ISR versus direct control

ISR does not mean that microorganisms directly kill pathogens. The effect is within the plant itself: the plant activates defense mechanisms more quickly, slowing disease development.

ISR leads to:

• faster activation of defense genes
• strengthened cell wall responses
• higher antioxidant capacity
• less damage during infection

How is ISR activated?

Induced systemic resistance usually arises after stimulation by beneficial root-colonizing microorganisms.

Key ISR trigger groups

• PGPR biostimulants (such as Bacillus and Pseudomonas)
• Microbial metabolites and signaling substances
• Elicitors such as oligosaccharides and chitosan
• Seaweed polysaccharides with a priming effect

These stimuli are recognized by the plant as a “warning signal” without actual infection, allowing defenses to be prepared preventively.

ISR as part of plant priming

ISR can be seen as a specific form of plant priming, focused on biotic stress. Primed plants respond faster and more controlled when a pathogen actually strikes.

This provides stress resistance without the plant continuously losing energy to active defense.

Physiological routes within ISR

Jasmonate and ethylene signaling

ISR is mainly regulated via jasmonate and ethylene pathways, which are involved in defense against necrotrophic fungi and insect herbivory.

Strengthening cell walls

ISR often leads to increased deposition of lignin and callose, making it more difficult for pathogens to penetrate.

Activation of antioxidant enzymes

Infection causes oxidative stress. ISR enhances the readiness of antioxidant enzymes, allowing ROS neutralization to proceed more quickly and limiting damage.

Production of secondary metabolites

ISR also stimulates biosynthesis of phenols, phytoalexins, and other defensive substances. These strengthen plant resistance and inhibit pathogen development.

ISR and microbiome-driven plant resistance

Induced systemic resistance demonstrates that the microbiome not only actively directs growth but also plant health. A well-functioning rhizosphere leads to:

• higher disease resistance
• better root continuity
• less yield loss due to biotic stress

ISR is therefore a key mechanism within microbiome-driven growth.

Synergy with biostimulant raw materials

ISR is often enhanced by combinations of microbial components and bioactive raw materials.

Examples of synergy

• PGPR + fulvic acid for uptake and rhizosphere condition
• PGPR + chitosan for elicitor-driven defense activation
• Seaweed extract + oligosaccharides for priming and stress buffering
• Microbial metabolites + protein hydrolysates for root health

ISR as a preventive cultivation strategy

ISR is particularly valuable because it works preventively. Instead of reactive control, the plant can develop a higher defense status in advance.

This fits perfectly within sustainable cultivation optimization:

• less dependence on chemical fungicides
• better resistance under field stress
• stability of yield and quality

From ISR to yield stability

Due to less infection pressure and faster defense response, ISR results in:

• lower disease damage
• less stress-related growth inhibition
• more uniform production
• more stable yield and quality

Overview: ISR within biostimulant strategies