Keeping crops healthy is the focus of farmers no matter what crop they cultivate or how they grow it. Although growers are familiar with disease preventative measures, protective treatments when coping with disease threats and treatment options when disease occurs, the plant’s own defenses are often less familiar.
Just what defense mechanisms do plants have to keep safe from infection or fight off serious illness?
Constitutive defenses are those which are always present in the plant, offering protection from organisms or environmental factors that might cause harm. The first line of defense for plants is their own structure – think waxy cell coatings, thorns, cell walls and bark or other structural components that offer protection. These can also include biochemical traits which are part and parcel of the plant, often warning off predators via smell or toxicity.
Plants also have inducible defenses, which are those that are triggered only when a threat is detected. This can include the production of some toxins or the fortifying of cell structures, but also includes a variety of other metabolic reactions produced only when a threat is detected.
Induced defenses operate in “the right place at the right time” mode, said Rick Bostock, professor of plant pathology at UC-Davis, speaking during a recent webinar. They can be structural or chemical, but the key is that they occur in response to an attack that has happened.
Induced defenses happen when a plant is challenged by a threat. These responses can be localized, occurring directly at the site of attack. Structural changes, such as lignin production, may occur in order to isolate out a pathogen. The lignin acts as a barrier to prevent the pathogen from consuming any more plant material, effectively starving the pathogen. Cell walls may be induced to grow thicker. “Shot holes” seen in diseased plants are a result of induced lignin barricading the pathogen.
But these induced defenses can also be systemically manifested, basically alerting the entire plant to protect itself from a threat. Systemic induced resistance (SIR) occurs when an attacker triggers a systemic plant response that protects other parts of the plant – not only the affected area – against that same pathogen as well as against other pests and pathogens. Insects, as well as certain pathogens and some root colonizing bacteria, can cause induced systemic resistance (ISR). Most pathogens cause systemic acquired resistance (SAR). Both are types of SIR.
The difference between the two has to do with the chemistry behind the response. In ISR, the chemical signal used is the jasmonic acid pathway, while the salicylic acid pathway is the backbone of SAR chemistry. These responses can also be elicited via the use of plant activators (natural or synthetic chemicals), which trigger the immune response.
Both jasmonic and salicylic acid act as plant hormones. The increase in these hormones occurs as a result of an attack then causes a lot of other chemical pathways to activate. The end result is an activation of gene expression as a direct result of the threat encountered. Either pathway uses distinct chemical signaling pathways and evokes a genetic response in the plant in an attempt to fend off invaders.
The plant is “being triggered to be on the lookout for new attacks,” Bostock said of SIR.
Both organic and synthetic plant activators prime the plant’s immune system. Chitosan from crab and shrimp primes both IRS and SAR pathways. Sygenta’s Actigard™, which was developed decades ago, operates on the salicylic acid pathway. Other commercial products, derived from various organic or synthetic sources, also act on these SIR pathways. The majority of products target the salicylic pathway, he said.
When growers use commercial plant activators, they are invoking a systemic induced response. “You’re harnessing natural plant defenses and in this way getting some control over diseases and pests that attack your plants,” Bostock said.
Do No Harm
There is a downside to activating the plant’s natural immune system. The plant will naturally lower its ability to photosynthesize and instead divert energy into immune responses. Plants that have had their defenses triggered can have lower yields than those that have not been triggered, particularly when disease pressure is low.
“Plant defenses are costly,” Bostock said, as they involve a “profound reorganization of gene expression. This is the tricky part.”
Activating one pathway can make the plant more susceptible to attacks on the other. When the salicylic pathway is activated, the plant can become more susceptible to insect damage, as the jasmonic pathway response can be minimized.
Utilizing plant activators – whether they are organic compounds or synthetic – can provide a way of enhancing the plant’s innate SIR disease defense without the plant having suffered a pathogen attack, in effect priming the plant to be prepared for future threats. Using products according to the label directions, and in conjunction with disease pressures at your location, can offer growers another means of keeping plants healthy.
“When you spray a plant with these, it basically short circuits things a little bit. You actually immediately get the chemistry going in turning on the gene expression that you need in order to produce these proteins that can make the various defense responses,” Bostock said.