi. 

Synthetic compounds can also be used to trigger SAR, inducing a similar pathogenic immune response. These compounds usually aim to encourage the accumulation of SA, the chemical signal that mediates SAR plant responses.

Recognizing Molecular Patterns

Both ISR and SAR are activated by pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs). These patterns, produced following infection, microbial priming, and infestation, are the signals plants recognize to initiate systemic resistance responses.  

Certain synthetic or biologically-derived chemicals can mimic the effects of these molecular pathways. For example, synthetic analogs of salicylic acid can simulate PAMPs to activate SAR, even without an initial pathogenic infection. Similarly, the application of jasmonic acid or its derivatives can simulate MAMPs, activating ISR. 

Mimicking these patterns is what most commercial inducers seek to do.

Implementing Induced Resistance in Crop Production

Induced resistance can potentially offer broad-spectrum pest management merely by leveraging a plant’s inherent resistance mechanism. Its potential in reducing chemical pesticide use—and the increasing threat of pathogen resistance to those chemical pesticides—is tremendous.

The first commercially developed chemical resistance inducer, acibenzolarS-methyl (ASM) has been used on cereal crops in Europe since the late 1990s. Since then, dozens of other chemical and microbial activators have been researched for their disease management potential in eliciting either ISR and/or SAR. 

Despite promising field studies, the implementation of IR is far from widespread. Part of this is because of the biological complexity involved. The efficacy of IR depends on many factors, including light, soil chemistry, temperature, humidity, plant genotypes, and nutrient availability. This means results can be highly variable and difficult to quantify.  

Another element holding back IR adoption is the way we conceptualize successful pest management. IR techniques and commercial products will likely never replicate what conventional pesticides have taught growers to expect crop protection to look like, in which results are easy to measure and often immediate.

Unfortunately, the measurable effectiveness of pesticide use comes at a high cost. Their overuse is highly polluting, detrimental to ecological diversity, and creates highly resistant strains of damaging pathogens.  

The success of IR as a crop protection strategy may look different on the surface, but implementing them can provide more holistic benefits. And it all begins in the soil.

How Healthy Soils Promote Induced Resistance

We have touched briefly on the role of beneficial microbes in inducing resistance. When present in the soil and the rhizosphere, they can have a significant impact in eliciting an ISR response.  

The soil microbiome and rhizosphere mediate ISR through the production of plant-beneficial compounds. Plants and microbes engage in a molecular dialogue in the rhizosphere, symbiotically exchanging nutrients and chemical signals. Microbes thrive and colonize the root zone, releasing a concoction of hormones and enzymes that trigger the plant to ramp up defensive capabilities. 

Healthy soils are diverse soils. They are replete with beneficial microbes but inevitably contain some pathogenic ones too. Fortunately, in healthy (disease-suppressive) soils, these pathogens are less likely to become a problem. Beneficial microbes will out-compete them for nutrients and space. This limits the severity and spread of infections while still exposing plants to them, thereby activating SAR immunity responses and protecting plants from future outbreaks. 

Streptomyces, Actinomyces, Trichoderma, Pseudomonas, non-pathogenic Fusarium, and Bacillus spp. are among the key microbes that contribute to the disease suppressiveness of soils.

Plant Growth-Promoting Rhizobacteria and Induced Resistance

Many of the beneficial microbes that effectively induce resistance are called plant growth-promoting rhizobacteria (PGPR). The Pseudomonas and Bacillus genera of bacteria are among the best-understood PGPR. They can improve plant growth and root development by producing certain hormones, outcompeting pathogens, and optimizing nutrient and water uptake. 

PGPR protects plants through a combination of facilitating more vigorous growth and eliciting IR responses. This makes them particularly appealing as part of an integrated crop protection strategy. 

Chemical or synthetic inducers, like ASM and 2,6-dichloro-isonicotinic acid (DCINA), have proven highly effective as a pest management solution for certain crops in certain conditions, but they tend to suffer from high variability in efficacy, depending on soil quality, environmental conditions and numerous other factors. Some may even impose a “fitness cost” by directing plant resources toward defense at the expense of yields.

