WILLAGRI
Induced resistance, a kind of “vaccination” of plants, is a promising approach that allows them to protect against pests without the use of pesticides. It consists of activating the natural immune systems of plants as soon as they are attacked by pathogens or insects or have to undergo environmental stresses. In this respect, it differs from the constitutive resistance naturally present in plants. This method helps to provide healthier food and preserve biodiversity. The creation of induced resistance in plants is an active field of research, combining knowledge in biology, agronomy and ecology. By integrating these concrete approaches, farmers can increase the resilience of their crops to diseases and pests, reducing their reliance on synthetic chemicals and promoting sustainable agriculture.
Pesticides have allowed, and still allow, agriculture to feed the planet. But at what cost? At the cost of side effects that are harmful to human health and serious damage to biodiversity. Worse, pests “know” how to adapt to pesticides by bypassing them and multiplying. Similarly, the technique of crossing plants with resilient genes faces the same problem. The competition between pests and pesticides seemed like a never-ending struggle until scientists became interested in harnessing induced resistance as a vaccine that boosts a plant’s immune system to prepare it for attacks during its growing period.
The induced resistance strengthens the plants’ ability to do what they already know how to do: defend themselves. For example, some plants release compounds that attract predators from pests that attack them. However, the most studied form of induced resistance is what is called “defense preconditioning”. In this scenario, when a plant experiences mild stress – such as an insect nibbling on one of its leaves, it partially activates its immune response. When the plant is attacked again, it remembers this and goes into full defense mode.
More interestingly, this “defense preconditioning” seems to last long enough to be passed on to the next generation of plants, probably through epigenetic mechanisms. Plants could pass on their immunity as a kind of hereditary armor.
The advantages of induced resistance are:
- Its energy efficiency: Rather than maintaining a constant defense (which can consume a lot of energy), the plant only strengthens its defenses when needed.
- Its adaptability : Induced resistance allows plants to adapt to various types of stress, increasing their survival and success in changing environments.
However, the induced resistance is not perfect. It does not offer complete protection on its own. You have to be attentive to the amount of energy a plant devotes to its defense. If this amount of energy is too much, it will take up the plant’s ability to grow, resulting in poor growth.
Finding the right balance between growth and defence
The safe and effective exploitation of induced resistance is not as simple as introgressing a single gene or spraying a single pesticide. This is because plants need to find a good balance between their growth and their defense mechanisms. Privileging one function is necessarily to the detriment of the other. There is no universal optimal balancing act. It differs depending on the plant, pest or environment. The induced resistance is tailor-made.
The game is won once the balance point has been found by plant, pest category and environment. Then the farmer will be able to do without pesticides completely. Better still, induced resistance is effective against a wide range of pathogens and pests against which it offers a more robust defense than chemical pesticides.
Methods and practices
Creating induced resistance in plants is done through a series of methods and practices that stimulate their natural defense mechanisms. Here are the main ones:
- Use of defense inducers such as salicyc acid, chitosan (derived from crustaceans) or plant extracts (garlic, laurels, aromatic herbs, etc.). Applied to plants, these substances provoke defense responses.
2.Foliar applications: application to leaves by spraying defense inducers.
3.Controlled environmental stress: moderate water stress (it can induce defense responses in some plants) and mechanical stress (friction or simulated injury can also cause defense reactions by stimulating the production of phytohormones).
4.Crop rotation and intercropping: Rotation with crops that do not harbour the same pests or pathogens can induce resistance.
5.Use of beneficial microorganisms: interactions with mycorrhizal fungi (they form symbiotic relationships with plant roots and improve resistance to stress) and soil bacteria induce defense responses.
- Varietal selection: selection and development of plant varieties that possess induced resistance mechanisms
7.Optimal growing conditions: provide adequate nutrients and proper water and lighting management.
Defense mechanisms
How does induced resistance work when a plant is attacked? Here are the principles:
- Signaling : When a plant is attacked, it releases certain molecules, such as volatile fatty acids or phytohormones (such as jasmonic acid, salicylic acid or ethylene), which act as signals to mobilize its defenses.
- Systemic response: This response is not limited to the area under attack; it can also influence other parts of the plant, making them more resistant to future attacks. This phenomenon is often referred to as “acquired systemic resistance” (RSA).
- Production of secondary metabolites : In response to attack, plants can produce secondary metabolites, such as flavonoids and alkaloids, which have antifungal, antibacterial, or insecticidal properties.
- Physiological changes : Induced resistance can also lead to changes in metabolism, cell structure, or other physiological characteristics.
Advantages and limitations
While induced resistance has proven its effectiveness in the laboratory or on small agricultural plots, it has never been exploited on a large scale. In other words, it has not yet been confronted with the conditions of the field, i.e. with use in large areas subject to the complex, unpredictable and diverse conditions of agriculture. Some plant varieties have been developed through energy selection to enhance their ability to induce defense responses and reduce the need for pesticides. Agricultural plots are treated with defence inducers (salicylic acid, chitosan, etc.). For the rest, traditional ecological agricultural practices are sufficient to cultivate plants with induced resistance: crop rotation and cover crops or biodiversity.
In conclusion, induced resistance reduces the use of chemical pesticides by improving the resistance of crops to biotic and abiotic stresses, which leads to increased yields. Its potential is widely recognized. But, large-scale implementation requires in-depth knowledge of induced resistance mechanisms and plant biology. In addition, the effectiveness of resistance inducers can vary depending on environmental conditions, plant species, and pathogens involved.