For more than 100 years scientists have known that once a plants survives a disease, it is often more resistant to other infections. It’s as if the plant’s immune system has become stronger, and actually, it has.
What was not known then was how it worked, and researchers have been working on teasing out the underlying mechanism for decades. Being able to harness plants’ natural ability to defend themselves would give growers new crop protection tools, and indeed, that has already happened.
There are two well-known types of induced resistance in plants, although there may be more. These two, systemic acquired resistance (SAR) and induced systemic resistance (ISR) are distinguished by the different ways in which the immune response is elicited. In addition, some plants have a localized, rapid response to invasion by certain insects, fungi or pathogens called the hypersensitive response (HR).
Some Plants Have Rapid Response
With the HR response, resistance genes (R) react to a corresponding avirulence gene (avr) in the invading organism. When the resistance gene recognizes the avr gene, chemical signals trigger the hypersensitive response, which quickly causes the death of plant cells at the point of invasion, sealing off the infected tissue and preventing spread to the rest of the plant. In addition, the HR response triggers expression of other genes (called pathogenesis-related genes), which produce proteins able to signal additional defense responses in the plant or to kill the pathogen directly. Some of the pairs of avr and R genes have been identified, which can lead to the introduction of those genes into other plants for protection.
Even plants lacking the HR response, however will still exhibit a wound response — changes in the cell wall that seal off water loss and prevent or slow further invasion of the pest or pathogen.
Systemic Acquired Resistance Is Response To Injury
The acquired immune response noticed a century ago can be compared to a human being getting chicken pox and developing antibodies that protect against future infection. But this comparison fits only loosely. The SAR response is less specific; there are no antibodies against a particular pathogen, and the protection period is shorter. But it does allow the plant to respond much more quickly the next time it is attacked — not only to this particular pathogen, but to others as well — because it is sensitized, or conditioned. And a quick response makes a critical difference between a plant surviving or becoming overwhelmed by the invader. Researchers have observed responses such as production of pathogenesis-related proteins (PRs), antifungal compounds called phytoalexins, and lignification (thickening) of cell walls in areas distant from the initial site of infection. The SAR response can be triggered by the HR response; however even without the HR response, SAR can be activated.
How does the plant “notify” these more distant parts to be on alert? A mobile signal is generated within 4 to 6 hours of infection and is translocated through the phloem (the vascular system that also translocates sugars throughout the plant). The pathway is far from simple or straight; it involves multiple chemicals and interactions. Salicylic acid (the active ingredient in aspirin) plays a key role, but there are many others.
It has also been demonstrated that SAR can be inherited by the next generation through changes in DNA.
Induced Systemic Resistance Triggered By Beneficial Microbes
Beneficial microbes in the soil can also induce systemic resistance in plants in both roots and foliage. Known as plant growth promoting rhizobacteria (PGPR), these microbes not only enhance growth by increasing absorption of nutrients, some of them also trigger systemic resistance. Some do this by triggering the same chemical pathways as SAR, others use completely different mechanisms. Because the method of eliciting the response is different (beneficial soil microbes vs. pathogen/pest) this type of resistance is called induced systemic resistance.
The most common studies on ISR have involved bacteria from the genus Pseudomonas. Bacillus spp. have also been shown to be effective as well as Trichoderma spp. and mycorrhizae; some use the same pathways as Pseudomonas, others activate entirely different ones. Bacillus has elicited an ISR response on a variety of crops against damping-off, root-knot nematode, leaf spots, cucumber beetles, and whitefly, among others. In addition, a combination of ISR and SAR can have a compounded effect; together they are more effective against a wider variety of pests than either is alone. PGPRs can also have direct impact on invading organisms, either with antibiotic activity, competition for space or other resources.