ISR versus SAR: the main difference

Within plant immunity, two systemic routes are distinguished:

• SAR – salicylic acid-dependent, often pathogen-driven
• ISR – jasmonic acid/ethylene-dependent, rhizosphere-driven

ISR is usually activated without large PR protein accumulation, but it does lead to increased preparedness of defense mechanisms.

Jasmonic acid and ethylene as ISR signal hormones

ISR is mainly driven by two hormonal routes:

• Jasmonic acid (JA)
• Ethylene (ET)

These hormones regulate defense against necrotrophic fungi, insect herbivory, and broad stress factors. ISR activates a different defense spectrum than SAR.

Which microorganisms activate ISR?

ISR is mainly elicited by plant growth-promoting rhizobacteria (PGPR) and beneficial fungi. Important examples include:

• Bacillus subtilis and Bacillus lipopeptides
• Pseudomonas fluorescens
• Trichoderma harzianum
• Mycorrhiza symbioses

These organisms produce metabolites that function as elicitors and priming signals.

ISR and plant priming: increased readiness

ISR is often considered a form of plant priming. The plant does not build up permanent defense activation but becomes “more quickly deployable” upon a subsequent stress trigger.

Primed plants exhibit:

• faster activation of defense genes
• higher production of phenols and lignin
• more efficient antioxidant response
• less yield loss under pressure

Microbial metabolites as ISR triggers

ISR is activated by various microbial signal substances, such as:

• lipopeptides (surfactine, iturine)
• siderophores (iron competition)
• cell wall fragments
• postbiotic fermentation metabolites

These molecules form a growing platform within next-generation biostimulants.

ISR and abiotic stress mitigation

Although ISR is often associated with disease resistance, it also strengthens tolerance against abiotic stress, such as:

• drought stress
• salt stress at high EC
• heat stress in greenhouse horticulture

This occurs via enhanced root activity, antioxidant protection, and osmoregulation.

Synergy with amino acids and metabolic energy

ISR activation requires metabolic building blocks and energy. Free amino acids provide a complete profile of all 20 amino acids, essential for enzymes, phenols, and defense compounds.

Additionally, amino acids support the citric acid cycle (Krebs cycle), making ATP available for:

• root growth and rhizosphere interaction
• stress recovery following defense reactions
• active nutrient transport

Commercial value of ISR in biostimulant formulations

For formulators and purchasers, ISR is a key mechanism because it leads to:

• preventive plant resistance
• stronger root zone and less loss
• lower dependence on chemical inputs
• sustainable yield security in specialty crops

Thus, ISR is a core concept within microbial biostimulant raw materials.

Overview: ISR mechanism in agriculture