Microbial Fertilizers
Systemic Acquired Resistance
Systemic Acquired Resistance (SAR), often referred to in Dutch as systemische verworven resistentie, is one of the most powerful defense mechanisms plants possess against diseases and pathogens.
SAR is a form of long-lasting, systemic immune activation where a local infection or elicitor trigger leads to increased resistance throughout the plant. Within modern biostimulant development, SAR is therefore a key concept in sustainable plant resilience.
What is Systemic Acquired Resistance (SAR)?
SAR is a defense strategy where plants build an “immune memory” after an initial stimulus. When a plant is locally attacked, this activates signaling pathways that spread to unaffected parts of the plant.
This results in:
- increased basal resistance against future infections
- systemic activation of defense genes
- long-lasting protection at the plant level
SAR and the role of salicylic acid
The core hormone within SAR is salicylic acid (SA). When pathogens are recognized, SA concentration rises, leading to activation of PR proteins (Pathogenesis-Related proteins).
Salicylic acid-driven SAR includes:
- signal transduction via SA accumulation
- systemic expression of PR genes
- strengthening of cell wall defenses
- higher tolerance against fungi and bacteria
PR proteins and systemic defense
A typical feature of SAR is the production of PR proteins, such as chitinases and glucanases, which can directly inhibit pathogens.
These proteins provide:
- breakdown of fungal cell walls
- antimicrobial protection
- systemic strengthening of plant tissues
SAR versus ISR: what is the difference?
Within biostimulants, a distinction is often made between:
- SAR – salicylic acid-dependent, often against biotrophic pathogens
- ISR – jasmonic acid/ethylene-dependent, often via rhizosphere bacteria
Both routes enhance plant resilience, but SAR is more strongly linked to immune memory and PR protein activation.
Elicitors as SAR trigger in biostimulants
SAR can be activated by elicitor raw materials, which act as controlled stress signals without disease infection.
Key elicitor sources in biostimulants are:
- seaweed polysaccharides (laminarine)
- chitosan and oligosaccharides
- postbiotic fermentation metabolites
- microbial cell wall fragments
These substances activate SAR pathways and strengthen plant immunity preventively.
SAR and oxidative stress control
Defense activation often accompanies ROS production. SAR therefore includes strengthening of antioxidant enzymes to mitigate oxidative damage.
- superoxide dismutase (SOD)
- catalase
- peroxidases
Synergy with amino acids and metabolic energy
SAR activation requires energy and metabolic building blocks. Free amino acids provide a complete profile of all 20 amino acids, essential for the synthesis of PR proteins, phenols, and defense components.
Additionally, amino acids support the citric acid cycle (Krebs cycle), ensuring ATP remains available for:
- systemic gene expression
- cell wall strengthening
- faster recovery from stress response
Commercial value of SAR-driven biostimulation
For formulators and buyers, SAR is an important concept within sustainable crop protection, as it leads to:
- preventive plant resilience
- less dependence on chemical inputs
- higher production continuity under disease pressure
- premium quality in specialty crops and horticulture
From SAR to yield certainty
The commercial goal of SAR activation via biostimulants is to increase plant resilience without growth inhibition. Effective application results in:
- more resistance against diseases
- faster recovery after infection pressure
- more stable yield and quality
- sustainable cultivation optimization
Overview: Systemic Acquired Resistance mechanism
| Component | Mechanism | Cultivation Value |
|---|---|---|
| Salicylic acid | SAR signaling hormone | Systemic resistance |
| PR proteins | Antimicrobial defense | Less infection |
| Elicitors | Preventive activation | Priming strategy |
| Amino acids + Krebs | Energy for defense | Yield continuity |