Induced Resistance In Plants: Do Your Plants Need An Aspirin?

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What Is Induced Resistance?

Induced resistance is a defense system within plants which allows them to resist attacks from pests such as fungal or bacterial pathogens or insects. The defense system reacts to the external attack with physiological changes, triggered by the generation of proteins and chemicals that lead to activation of the plant's immune system.

Think about this in the same way as you would consider the reaction of your own immune system to attack, from, for example, a cold virus.

The body reacts to the presence of an invader through several different mechanisms; however, the result is the same. The alarm has been sounded, and the system mounts a defense to the attack.

Two Types Of Induced Resistance

Two main types of induced resistance exist: systemic acquired resistance (SAR) and induced systemic resistance (ISR).

  • Systemic acquired resistance occurs when a localized wound is created on the plant, causing necrosis. The resistance is stimulated when a treatment designed to induce the resistance is applied to the spot where the pathogen has invaded the plant. The treatment can come in the form of another microbe, or as a chemical, such as salicylic acid. (An interesting fact: salicylic acid is also used to make aspirin!) The treatment triggers a systemic response in the plant, and the immune response is signaled. Obviously, this process takes some time to occur, depending on the plant species, the environmental conditions, and the nature of the pathogenic attack.
  • Induced systemic resistance occurs when plant roots are colonized by plant growth promoting rhizobacteria (PGPR), soil bacteria which directly and indirectly influence plant growth. When the PGPR sense a change in the plant, a physiological response is triggered via a pathway involving (again!) salicylic acid. The chemicals jasmonate and ethylene are also involved as signaling chemicals. Unlike SAR, necrotic lesions on the plant are not involved in ISR.

    Both resistance pathways lead to the same final ending -- the genes are different, the pathways are different, the chemical signals are different -- but they both induce the resistance of plants to attack by pests. Although the pathways are not alike, they can work synergistically, and therefore the scientific community decided in the early 2000's to consider ISR and SAR as synonyms.

    History Of Induced Resistance Research

    The phenomenon of induced resistance has been realized for many years, but only since about the early 1990's has it been studied as a valid method of plant disease management. The most prophetic early paper on induced resistance was published in 1901 by Beauverie. Titled "Essais d'immunization des vegetaux contre des maladies cryptogamiques", or "Testing the immunization of plants against fungal diseases", Beauverie's research involved adding a weakly virulent strain of the fungus Botrytis cinerea to begonia plants, and discovering that this imparted resistance to more virulent strains of the fungus. This research was followed up by Chester in 1933, who outlined the first general concept of plant defense systems in his publication titled "The problem of acquired physiological immunity".

    The first biochemical evidence for induced resistance, however, was discovered in the 1960's. Joseph Kuc, widely considered to be the "father" of induced resistance research, demonstrated for the first time the induction of systemic resistance using the amino acid derivative phenylalanine, and its effect on imparting resistance of apples to apple scab disease (Venturia inaequalis).

    Recent Work And Commercialization Of The Technology

    Although the presence and identification of several pathways and chemical signals have been elucidated, scientists are still unsure of the mechanisms involved for many plant species and many of their diseases or pests. For example, the resistance mechanisms involved for plant viruses are still not well understood.

    There are several resistance inducers -- called plant activators -- on the market.

    ActigardTMV was the first resistance inducer chemical on the market in the USA. It is made from the chemical benzothiadiazole (BTH) and registered for use in many crops, including garlic, melons, and tobacco.

    Another product involves proteins called harpins. Harpins are proteins produced by plant pathogens. Plants are triggered by the presence of harpins into a warning system to activate resistance responses. Currently, a company called Rx Green Solutions is marketing harpins as a product called Axiom.

    Key Terms To Know

    • Phytoalexins -- antimicrobial proteins that accumulate in plant cells following microbial infection. They do not appear in healthy tissues; they are only formed after infection or injury.
    • Hypersensitive response -- the rapid response triggered by a plant in response to pathogen attack.