Microbial Inoculants, Induced Resistance, and the Future of Disease Management

Microbial inoculants are a cost-effective and low-impact method to build up plant resilience to pathogens. By adding PGPR to the rhizosphere, inoculants can reduce conventional pesticide use, limit the buildup of pesticide-resistant pathogen strains, and contribute to a healthy soil microbiome that keeps those pathogens in check.

Climatic instability is compounding biotic and abiotic stress facing crops, making them more susceptible to pests and disease. Innovative solutions will be needed if we are to sustain and increase food production without sacrificing the ecological integrity of our soils.

More research is needed to better illustrate a causal relationship between beneficial microbes and ISR and SAR responses, but their ability to provide plant growth alongside disease suppression is promising. Beneficial microbes and other induced resistance strategies are likely to be critical players in the agricultural revolution that these enormous challenges demand from us.

For a deeper dive into induced resistance, and the role of beneficial microbes in particular, explore the sources referenced in this article:

Abdul Malik, N. A., Kumar, I. S., & Nadarajah, K. (2020). Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity. International Journal of Molecular Sciences, 21(3), 963. https://doi.org/10.3390/ijms21030963 

Bektas, Y., & Eulgem, T. (2015). Synthetic plant defense elicitors. Frontiers in Plant Science, 5. https://doi.org/10.3389/fpls.2014.00804

Choudhary, D. K., & Johri, B. N. (2009). Interactions of Bacillus spp. and plants – With special reference to induced systemic resistance (ISR). Microbiological Research, 164(5), 493–513. https://doi.org/10.1016/j.micres.2008.08.007

Choudhary, D. K., Prakash, A., & Johri, B. N. (2007). Induced systemic resistance (ISR) in plants: mechanism of action. Indian Journal of Microbiology, 47(4), 289–297. https://doi.org/10.1007/s12088-007-0054-2

Kloepper, J. W., Ryu, C. M., & Zhang, S. (2004). Induced Systemic Resistance and Promotion of Plant Growth by Bacillus spp. Phytopathology®, 94(11), 1259–1266. https://doi.org/10.1094/phyto.2004.94.11.1259 

Kamle, M., Borah, R., Bora, H., Jaiswal, A. K., Singh, R. K., & Kumar, P. (2020). Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR): Role and Mechanism of Action Against Phytopathogens. Fungal Biology, 457–470. https://doi.org/10.1007/978-3-030-41870-0_20

Leadbeater, A., & Staub, T. (2014). Exploitation of Induced Resistance: A Commercial Perspective. Induced Resistance for Plant Defense, 300–315. https://doi.org/10.1002/9781118371848.ch13

Maksimov, I. V., Abizgil’dina, R. R., & Pusenkova, L. I. (2011). Plant growth promoting rhizobacteria as alternative to chemical crop protectors from pathogens (review). Applied Biochemistry and Microbiology, 47(4), 333–345. https://doi.org/10.1134/s000368381104009

Reglinski, T., Dann, E., & Deverall, B. (2007). Integration of Induced Resistance in Crop Production. Induced Resistance for Plant Defence, 201–228. https://doi.org/10.1002/9780470995983.ch11

Pieterse, C. M., Zamioudis, C., Berendsen, R. L., Weller, D. M., Van Wees, S. C., & Bakker, P. A. (2014). Induced Systemic Resistance by Beneficial Microbes. Annual Review of Phytopathology, 52(1), 347–375. https://doi.org/10.1146/annurev-phyto-082712-102340

van Loon, L. C. (2007). Plant responses to plant growth-promoting rhizobacteria. European Journal of Plant Pathology, 119(3), 243–254. https://doi.org/10.1007/s10658-007-9165-1 

Walters, D. R., Ratsep, J., & Havis, N. D. (2013). Controlling crop diseases using induced resistance: challenges for the future. Journal of Experimental Botany, 64(5), 1263–1280. https://doi.org/10.1093/jxb/ert026

Yu, Y., Gui, Y., Li, Z., Jiang, C., Guo, J., & Niu, D. (2022). Induced Systemic Resistance for Improving Plant Immunity by Beneficial Microbes. Plants, 11(3), 386. https://doi.org/10.3390/plants11030386

Zhou, M., & Wang, W. (2018). Recent Advances in Synthetic Chemical Inducers of Plant Immunity. Frontiers in Plant Science, 9. https://doi.org/10.3389/fpls.2018.01613


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